High Level Science Goals, Key Science Requirements, Operational Concept · 2014. 7. 16. · argument arise from increased collecting area; the other two arise from improved image

GMT SYSTEM LEVEL PRELIMINARY DESIGN REVIEW December 17, 2013

High Level Science Goals, Key Science Requirements,

Operational Concept Section 2


HIGH LEVEL SCIENCE GOALS, KEY SCIENCE REQS, OPERATIONAL CONCEPT 2–2

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Table of Contents 2 HIGH LEVEL SCIENCE GOALS, KEY SCIENCE REQUIREMENTS, OPERATIONS

CONCEPT ................................................................................................................................... 5 2.1 High Level Science Goals ..................................................................................................... 5

2.1.1 Discovery Space Opened Up by the GMT ..................................................................... 5 2.1.2 Contemporary Science Goals ......................................................................................... 7

2.1.2.1 Formation and Evolution of Planetary Systems ................................................... 7 2.1.2.2 Stellar Populations and Chemical Evolution ...................................................... 14 2.1.2.3 Galaxy Assembly and Evolution ........................................................................ 18 2.1.2.4 Dark Matter, Dark Energy, and Fundamental Physics ....................................... 25 2.1.2.5 First Light and Reionization ............................................................................... 28 2.1.2.6 Transient Phenomena ......................................................................................... 33

2.1.3 Scientific Synergies with Other Major Facilities ......................................................... 38 2.1.3.1 Synergy with Ground-Based Facilities ............................................................... 39 2.1.3.2 Synergy with Space-Based Missions .................................................................. 41

2.1.4 Summary of High Level Science Goals ....................................................................... 42 2.2 Top-Level Science Requirements ...................................................................................... 43

2.2.1 Mapping Science Goals and GMT Requirements ........................................................ 43 2.2.2 Telescope and Subsystem Requirements ..................................................................... 47

2.2.2.1 General Requirements ........................................................................................ 47 2.2.2.2 Spectral Range .................................................................................................... 47 2.2.2.3 Seeing-Limited Image Quality ........................................................................... 47 2.2.2.4 Motion Control ................................................................................................... 48 2.2.2.5 Adaptive Optics Requirements ........................................................................... 48 2.2.2.6 Instrument Requirements.................................................................................... 50

2.3 Operational Concept .......................................................................................................... 50 2.3.1 Organization ................................................................................................................. 50 2.3.2 Facilities and Infrastructure .......................................................................................... 51

2.3.2.1 Summit Facilities ................................................................................................ 52 2.3.2.2 Operations Center ............................................................................................... 52 2.3.2.3 Science Operations ............................................................................................. 53

2.3.3 Operating Modes .......................................................................................................... 53 2.3.3.1 Investigator Directed - On-Site (“Classical”) ..................................................... 53 2.3.3.2 Investigator Directed – Remote .......................................................................... 53 2.3.3.3 Service Observing .............................................................................................. 54 2.3.3.4 Queue Scheduled Service Observing ................................................................. 54 2.3.3.5 Survey and Campaign Modes ............................................................................. 54 2.3.3.6 Interrupt Mode / Target of Opportunity ............................................................. 55 2.3.3.7 GMTO Support for Operational Modes ............................................................. 55

2.3.4 Observing Modes ......................................................................................................... 55 2.3.5 Time Allocation ........................................................................................................... 56

2.3.5.1 Engineering Time ............................................................................................... 56 2.3.5.2 Contributors and Others’ Observing Time ......................................................... 56 2.3.5.3 Director’s Discretionary Time ............................................................................ 57 2.3.5.4 Time Allocation Process..................................................................................... 57

2.3.6 User Support ................................................................................................................. 58 2.3.6.1 Observing Assistance ......................................................................................... 58 2.3.6.2 Instrument Handbooks ....................................................................................... 58 2.3.6.3 Data Reduction Pipelines ................................................................................... 58 2.3.6.4 Quick-Look Reduction and Analysis Tools ....................................................... 59



2.3.7 Instrumentation and Adaptive Optics .......................................................................... 59 2.3.7.1 Multiple Instruments and AO Available............................................................ 59 2.3.7.2 Configuration ..................................................................................................... 60 2.3.7.3 Calibrations ........................................................................................................ 60 2.3.7.4 Performance Monitoring.................................................................................... 60

2.3.8 Performance and Success Metrics ............................................................................... 60 2.3.9 Science Data Management .......................................................................................... 61 2.3.10 Data Archive and Distribution .................................................................................... 61

2.3.10.1 Common Data Formats ...................................................................................... 61 2.3.10.2 Data Compatibility ............................................................................................ 62 2.3.10.3 Remote Networking ........................................................................................... 62 2.3.10.4 Engineering Data Management ......................................................................... 62 2.3.10.5 Workstations ...................................................................................................... 62

2.3.11 Environment ................................................................................................................ 62 2.3.11.1 Environmental Data Gathering and Statistics .................................................... 62

References .................................................................................................................................... 64



2 HIGH LEVEL SCIENCE GOALS, KEY SCIENCE REQUIREMENTS, OPERATIONS CONCEPT

2.1 High Level Science Goals The GMT science case is structured in three parts: new discovery space opened by the GMT (Section 2.1.1) contemporary science goals that will be addressed by the GMT (Section 2.1.2) and scientific synergies with existing and planned facilities (Section 2.1.3). The most important aspects of the science case for this review document are those that drive requirements for the facility, scientific instruments, and operations.

The discussion begins with the new discovery space opened by increased angular resolution and the large primary collecting area of the GMT. The bulk of the discussion that follows details how the GMT can be used to address contemporary problems in astronomy and astrophysics. This is naturally where the bulk of the effort in developing the scientific requirements for the facility is focused. Since the precise mix of questions of interest at the time of first light with GMT will surely differ from what is outlined here, the discussion is focused at a level such that the derived requirements will be robust against changes in the details of the scientific questions.

2.1.1 Discovery Space Opened Up by the GMT The gain from an increased aperture in seeing-limited applications is easily characterized. The number of photons collected per unit time increases as the collecting area, or D2. Thus the signal to noise per unit time increases as the first power of the diameter in background- and source-limited applications, and as the square for detector-limited and other fixed-noise environments. The time needed to reach a given signal-to-noise ratio for a fixed flux, often called the “sensitivity”, decreases as the first or second power of the diameter in the background or detector-noise limited regimes, respectively. Many science applications with large telescopes are sky or background-limited. High-dispersion spectroscopy is occasionally detector-noise limited, while high signal-to-noise high-resolution spectroscopy of bright targets is often source-noise limited.

Adaptive optics (AO) allows one to concentrate the light from point or compact sources against the foreground sky. This improved image concentration provides additional gains in sensitivity as a function of aperture size. In the sky- or background-limited regime, the signal to noise ratio for a point source per unit time increases as D2, while the time needed to reach a given signal to noise ratio decreases as D4. The slightly dilute pupil provided by the GMT makes this scaling somewhat more complex. Two of the powers of D in the D4 argument arise from increased collecting area; the other two arise from improved image concentration due to diffraction. The effective diameters for collecting area and diffraction differ for the GMT. Thus, when comparing to an 8 m aperture, one should consider the GMT AO sensitivity as scaling like (24.5/8)2 * (21.9/8)2=70.

This D4 factor is often cited as one of the primary drivers for extremely large telescopes (ELTs). Some of the most interesting applications of adaptive optics today, however, will likely not scale as D4, as they deal with objects that are partially or fully resolved at ~100 mas resolution. Integral field spectroscopy of distant galaxies, for example, is one of the key science drivers for ELTs. While distant galaxies have a significant flux in compact structures, they also have significant flux in resolved structures at HST resolution and so one will not achieve the same gains in sensitivity realized for point sources. We expect gains in speed that will scale as roughly ~D3 in this case.

There are regimes where ELTs will deliver gains in sensitivity that scale as a higher power of the aperture diameter. These include crowded fields and contrast-limited applications. Imaging of partially resolved stellar populations in crowded regions will benefit greatly from the increased



image concentration for point sources and the reduced crowding and confusion noise associated with the unresolved component. Imaging of exoplanets near bright stars will also benefit from increased aperture through concentration of the faint exoplanet signal, reduced PSF wings from the parent star and improved stability of the PSF. Gains as steep D6, or steeper, have been posited for such application.

Figure 2-1. Discovery space opened by gains in sensitivity. We compare the 5σ depths in an hour of integration for current 8-meter apertures (tops of bars) with the depths that will be achieved by the GMT (bottoms of bars) across its entire operating wavelength range. Seeing-limited applications are shown in blue, while AO applications are shown in red. Spectroscopic limits are indicated by the curves in the visible and near-IR. Sensitivities are shown as micro-Janskys (Left) and AB magnitudes (Right).

Figure 2-1 illustrates the discovery space opened by the GMT in terms of gains in sensitivity. In this context, sensitivity refers to the depth achieved at a fixed signal-to-noise ratio in a fixed time as a function of wavelength. Figure 2-2 illustrates the discovery space opened by the increased angular resolution offered by the GMT compared to current generations of telescopes.

Figure 2-2. Angular resolution discovery space opened by the GMT using AO. The red bar shows the difference between the angular resolution of 8 m diffraction-limited AO systems and the GMT AO system, as defined by the Rayleigh criterion. We also show lines for 3 and 5 λ/D for contrast limited AO applications. The linear scale corresponding to the angular scale on the left is shown on the right for distances appropriate to exoplanets (100 pc) and distant galaxies (z=1) in AU and kiloparsecs, respectively. Seeing-limited and ground-layer adaptive optics resolutions do not scale with aperture and are shown as blue lines for reference.



Adaptive optics also opens up a discovery space in spatial resolution (Figure 2-2). The gains in angular resolution scale as the diameter of the aperture. The factor of 3 increase in resolution compared to an 8 m aperture opens up a number of interesting areas of parameter space. The GMT will dramatically increase the volume of space over which one can image the sphere of influence for massive black holes. Similarly, the mass range that one can probe for central black holes in nearby galaxies is improved. In the case of exoplanet imaging, a factor of 3 reduction in the inner working angle opens up a large number of radial velocity detected planetary systems for imaging in reflected light, where very few are within reach today. Similarly, in the case of thermal emission, the improved angular resolution will allow the GMT to reach to stellar nurseries in the southern Milky Way and to probe terrestrial zones in nearby stars and protoplanetary and debris disks.

2.1.2 Contemporary Science Goals The following sections deal with contemporary science topics that are both of interest and are relevant to the ELT user community. The topics covered are:

• Formation and Evolution of Planetary Systems

• Stellar Populations and Chemical Evolution

• Galaxy Assembly and Evolution

• Dark Matter, Dark Energy, and Fundamental Physics

• First Light and Reionization

Each section focuses on a small number of topics of interest to illustrate where the GMT can have a high impact. Examples include using its great collecting area to probe abundance patterns in the most metal poor stars, using its increased angular resolution and sensitivity to measure dynamical masses of young galaxies, and using its sensitive near-IR spectroscopy to detect Lyα and HeII emission from galaxies in the first few hundred million years after the Big Bang.

2.1.2.1 Formation and Evolution of Planetary Systems Two paradigms currently dominate planetary formation models: core accretion (see e.g., Ida & Lin 2004 and references therein)1 and gravitational disk instability (e.g., Boss 1997)2. The former involves the collisions and sticking of rock-ice planetesimals, which grow to Earth-size and beyond; the latter posits that planets form through gravitational instabilities in the proto-planetary disk. Observations currently favor the core accretion model for inner regions, and the instability model at larger separations (i.e., >5 AU). Many current questions about planet formation will likely remain the targets of active research for a decade or more. These include:

• In which environments do the core accretion and/or disk fragmentation mechanisms dominate?

• To what extent are the planetary properties we observe a result of formation processes, as opposed to migration and dynamical evolution?

• How do formation mechanisms impact the composition and structure of exoplanets and their atmospheres?

• What is the full range of planetary system multiplicities and structures (i.e., the range of system architectures) produced?

• How common are Earth-analogue planets and how often are they hospitable to life?



Known planets today have masses that range from a few to a few thousand Earth masses, and locations that range from a few hundredths of an AU to ~100 AU from their parent stars. Understanding how this panoply of systems formed is one of the great challenges facing astrophysics. Constraining models for the distribution of planetary orbits and masses requires better statistics on the locations and timescales of planet formation, planetary migration rates, and disk dissipation times.

Kepler has identified thousands of candidate transiting planets (Borucki et al., 2011)3 and our understanding of the statistical properties of planetary systems has been greatly expanded. Kepler has revealed that Earth-sized planets in the habitable zone are common around solar type stars (Howard et al., 20114 ; Petigura et al. 20135).

Direct imaging techniques are producing their first exoplanetary detections (Marois et al., 20086, 20107; Kalas et al., 20088) and over the next decade these methods promise to find more giant planets in the outer regions of planetary systems. We can also expect that over the next 10 years, Doppler planet searches will continue to push their precision limits to lower masses, identifying terrestrial-mass planets with orbits of under a year (e.g., Howard et al., 20109, Wittenmyer et al., 2011b10), while also determining the frequency with which Jupiter-mass planets orbit Sun-like stars in orbits comparable to those of gas giants in our Solar System (e.g., Wittenmyer et al., 2011a11).

The GMT will possess multiple capabilities critical for breakthroughs in exoplanetary science. Its huge aperture will enable acquisition of spectra of transiting planets 7.5 times faster than current 8.m telescopes, enabling a new generation of spectroscopic studies of exoplanet atmospheres in the short windows allowed by primary- and secondary-transit durations. The GMT’s 24.5 m effective diameter will allow it to achieve unprecedented spatial resolution. The potential for synergy when these capabilities are combined in one facility will be powerful. For example, while transit observations and spectroscopy with G-CLEF will identify and measure the masses of habitable planets, the GMT spectrographs will study their atmospheres, and near-IR imagers will be able to image the outer planets directly.

This section addresses key questions in planet formation and evolution for which it is believed the GMT will make major contributions. These include: (1) the disk-planet connection, (2) characterization of exoplanet atmospheres, (3) probing the architecture of planetary systems through imaging and precision Doppler measurements, and (4) characterizing habitable worlds. The GMT Science Book12 also discusses the role of the GMT in advancing our understanding young stars, their jets, transport of volatiles within disks, and studies of our own solar system.

2.1.2.1.1 Young Stars and the Disk-Planet Connection Stars form and evolve hand-in-hand with planets, and stellar properties such as mass, rotation, and magnetic field strength control their co-evolution. Star formation begins with molecular clouds where discrete clumps of gas and dust gravitationally collapse to form protostars. Conservation of angular momentum during the collapse results in gas and dust settling into circumstellar and protoplanetary disks, in which planets may coalesce and grow (Williams & Cieza, 2011)13.

Obscuration and shadowing by disks put a large fraction of protostars in dark clouds beyond the reach of 8 m telescopes. It is widely thought that embedded stars are younger on average than their unobscured counterparts in the same regions, but this paradigm has yet to be tested. Winds, driven by X-ray and UV radiation ultimately limit the time available for forming planets around young stars. The GMT’s high spatial and spectral resolution capabilities will enable multiple, simultaneous astrophysical measurements that address the key issues of obscuration, age, winds and magnetic fields.



Figure 2-3. The spectrum of a young planet such as β Pic b can reveal the composition of the atmosphere and the presence of dusty clouds. The black bars show a simulated GMT/GMTNIRS spectrum for a 1800 K planet observed around β Pic in 1 hour at a spectral resolution of 30,000, as it would look if the atmosphere were cloudless (dark blue curve) or cloudy (light blue curve). Models courtesy of Travis Barman.

Disks with masses sufficient to form a planetary system like the Solar System have been detected around more than 80% of Sun-like stars at young ages (~1 Myr) but are essentially non-existent for stars older than 10 Myr (Strom et al., 198914, Williams & Cieza, 2011). Only a few tens of these disks have been spatially resolved, largely due to the high contrast between protostars and disks, and the great distances to the stars.

If planets form early by gravitational instability (rather than by core accretion) the GMT will be able to image them around very young stars and subsequently study their orbits and atmospheres. For example, the recently detected 8 MJup planet around the 12 Myr old star β Pic has an L-band apparent magnitude of 11. Figure 2-3 shows that the GMT would easily obtain spectra of this planet in under one hour. GMTNIRS’ high-spectral resolution will enable the determination of the molecular composition of the planet, including the abundances of important molecules such as water, methane and carbon monoxide, as well as evidence for auroral emission due to a planetary magnetic field producing H3+. As more planets of lower masses and fainter magnitudes are discovered closer to their stars by direct imaging at smaller separations, GMT spectra will facilitate further studies of their physical conditions.

Studying young disks requires high spatial and spectral resolution. The GMT’s smaller inner working angle compared to 8–10 m apertures will allow it to carry out observations that probe to 2.AU at 1.6 μm or 4 AU at 3.8 μm (for the typical 140 pc distance of nearby star forming regions) thereby discovering large samples of young disks. High contrast (10-6) AO images with ~4 AU resolution will enable the first direct views of the terrestrial and gas giant formation zones in disks around the youngest nearby stars. Thermal infrared imaging at ~10 μm may further reveal temperature and structure perturbations in disks, as in Figure 2-4 and Figure 2-5. Gravitational instability leads to the formation of spiral arms that may be observable in scattered light. These features can change on short timescales (



Figure 2-4. Simulated GMT image of protoplanetary disk inclined at 45 degrees to our line of sight, with a 30 MEarth planet at 10 AU that carves a gap in the disk. The left panel shows the model. The right panel shows the disk around a star at 100 pc as imaged at 3.8 µm with the GMT mid-IR imager concept, TIGER. The GMT has the power to reveal planets in formation and their interactions with the disk from which they form.

Figure 2-5. Simulated GMT image of the disk around HR4796A at 3.8 μm. Left: The Kuiper Belt-like disk at 1’’ (73 AU) has been imaged at 5x lower spatial resolution with HST, but will be revealed spectacularly by the GMT. Modeled here as smooth, the GMT would be able to reveal any clumps or asymmetries not visible from HST. Right: A close-up of a simulated TIGER image of a hypothetical inner disk–a model of zodiacal-like emission 1500x the Solar System Zodi sitting in the habitable zone of the star. The GMT would not only detect such dust but also spatially resolve it. The true distribution of dust around this star depends on the architecture of its planetary system, which could be inferred from GMT images even if the planets are too small to detect directly.

GMT observations of the CO fundamental lines will probe the kinematics of warm gas and may allow separate observations of the Keplerian gas motion, and hence a direct constraint on the stellar mass and radial motions of gas such as streamlines past a giant planet in formation.

2.1.2.1.2 Probing Exoplanet Atmospheres Observing primary and secondary eclipses of transiting planets yields insights into their atmospheric compositions that cannot be obtained with other techniques. During primary transit, when the planet crosses the stellar disc, spectroscopic observations of the depth of the transit can be used to determine the planet’s radius at a variety of wavelengths, and subsequently the properties of the transiting planet’s atmosphere (Figure 2-6). The key factor presently limiting such work is the difficulty in obtaining a high enough S/N ratio during short (typically ~1 h) transit durations. The GMT’s aperture will make it possible to achieve the required sensitivity during transits. It will also



make it possible to use this technique on smaller planets with thinner atmospheres, thus opening the door for probing the atmospheres of the potentially habitable planets that will be discovered by missions like TESS over the coming decade.

Figure 2-6. Primary transit spectroscopy from the VLT has been used to probe the atmosphere of the 6

MEarth planet GJ1214b (Bean et al., 2011)15

In secondary transit, when a planet passes behind the star, observations of the stellar light can be subtracted from the Planet+Star observations (obtained out of transit) revealing a planetary emission/reflection spectrum. The keys to these observations are high precision and high stability near-IR spectroscopy sufficient to measure contrast levels of Fplanet/Fstar1 Gyr) planets detectable via their thermal infrared emission or reflected light (Figure 2-7). The GMT will provide high contrast, high resolution imaging capabilities in the near and mid-infrared enabling the detection of exoplanets in each of these categories.



Figure 2-7. The multiple planet system orbiting HR8799 has been resolved by multiple ground-based AO

systems, including these 1.6 and 3.3 micron images taken with the LBT (Skemer et al. 201216). 2.1.2.1.3.1 Young Gas Giant Planets The L’-band is likely to be the most sensitive window for imaging giant planets. In this band, the GMT will have a resolution limit (2 λ/D) corresponding to 46 mas, and in a 1-hour observation will be able to reach a sensitivity limit of 20th magnitude. In Table 2-1 we list some representative limiting masses for GMT observations of exoplanets with ages ranging from 10 Myr to 1 Gyr. In Figure 2-8 we show simulated GMT observation of planets in the β Pic system (already known to host one 10 MJup planet). A design reference survey to L’=20 for some 50 nearby stars with a median age of 0.5 Gyr would be expected to detect some 15-20 new exoplanets with separations of 1–20 AU. Follow-up photometry and low resolution spectroscopy at wavelengths between 1 and 13 μm will be particularly critical for determining atmospheric and surface properties of these planets.

Figure 2-8. Simulated GMT observations of the β Pic system (compared to current state-of-the-art VLT L’ data). GMT will not only trivially detect the known 10 MJup planet, but will be able to detect planets of Saturn mass beyond 3 AU in this system.

Table 2-1. Detectability of planets via thermal IR imaging

Distance (pc) Age Separation (AU) Mass Limit 50 10 Myr 2.3



2.1.2.1.3.2 Thermal Imaging of Rocky Planets It is likely that by the time the GMT goes into operation, nearby stars hosting rocky Earth-mass planets will have been identified by transit searches (e.g., TESS) or on-going Doppler survey programs, providing a wealth of new targets for GMT direct imaging studies. The nearest stars (d



2.1.2.1.4 Probing the Nearest Habitable Planets Using Doppler Spectroscopy (Earth Analogues Orbiting Sun-type Stars)

Doppler velocity measurements will remain critical in the decade ahead, since only the combination of Doppler velocity measurements and transit searches can reveal both exoplanet mass and size. This combination delivers exoplanet bulk density, which is a critical parameter in determining whether the potentially habitable low-mass planets we find orbiting other stars are rocky, small ice-giants, or water worlds.

With its high-precision G-CLEF spectrograph, the GMT will be able to carry out a census of the population of Earth-like planets orbiting the nearest Sun-like stars. Such a census requires sub-10_cm/sec Doppler velocity precisions at multiple epochs over a planetary orbital period. Recent Doppler detections by the HARPS spectrograph indicate that these extraordinary levels of precision are achievable. Indeed, the ability to detect planets down to a few Earth masses at periods of up to 90 d has been demonstrated by obtaining multiple observations averaged over a single night, and 40-50 epochs per year over several years (Pepe et al., 2011)18. Moreover, when those detected planets are removed from the data sequence, and the resulting residuals binned over periods of 40–50 d, the resulting data set displays residual velocity dispersions that drop below 20 cm/sec.

This result shows that there are stars in the solar neighborhood with the intrinsic Doppler stability (at periods of interest) to detect Earth-like planets, as long as sufficient photons can be detected, and astrophysical sources of noise can be averaged over. Observations that achieve these goals can potentially detect the much sought-after habitable Earth-analogue planets in ~1 AU orbits.

Obtaining such data for a meaningful sample will require the combination of the GMT’s massive aperture and G-CLEF’s extraordinary stability and precision. The HARPS data described above for HD20794 was collected by integrating nine 4-m telescope nights over 3 years, all dedicated to this one target, delivering a ‘per night’ dispersion of 82 cm/s. Dropping this dispersion down to 10_cm/sec will require averaging over some 70 epochs to acquire the extra factor of 8.2 in signal-to-noise required. This detection is completely unfeasible on 4- or 8-m telescopes. The GMT, however, will collect photons 37 times faster than HARPS’, making it possible to study a sample of 10 Earth-analogue host stars in less than 36 nights per year over the first 5 years of operation. It bears noting that this represents the most conservative possible calculation, based only on presently achieved Doppler precision. Recent history suggests that further gains in precision are likely.

2.1.2.2 Stellar Populations and Chemical Evolution Baade’s early recognition of distinct stellar populations has provided an enduring framework for studies of galaxy formation and evolution, stellar chemistry, and stellar dynamics. Today, any viable model for the formation of the Milky Way must address the origin of thin disk, thick disk, bulge and halo populations, and the diversity of their dynamical, chronometric, and chemical properties. Our understanding of stellar populations in external galaxies is still quite limited and is based largely on integrated light spectra. With its southern location and well-suited instruments, the GMT (in concert with SkyMapper, VISTA, VST, LSST, DES and other southern surveys) will open a number of avenues for detailed studies of stellar populations in the outer halo, the Local Group, and beyond.

The GMT will greatly enhance stellar science through the acquisition of high-resolution spectra of brighter targets with S/N ratios that cannot be achieved today and by probing stellar systems at larger distances than possible with 8-10 m apertures. Diffraction-limited imaging provides an additional avenue for stellar population studies with the GMT. The sections below highlight a few of the topics to be explored with the GMT in the area of stellar populations and chemical evolution.



2.1.2.2.1 Characterizing the Most Metal-Poor Stars Extremely metal-poor stars ([Fe/H] < –3.0) predate the main halo population, possibly from a pre-Galactic era. Some stars are so metal-poor and have such unusual compositions that they are thought to reflect the composition of individual supernova events after the Big Bang (e.g., McWilliam, 199519; Beers & Christlieb, 200520; Frebel, 201021). These objects are profoundly important for understanding the very first chemical enrichment in the Universe, and provide important constraints on the nature of Population III stars, their IMF, and the yields of the first massive core-collapse supernovae. Unfortunately, since these stars are so metal-poor, (the most metal-poor stars currently known have [Fe/H]~ -5.5), the strongest absorption lines of the most important elements will be too weak to measure in normal high dispersion spectra with S/N~100 (Frebel et al., 200522, Aoki et al., 200623).

Figure 2-9 shows the spectrum of the extremely metal-poor warm subgiant HE1327-2326, which has Teff ~6200 K and [Fe/H]= -5.4. Also shown, for comparison, is the spectrum of extremely metal-poor turnoff star, G64-12 for which [Fe/H]= -3.2. The region around the strongest Fe line is displayed and its equivalent width is only ~7 mÅ in the HE1327-2326 spectrum. If this object had been somewhat hotter or somewhat more iron deficient, no Fe line could have been measured with current facilities. Given the odd chemical abundance pattern of extremely high [C, N, O/Fe] element ratios and elevated [alpha/Fe] found in both of the currently known [Fe/H]< -5.0 stars, accurate measurements of the Fe abundance from several extremely weak Fe lines are crucial.

Figure 2-9. Spectral region around the strongest Fe line in the optical wavelength regime at 3860 Å. It illustrates the effect of metallicity on line strengths for the Sun and three additional stars with decreasing metallicity. Compared to G64-12 (2nd from bottom), this line in HE 1327-2326 (bottom), with [Fe/H] = -5.4, is hardly detectable. Figure from Frebel & Norris (2012)24.

The next generation of photometric and spectroscopic surveys will unveil large samples of extremely metal poor stars. The SkyMapper Southern Sky Survey (Keller et al., 200725), for example, expects to provide a 100-fold increase in the numbers of [Fe/H]< -3 stars, as well as up to 50 objects with [Fe/H]< -5.0. Most of these stars, however, will be too faint for high resolution spectroscopy with current facilities. Only with the light-collecting power of an ELT such as the GMT will it be possible to study these fossil stars with the required precision to generate a much-improved understanding of the early Universe and the processes governing nucleosynthesis.



The GMT could make major contributions to stellar archaeology through a two-component observing program. A discovery mode (R~20,000, S/N~30 at 4000 Å) would be used to confirm the metallicity of pre-selected targets from large surveys and to quickly identify any unusual abundance characteristics (e.g., ultra-metal-poor, or r-process enhancement). Important targets would then be observed with a resolution of R~40,000 and S/N~100 to acquire data required for a detailed chemical abundance analysis.

In a modest allocation of nights over 2-3 years, one could survey several hundred stars to V~18, selecting a sample of a few tens of stars for detailed abundance work. Over the course of a few years, a modest sized group working within the GMT community could then extract a great deal of value-added science from the large-scale photometric and spectroscopic survey programs planned for the coming decade. In particular, such studies would permit a much more accurate definition of the metallicity distribution function (MDF) at the lowest metallicities. The MDF is currently poorly defined as the number of known extremely metal-poor stars is small: for example, at the present time there are only a handful of stars known with [Fe/H] < -4.0. Characterizing the extremely metal-poor tail of the MDF is, however, pivotal for constraining both models of the formation of the first stars and the role of processes such as SNe feedback in the earliest stages of galaxy formation (e.g., Salvadori et al., 2007)26.

Such studies will also play a vital part in defining the role of carbon abundances at the lowest metallicities. Approximately 20% of stars with [Fe/H] ~ -2 are significantly enhanced (by a factor of ~10 or more) in carbon relative to the scaled solar value, and this percentage rises to ~100% at the lowest abundances. Carbon is probably the first element to enter the ISM and is therefore likely to be a major driver of dust formation at the earliest epochs. Yet the origin of the substantial carbon enhancements at the lowest metallicities is not well understood, as there are potentially a number of production mechanisms involving stars of different mass. Nevertheless, with detailed abundance studies of a large sample of extremely metal-poor stars, we can expect to untangle the role of carbon enhancements and thus advance our understanding of the physical processes that govern star formation at the earliest times.

2.1.2.2.2 Age Dating the Oldest Stars The ages of the oldest stars can be directly determined by measuring the abundances of long-lived radioactive isotopes such as 232Th (half-life 14 Gyr) and 238U (4.5 Gyr). Specifically, the age of a star can be derived by comparing the abundances of Thorium (Th) and/or U with those of stable r-process nuclei, such as Europium (Eu). While Th is often detectable in metal-poor stars that are enhanced with r-process elements, determining U abundances is particularly challenging because the only U line in the optical and near-IR spectral region with sufficient strength to permit abundance determinations is extremely weak and lies in the UV-blue spectral region (~3860 Å) where there are many other contaminating lines from more abundant species.

Currently, 238U has only been detected in three stars, and one of these detections is tentative. For HE1523-0901 (V=11.1) and CS 31082−001 (V=11.3), high-resolution spectra with R~80,000 and S/N~350-500 at 4000 Å were required to successfully measure the optical U line from which the abundance was deduced (Frebel et al., 2007). The spectral region around the U line in these stars is shown in Figure 2-10. HE1523-0901 was determined to have an age of 13.2 Gyr by averaging the results of several nucleochronometers involving combinations of Eu, Os, Ir, Th and U (Frebel et al., 2007), while for CS31082-001, the U/Th chronometer yielded an age of ~14 Gyr (Cayrel et al., 2001)27. These ages provide a lower limit to the age of the Galaxy and hence, the Universe. They are consistent with the age of 13.75 Gyr derived from cosmological parameters measured in the WMAP experiment (Larson et al., 2011)28.



Figure 2-10. Spectral region around the only available uranium line in HE1523-0901, the r-process enhanced star with the strongest overabundance in heavy neutron-capture elements in comparison to CS31082-001, the only other star with a U detection (but 200 K cooler than HE1523-0901, resulting in a significantly weaker U line). A larger region is shown on the left, with a zoom-in version of the right (Frebel et al., 2007)29.

The stars most suitable for direct age determinations are cool metal-poor giants that exhibit strong overabundances of the r-process elements. Unfortunately, such stars are often also very Carbon-rich and the 238U line is blended and not measureable. For example, in the extreme r-process star CS22892-052, only the Th/Eu ratio could be employed yielding an age of ~14 Gyr (Sneden et al., 2003)30. Compared to Th/Eu, the U/Th ratio is much more robust, however, with respect to uncertainties in the theoretically derived production ratio, as a result of the similar atomic masses of Th and U (Schatz et al., 2002)31. Hence, old metal-poor stars displaying both Th and U are the most valuable for nucleochronometric age determinations. Candidates for further study will come from large area surveys of very metal-poor stars such as SEGUE, SkyMapper and HERMES.

With the GMT’s optical high resolution spectrograph it will be possible to obtain spectra of stars suitable for nucleochronometric age determinations with S/N of 500 or more in just a few hours. This S/N and a resolution of R>50,000 are required to detect and adequately measure the weak U line. Spectra of this quality will also enable the detection of weak lines of a number of other rarely studied elements permitting a full chemical characterization of the r-process element enhanced metal-poor stars. For example, Lead (Pb) measurements in these stars (Pb being the decay product of Th and U) are even more challenging (S/N>500 required), but they will provide the ultimate empirical constraint for r-process modeling and thus nuclear astrophysics. Ultimately, a large stellar database with measurements of the heaviest chemical elements will not only provide a constraint on cosmological models but will also further our understanding of the production of the heavy elements in the Universe.

2.1.2.2.3 Globular Clusters in Local Group Galaxies and Beyond With its large collecting area and high angular resolution in the diffraction-limited mode, the GMT will be able to uniquely contribute to our understanding of dense stellar systems. Globular clusters in Local Group galaxies and other close neighbors can be studied in great detail with the GMT, while integrated light studies can probe the dynamics and abundances in globular cluster systems associated with large galaxies and high stellar densities in the Virgo and Coma Clusters.



Figure 2-11. Simulated images of a globular cluster at the distance of Centaurus A (3.8 Mpc). The cluster has a core radius of 3 pc and the field shown corresponds to 2” on a side. The three panels show the cluster as imaged with PSFs appropriate to HST, Gemini and GMT at 1.5 µm. The GMT simulation uses a PSF with a Strehl ratio of 0.7.

Figure 2-11 shows a simple graphical simulation of diffraction-limited images of a globular cluster with a core radius of 3 pc at the distance of Centaurus A (NGC 5128). The three panels show the cluster as imaged with Hubble, a diffraction-limited 8 m, and the GMT using the LTAO adaptive optics mode. The reduction in crowding noise is dramatic and illustrates one of the strengths of the ELTs–their ability to access crowded environments beyond the reach of Hubble, current ground-based telescopes, and the James Webb Space Telescope.

Color-magnitude diagram studies based on near-IR imaging with LTAO have the potential to provide much new information on the ages (via the luminosities of thermally pulsing AGB stars) and metallicities (via red giant branch colors) for both the star clusters and the field-star populations in many galaxies within the local Universe (D



2.1.2.3.1 Near-Field Studies of Galaxy Assembly Dwarf galaxies, and ultra-low-mass dwarfs in particular, provide a unique laboratory for studying the building blocks of today’s galaxies. The lowest mass systems present today are believed to be relics of early dark matter halos that have avoided merging into larger systems. These heavily dark-matter dominated systems provide a unique opportunity to study the small-scale properties of dark matter halos and to test the CDM paradigm (Simon & Geha 200732; Geha et al., 200833).

The number of known ultra-low mass systems has grown rapidly with the advent of large sky surveys (e.g., Willman et al., 200534; McConnachie et al., 200835). Future surveys, and LSST in particular, will greatly increase the available volume for dwarf-galaxy studies. In the near-term, SkyMapper and the Dark Energy Survey (DES) will open the southern sky to dwarf galaxies to depths greater than those of SDSS in the north and the equatorial survey strip.

The shallow potential wells in ultra-low mass dwarfs make them unique environments for studying the enrichment process. Single supernovae events can leave a clear imprint in the abundance patterns of subsequent stars. The number of stars accessible to detailed abundance work is presently frustratingly small, as Figure 2-12 illustrates. The visible and near-IR echelle spectrographs being developed for the GMT are particularly well-suited to this science, and the facility multi-fiber system (MANIFEST) could greatly increase the power of the visible echelle in studying the dark matter content and stellar populations in low-mass systems.

Figure 2-12. CM diagrams for known dwarf and ultralow mass dwarf galaxies. The current limit for a few hours of integration on an 8 m telescope is shown by the red line. The dashed blue line is the analogous limit for the GMT using the G-CLEF fiber fed echelle spectrograph; the green shows the sensitivity with only 4 primary mirror segments. The number of stars within reach of velocity and abundance determinations is greatly improved and some systems that are beyond the reach of 8 m Echelle spectrographs (e.g., CVn I, Leo IV) will be accessible for the first time with the GMT. Even in the 4-mirror configuration, many more stars are accessible, although we cannot reach the red clump stars in Sculptor or Sextans until the primary mirror array is fully populated. (J. Simon, priv. comm.).



Figure 2-12 shows color-magnitude diagrams for dwarf galaxies ranging from fairly rich systems (such as Fornax) to those with only a handful of constituent stars (such as Segue 1). The reach of current echelle spectrographs is marked by a fiducial line at V=19. The GMT and the G-CLEF echelle will allow one to reach ~1.5 magnitudes fainter, greatly increasing the number of accessible stars, particularly for the lowest mass dwarfs. Some systems that are beyond the reach of 8 m echelle spectrographs (e.g., CVn I, Leo IV) will be accessible for the first time with the GMT.

2.1.2.3.2 The Galaxy Building Epoch The process of galaxy building spanned several Gyr, with the peak period of mass growth occurring between redshifts of ~5 to as low as ~0.5, with the bulk of the mass growth in M* galaxies occurring in the interval from 1



A broad range of processes contribute to the growth of galaxies: conversion of in-situ gas into stars, accretion of outside gas through cold flows, and the build-up of stars and gas through major and minor galaxy mergers. These processes are modulated by feedback from massive stars and supernovae, large-scale galactic winds, and outflows powered by nuclear AGN. Understanding the interplay between these various processes and their impact on the present-day properties of galaxies is one of the most active areas of research today.

2.1.2.3.2.1 Dynamical Masses Theoretical treatments of the growth of structure on galactic scales and greater deal primarily with total rather than baryonic masses. The underlying dark matter distribution drives the growth of galaxies, groups, and clusters through accretion of gas along dark matter filaments, and through hierarchical merging of structures in over-dense regions. Comparison between theory and observation is greatly facilitated by accurate determinations of the dynamical masses for galaxies, groups, and clusters. Several methods are employed to acquire dynamical masses for galaxies at z>1. Determining dynamical masses from stellar velocity dispersion measurements is best achieved in the near-IR (where spectroscopy is sensitive to stellar absorption features in the rest-frame optical at redshifts 1.5



Figure 2-14. Simulated velocity channel maps from an observation of a z=1.5 galaxy with the GMTIFS instrument on the GMT. An ordered velocity field with an amplitude of +/- 200 km/sec was imposed on an ACS i-band image from the Hubble Ultra Deep Field. This simulation assumes 50 mas spatial elements and a total line flux of 1.1x10-16 erg cm-2 s-1 (Bournaud et al., 2008) which was observed over a period of 12 h (9 h on source +3 h on sky).

The GMT and other ELTs will improve the photon rates at fixed spatial sampling by an order of magnitude thus allowing robust determinations of galaxy velocity fields over a range of masses and luminosities. GMTIFS will provide a gain in both surface brightness sensitivity and spatial sampling, allowing more detailed studies of high surface brightness systems and access to a broader range of surface brightness levels. Figure 2-14 shows velocity channels from a simulated GMTIFS observation of a galaxy at z=1.5.

2.1.2.3.3 Feedback and the Galaxy-IGM Connection Feedback (the injection of energy into gas in galaxies and the resulting regulation of star formation) is thought to be the key to understanding how present day massive galaxies acquired their distinctive morphologies, masses, and stellar content. The IGM and CGM (defined to be the gas within about 300 kpc of the galaxies) provide a laboratory in which the feedback effects from galaxy formation and AGN accretion can be measured. One promising route to understanding the relevant baryonic processes is to survey galaxies and gas in the IGM in the same cosmic volumes during the epoch when they were exerting the most influence on one another, near the peak star-forming and black hole accretion era at 2



galaxies allow for detailed study of galaxy-scale outflows and/or inflows of cold gas via strong interstellar absorption lines and Lyα emission. Current surveys using large samples of spectroscopic data for LBG galaxies at 2



sensitivity over a wide wavelength range (~0.3-1 micron) to detect the rest-frame UV spectral features of the gas associated with the ISM and CGM of galaxies for 2



We expect that exposure times with the GMT and an IFU at z~0.5 will be around 8 hours for a spatial sampling of 12 mas. This will Nyquist-sample the K-band PSF in the diffraction-limited mode. Using the finest scale for GMTIFS (6 mas) will likely increase the required exposure time. At z~1 the exposure times should be comparable because one can access the strong CaII triplet features in the H-band. At redshifts of z~2 stellar features become very difficult to discern due to the strong surface brightness dimming, but narrow emission lines, particularly [OIII]5007 and Hα, will be very useful velocity tracers.

Figure 2-17. The angular size of the sphere of influence as a function of black hole mass. The diffraction

limit of GMT in the J- and H-bands is shown by the dotted line.

A well-balanced GMT survey of the evolution of the MBH-σ relation will contain a mix of galaxies at z~0.25, 0.5, and 1 with a sampling of narrow-lined AGN at z~2. Such a program can be carried out in 40-50 nights spread over a few years.

2.1.2.4 Dark Matter, Dark Energy, and Fundamental Physics Our understanding of the constituents of the Universe and the evolution of structure has improved dramatically in recent decades. This has come about through a combination of theory and multi-wavelength observations of phenomena as diverse as Cepheid variable stars, supernovae, galaxy clusters, and diffuse background radiations. Giant telescopes will continue to play a leading role in cosmological studies through their access to key diagnostics and sensitivity to objects at great distances.

This section considers the GMT’s role in advancing our understanding of dark matter and dark energy through calibration of large-scale cosmological probes, dynamical studies of dark matter in massive galaxy clusters, and the structure of dark matter halos in low mass dwarf galaxies.

2.1.2.4.1 Dark Energy Probes Several ambitious projects are underway, or in the planning stages, that aim to probe the expansion history of the universe using standard rulers. Baryon Acoustic Oscillation (BAO) experiments rely on a fixed scale imprinted on the matter distribution at recombination. Determinations of the



angular size of the fixed 150 Mpc co-moving scale of the oscillations at high redshifts provide a direct distance determination on cosmological scales. The distance-redshift relationship is a powerful probe of the underlying cosmology and hence, the basic cosmological parameters.

Two large-scale BAO programs (LSST and the DES) rely on photometric redshifts to determine the two-point correlation function in redshift bins. Photometric redshifts can be quite precise, approaching errors of a few percent rms. These surveys must be calibrated with spectroscopic samples as they probe redshifts and luminosities beyond current training sets. Hence, there is a continuing need for spectroscopic redshift surveys that are complete to the LSST detection limit and that are large enough to sample the full range of galaxy types with good statistics. The GMT can provide these data with optical and near-IR multi-object spectrographs. Samples of 10,000–20,000 redshifts could be assembled with a modest investment of observing time on the GMT and these could play a vital role in determining cosmological parameters from large imaging surveys.

2.1.2.4.2 Clusters and Dark Matter The distribution of mass within rich clusters of galaxies is diagnostic of the history and assembly of these large structures. The value of the cosmological parameter σ8, the normalization of the matter power spectrum to 8-Mpc scales, is exquisitely sensitive to the number of high-mass clusters of galaxies, while the rate of evolution of the cluster mass function depends powerfully on Ωm. Therefore, cluster numbers and their evolution have proved to be reliable tools for probing the world-model and key cosmological parameters. Several large cluster surveys are underway (South Pole telescope) or planned for the coming decade (e.g., EUCLID). The GMT can provide spectroscopic redshifts, velocity dispersions, and weak lensing maps–essential data for cosmological studies based on the cluster mass and redshift distributions.

Figure 2-18. Example of a strong cluster lens from the SDSS survey (SDSSJ1038+4849). Four galaxies with redshifts ranging from 0.8 to 2.8 are each imaged multiple times by the rich foreground cluster. The location and shapes of the various lensed images can be used to map the distribution of dark matter in the cluster and compare it with the distribution of luminous matter. See Bayliss et al., 201140 for details.

Strong lensing, illustrated in Figure 2-18, provides complementary and important information about mass distribution down to the scale of the cluster cores, while small distortions in the shapes of background galaxies (weak lensing) probes the overall mass profile. The combination of weak and



strong lensing allows one to measure substructure and the concentration of the mass in the clusters, as well as the overall ellipticity of dark matter in the clusters. In at least some cases, the central concentration appears to be higher than predicted by ordinary CDM, possibly suggesting an earlier formation for these structures. Most clusters arcs are beyond the reach of spectroscopy with 8 m telescopes, but can be studied with the GMT. Samples of strong lenses will come from DES and LSST for z1.

2.1.2.4.3 Dark Matter Profiles in Dwarf Galaxies A powerful test of the properties of dark matter comes from the shape of the dark matter density distribution. Cold Dark Matter (CDM) predicts that dark matter halos should show steep central density “cusps” (Navarro, Frenk & White 199641, NFW). Dwarf galaxies are ideal test subjects to measure dark matter profiles as they are highly dark-matter dominated (with mass-to-light ratios approaching 1000). Since the baryons make a negligible contribution to the mass even in the inner regions, the dark matter dictates all kinematics.

An ongoing debate centers on whether dark matter halos exhibit “cusps” or show shallow density-profile “cores”. Velocity measurements of stars in low mass dwarf galaxies allow one to derive the distribution of the dark matter (e.g., Walker et al., 2009)42. As illustrated in Figure 2-19, the current data allow for mass profiles including a cuspy NFW profile as well as a halo+core. The current limits are restricted by both the number of dwarf galaxies studied and the number of stars per galaxy for which we have accurate kinematic information.

Figure 2-19. The left panel shows the mass interior to the half-light radius and the right panel shows the mean density within the half-light radius for dwarf galaxies (adapted from Walker et al., 2009). The curves show the best-fitting mass profiles, including the NFW profile with a cusp (γ = 1) and a cored model (γ = 0). Currently both dark matter mass profiles are consistent with the observations.

A wide-field optical spectrograph such as GMACS/MANIFEST will provide a technological leap for measuring the kinematic properties of dwarf galaxies. Confirming that dwarf galaxies are gravitationally self-bound systems generally requires kinematic information of at least 100 stars to R~23 mag at S/N=5. Since dwarf galaxies are low mass objects, their internal velocities are small and subsequent kinematic observations require precision on the order of 3 km/s, setting an observational spectral resolution requirement of R~6000. This could be achieved on the GMT with MANIFEST feeding an echelle, such as G-CLEF, or by an echellette mode for GMACS.



2.1.2.5 First Light and Reionization A small number of seminal events mark sharp and distinctive transitions in the history of the Universe: the end of inflation, the quark-Hadron phase transition, recombination, and reionization (Figure 2-20). The latter, the time when the intergalactic medium (IGM) was reionized, is intimately connected to the formation of the first massive collapsed objects and the birth of galaxies. Exploring the era when galaxies first formed has been among the forefront goals of extragalactic astronomy since the discovery of the hot Big Bang.

Figure 2-20. A graphical history of the Universe (Robertson et al., 2010)43. Key events relating to the early history of the Universe–recombination, the cosmic dark ages, the birth of galaxies and reionization of the IGM are illustrated. The GMT will allow us to probe the formation of the first galaxies near the end of the dark ages.

The search for the first galaxies, both from empirical and theoretical perspectives, has been underway since the 1970’s and we are now able to observe and crudely characterize galaxies in the first few hundred million years after the Big Bang. The GMT, working in conjunction with JWST, SKA and other facilities, will provide new and powerful observational tools for studies of the reionization epoch and the period of early galaxy growth that followed. In this section we illustrate a few areas in which the GMT is expected to have a significant impact and highlight ways that GMT instruments and other facilities can work together.

2.1.2.5.1 The First Dark Matter Halos, Stars, and Galaxies State-of-the-art simulations suggest that the first stars formed in gravitationally bound clumps of dark matter at redshifts between z~30 and 20. However, these mini-halos likely formed few stars, since the ionizing radiation from a single star in such a small halo may be able to dissociate and ionize molecular gas and, in some cases, unbind the halos altogether. The enriched material from these first stars seeded subsequent generations of star formation and led to the galaxies that we see at z~6-7.

Empirical characterization of the first galaxies will require observations in the near-infrared (NIR). Samples of galaxies at z>6 are being produced by HST; JWST will provide samples of galaxies at even larger redshifts. Deep NIR spectroscopy is needed to robustly measure the rest-frame ultraviolet luminosity functions and infer their contribution to reionization (Figure 2-21). Near-infrared spectroscopy is also crucial to probing these galaxies for the presence of metal-free stars via high-ionization emission lines such as HeII1640.

2.1.2.5.2 Galaxies in the Early Universe Galaxies at large redshifts are generally selected on the basis of their flat (in fν) rest frame ultraviolet continuum or their strong Lyα emission lines. Ultra-deep visible and near-IR imaging



surveys with HST have allowed us to characterize the overall evolution in the galaxy density from z~8 to the z~1-2 where ground-based surveys provide good statistics.

Figure 2-21. The evolution of the UV luminosity density and derived global star formation rate from z~10 to the present day (from Bouwens et al., 2011)44. The rate of star formation grew rapidly in the first ~1 billion years after the Big Bang and peaked when the Universe was around 2-3 Gyrs old. Observations with the GMT and the James Webb Space Telescope will allow us to probe the first ~500 Myr for signatures of the reionization.

At the time of this writing, only a few galaxies at z>7 have been spectroscopically confirmed via Lyα emission (e.g., Vanzella et al., 2011)45. Very long integrations (e.g., 15-25 hours) with 8 m telescopes have been required to yield the few detections to date.

Figure 2-22. Left: VLT/SINFONI 15 h spectrum of the z=8.56 galaxy, UDFy-38135539 (from Lehnert et al., 2010)46. Right: A simulated GMT/NIRMOS spectrum of the same object with the same exposure time in black, with the 1-sigma noise spectrum in red. The significance of the detection in the simulated GMT spectrum is ~70 sigma and the input Lyα flux is 6 x 10-18 erg/sec/cm2.

In Figure 2-22 a comparison is observed as a spectrum of UDFy-38135539 along with a simulated observation of the same galaxy with a multi-object NIR spectrograph on the GMT. Not only is the significance of the detection vastly improved, but also the sensitivity is sufficient to allow useful searches for other features.

The James Webb Space Telescope (JWST) should produce large samples of galaxies at z>8. The GMT with its near-IR spectrographs will have the sensitivity to detect Lyα emission from galaxies



to z~12 and perhaps beyond. At z=12 a galaxy with the same intrinsic Lyα luminosity as UDFy-38135539 will have a Lyα flux of 3 x 10-18 erg/sec/cm2, and thus be detectable with the GMT in a practical exposure time if the IGM is reasonably transparent at these wavelengths.

The JWST spectrograph, NIRSPEC, will have great sensitivity in its lowest resolution mode (R=100) and will be highly competitive with ground-based ELTs in its R=1000 mode. The GMT, with a larger field of view and high spectral resolution will excel at studies of larger samples, line profiles, and detection of faint narrow spectral lines in clear windows of atmospheric transmission and emission.

2.1.2.5.3 Discovering the First Stars By z~15 the average metal abundance of the Universe will likely have increased above the critical value necessary for Population III star formation. Pop III stars could, however, still form in primordial gas clouds at later times and, hence, lower redshifts. HeII recombination emission is a tracer of Pop III. The HeII1640 emission line can be very bright and does not suffer from resonant scattering. Scannapieco et al. (2003)47 predicts that ~30% of L(Lyα) galaxies at z=6-10 host Pop III star formation.

Table 2-3. Lyα fluxes and Lyα/HeII flux ratios for different abundances and IMFs

IMF F(Lya) (1E-18) Lya/HeII Z = 0, Top-Heavy 85.0 8 Z = 0, Salpeter 8.5 40 Z = 0.0005, Salpeter 8.5 80,000

In Figure 2-23 the most recent models from Pawlik et al. (2011)48 are used to estimate the likely levels of HeII1640 emission from Pop III star-forming galaxies at high redshift. They consider three stellar populations: zero metallicity with a top-heavy initial mass function (IMF); zero metallicity with a Salpeter IMF; and Z=10-3.3 with a Salpeter IMF. For a z=9 galaxy with M*=107 MSun, these three models correspond to the Lyα fluxes and Lyα/HeII flux ratios shown in Table 2-3. A Salpeter IMF with a non-zero metallicity will not produce a detectable level of HeII flux, but the two metal-free IMFs may.

Figure 2-23. Left: Model of metal-free disk formation at high redshift (from Pawlik et al. 2011). Right: A simulated NIRMOS 2 h integration of a z=9 metal-free galaxy with a top-heavy IMF. HeII 1640 emission is clearly detectable. With a Salpeter IMF, this same galaxy is not detectable in HeII, thus a top-heavy IMF may be necessary to detect HeII at the highest redshifts with the GMT.



2.1.2.5.4 Probing Reionization 2.1.2.5.4.1 Constraints from Lyα Emission One probe of reionization is the frequency of the occurrence of Lyα photons, a clear signature of ionizing radiation. As Lyα photons are resonantly scattered by neutral hydrogen, the frequency of Lyα emission in galaxies during reionization is sensitive to IGM neutral fractions ranging from 10 to 100% (McQuinn et al., 2007)49.

Large surveys of star forming galaxies at 46 galaxies using ultra-deep (12–18.h) exposures with optical and near-IR spectrographs on Keck and the VLT (Schenker et al., 201150; Ono et al., 201151, Vanzella et al., 2011). These studies find few objects with detectable Lyα emission at these redshifts and suggest a drop in the fraction of galaxies with EW(Lyα)>25 Å from ~50% at z=6 to ~10-40% at z=7. One interpretation of this downturn is that we are on the cusp of detecting the signature of reionization. However, the current small samples and poor limiting flux sensitivity produce large error bars on this measurement, which prohibits any strong conclusions regarding the timing of reionization.

The wide-field spectrographs on the GMT will provide the collecting area and large fields-of-view needed to sample large numbers galaxies with faint Lyα emission. A 2-hour GMACS observation will reach a limiting line flux of 2-3 x 10-18 erg/sec/cm2 comparable to the measured line flux in the few confirmed z=7 sources, which required integrations over 5 times longer. In a modest investment of time (e.g., five nights) one could assemble a sample of ~200 Lyα galaxies at z>7 with the GMT.

2.1.2.5.4.2 High Resolution Spectroscopy of Quasars Lyα absorption from over-dense regions of the IGM imprints a forest of dense features on the spectra of distant quasars. As the Universe transitions from ionized to neutral with increasing look-back time, the “forest” becomes a deep opaque trough in the spectra of quasars and gamma-ray bursts (GRBs) at wavelengths below Lyα in the rest-frame of the object. Detailed studies of intergalactic absorption lines enable measurements of quantities such as the IGM temperature and metal abundances, as well as the ionization state of the IGM in the early Universe.



Figure 2-24. Synthetic Lyα absorption spectra of a quasar at z=6.15, shown with the resolution and signal to noise typical of spectra obtained on 8-m class telescopes (upper), and (lower) as it would look with GMT using a high resolution (R~40,000) optical spectrograph such as G-CLEF

Useful observations with 8-10 m apertures require very long integration times (>10 h). Figure 2-24 displays detailed simulations of intergalactic Lyα absorption in the spectrum of a z=6.15 quasar. The upper panel shows the normalized transmission blue-ward of the Lyα emission line against observed wavelength. The spectrum has resolution R=2800 and a S/N=20 per 3.0_Å/pixel, representative of moderate resolution and signal-to-noise data obtained with Keck/ESI for the brightest known quasars (i~22) at z~6. The lower panel shows the same spectrum, but for R=40000 and S/N=30 per 0.2 Å/pixel, representative of the improvement in resolution and signal-to-noise attainable with a high-resolution optical spectrograph on the GMT such as G-CLEF (in an 8-hour integration).

The much higher spectral resolution enables the intrinsic widths of Lyα absorption lines in the quasar proximity zone (orange shading) to be fully resolved, allowing their thermal widths to be measured; the thermal state of the IGM provides a valuable indirect probe of reionization. Narrow features indicating a highly ionized IGM (which are difficult to observe in the trough of absorption in the lower resolution spectrum) are clearly apparent in the high-resolution simulation (cyan shading). The statistics of these regions may be used to place constraints on the reionization history and the ionization state of the IGM. Furthermore, the overall shape of the Lyα transmission close to the quasar rest frame is well-resolved; this may be used to identify the possible damping-wing signature of neutral gas along the quasar or GRB sight-line (Miralda-Escude, 1998)52.

VISTA, Euclid, WFIRST and other large near-IR surveys are expected to produce large samples of z>7 quasars, most of which will be beyond the reach of spectrographs on 8-10 m telescopes. The GMT will be well suited to IGM studies using these quasars as probes.

GRBs may provide even more powerful probes of the IGM as they can reach very bright apparent magnitudes and are known to occur at very high redshifts.



2.1.2.6 Transient Phenomena Investigating transient phenomena (those with variability time-scales ranging from a few minutes to a few months) is a new frontier in astrophysical research. They represent a vast, unexplored parameter space for testing fundamental physics in powerful cosmic explosions such as γ-ray bursts and supernovae (SNe). Various synoptic all-sky surveys, such as Pan-STARRS and LSST, are designed to study the dynamic Universe by documenting new transient events, but a physical understanding of the origin and nature of these transient phenomena requires spectroscopic follow-up with large telescopes. The GMT will offer unique opportunities to exploit these survey databases to probe the early Universe and the most energetic phenomena known.

2.1.2.6.1 Optical Transients of Long-Duration Gamma-Ray Bursts GRBs are among the most energetic events in the Universe. In particular, long-duration GRBs are believed to originate in the catastrophic deaths of massive stars (see Woosley & Bloom, 2006 for a review)53. Some bursts are followed by optical afterglows (e.g., Akerlof et al., 199954; Kann et al., 200755; Bloom et al., 200956) that can briefly exceed the absolute brightness of any known quasar by orders of magnitude (Figure 2-25). Similar to high redshift quasars (QSOs), GRB afterglows can serve as a sensitive probe of “dark” intervening gas that is either local or external to the burst progenitor’s environment. Since GRB afterglows rapidly decline however, an effective exploitation of these lighthouses requires rapid follow-up spectroscopy (within 2 days) with 8-m class telescopes.

Figure 2-25. Rest-frame optical light curves of luminous GRB afterglows and SN2006gy (Smith et al., 2007)57, compared to rest-frame, absolute r-band magnitudes of known QSOs from SDSS (Schneider et al., 200758; the grey horizontal band). The most luminous QSO to date is marked by the dashed horizontal line. The brightest GRB afterglow recorded was GRB 080319B. At early times, the optical transient was ~103 times more luminous than the most luminous QSO (Bloom et al., 2009).

GRB afterglows are suitable cosmic probes because a large fraction (>50%) are known to originate at redshift z>2, including a growing fraction at z>6 when the Universe was less than one billion



years old. The redshift distributions of known GRBs and QSOs in Figure 2-26 shows that GRB afterglows are better than QSOs for probing the re-ionization epoch at z>6. Rapid echelle or moderate-resolution spectroscopy of the afterglows is critical both for constraining the distances (and therefore the energy output) of individual burst events, and for probing the physical conditions of gaseous clouds along the lines of sight.

Figure 2-26. Redshift distribution of 219 GRB afterglows identified as of November, 2010 (red solid curve), compared to the redshift distribution of i



Objects Monitor (SVOM) (a Chinese-French space mission) is expected to be launched. Similar to Swift, SVOM is also designed to deliver rapid localization of new GRBs for ground-based follow-up. Other proposed space missions include the Joint Astrophysics Nascent Universe Satellite (JANUS; Fox et al., 201062) and the Energetic X-ray Imaging Survey Telescope (EXIST; Grindlay et al., 201063). These new satellites will deliver new, localized GRBs well into the decade beyond the Swift era.

Figure 2-27. The afterglow spectrum of GRB050730 at z=3.968 obtained using MIKE on the Magellan Clay

Telescope (Chen et al. 2005)

Once a GRB is detected by one of these instruments, rapid follow-up observations from available ground based telescopes are vital. But the windows of opportunity for making these observations are short. The decay curves and apparent magnitudes for a large sample of GRBs strongly argue for a 15-minute response time for the most effective use of GRBs and probes of the IGM/ISM.

2.1.2.6.1.1 GRBs and the Reionization Epoch Empirical studies of the reionization epoch have focused primarily on observing the most distant QSOs at z>6. However, QSOs are powered by supermassive black holes in ~109 MSun dark matter halos, which form presumably through mergers of smaller halos under the ΛCDM hierarchical formation paradigm and are substantially less common at earlier times. In contrast, the first stars are expected to form in ~106 MSun halos (e.g., Barkana & Loeb, 2001)64, which are already present at z>7. Since these stars subsequently generate GRBs, GRB afterglows can serve as a more abundant and sensitive probe of the intergalactic medium during the epoch of reionization.

Observations of z>6 GRBs (e.g., Kawai et al., 200665; Tanvir et al., 200966) help to unveil the sequence in which the Universe became reionized (Mesinger et al., 2004; 2005)67 68. Figure 2-28 shows the afterglow spectrum of a GRB at z=6.295. In addition to strong and narrow metal-line absorption features that allow us to determine the redshift of the source, a damping trough is observed at 8900 Å. Interpreting the red damping wing as entirely due to the interstellar medium of the GRB host leads to a gas surface density of N(HI)~4x1021/cm2 (the solid curve and inset of Figure 2-28) in the host medium. Such high density is comparable to what is seen in the denser part



of star-forming regions in the Milky Way. However, a neutral intergalactic medium is also expected to contribute to the absorption trough (e.g., Miralda-Escude, 1998).

Figure 2-28. A combined afterglow spectrum of GRB 050904 at z=6.295 (Kawai et al. 2006). The data were taken 3.4 d after the initial burst using the Faint Object Camera and Spectrograph (FOCAS) on the 8 m Subaru Telescope with a total exposure time of 4 hours and a spectral resolution of FWHM ~ 8.5 Å at ~9000.Å. The strong, narrow absorption features due to O0, Si+, and C+ allow an accurate redshift measurement of the host galaxy. Interpreting the damping trough at 8900 Å as entirely due to the host ISM (the red solid curve; see also the inset), Totani et al., (2006)69 estimates a host-DLA of log N(HI) ~21.6. Analyzing the red damping wing also allows us to constrain the neutral fraction of the IGM. Different curves show different model expectations.

2.1.2.6.2 Supernovae The frontier for supernova (SN) research in the GMT era will be routine, temporally well sampled optical and NIR spectropolarimetry to address key questions regarding the nature of the progenitors and explosion mechanisms. Answers to these questions bear significantly on studies of dark energy (Riess et al., 199870; Perlmutter et al., 199971).

About 1 in 1000 galaxies will have a live supernova in it. Efforts should begin now to construct an efficient pipeline to deconvolve SN spectra from galaxy spectra and hence to discover and identify supernovae from spectra alone. The rate of discovery of supernovae promises to expand dramatically with the Palomar Transient Factory and Pan-STARRS. The LSST alone is expected to discover more than 100,000 supernovae per year. One in a thousand of these (or about 100 per year) may also be strongly lensed. An important task will be to decide which of this plethora of events should be studied in more detail with spectroscopy on a 20 m class telescope like the GMT.

2.1.2.6.2.1 Type Ia Supernovae Among the major outstanding issues in the study of SN Ia is proof that they arise in binary systems, and if so, in single degenerate systems, double degenerate systems, or some mix of the two. Clues to the progenitor system may arise by the detection of circumstellar matter. Search for and detection of variable Na D absorption in the early spectra of SN Ia is currently a topic of great interest (Simon et al., 2009)72.



Figure 2-29. Time series of low-resolution spectra of Type Ia supernova SN 2010kg obtained with the Hobby-Eberly Telescope showing the evolution of the high-velocity CaII infra-red triplet. Note the especially broad feature in the top spectrum (1202) that may be blended with OI 7774 and the evolution of this feature to lower velocities (G. H. Marion, unpublished). Observing these features well before the peak magnitude is critical. LSST should find many SN Ia very early after explosion.

Another important clue has come from the discovery and study of high-velocity Ca and Si features moving at 20,000-30,000 km/sec, as shown in Figure 2-29. The high-velocity calcium lines may arise from gas at solar abundance associated with some circumstellar medium, but the high-velocity silicon must represent freshly synthesized matter from the explosion. The observed kinematics may be associated with the collision of the ejecta with a shell of about 0.02 solar masses that lies at substantially less than 1015 cm in order not to contaminate the rising light curve (Gerardy et al., 2004)73. No such collisionally induced luminosity is seen (Hayden et al., 2010)74. These high-velocity features are highly polarized. Similar features are also seen in Type Ib/c supernovae.

Observing these features early (i.e., well before maximum) is critical. Observing at larger redshifts with the associated time-delay may make this task somewhat less challenging and LSST should find many SN Ia very early after explosion. These features are under close study now, but the sample remains small and there will remain much to do in the GMT era.

2.1.2.6.3 Other Transient Sources In addition to long-duration GRBs and supernovae, other known transient events include short-duration GRBs and flares near galactic nuclei. Little is known about these phenomena, and significant progress is expected to occur during the GMT era with its imaging and spectroscopic follow-up capabilities.



Unlike long-duration GRBs, which often display bright X-ray and optical afterglows, only a few short-duration GRBs have been detected in afterglow radiation in X-ray and optical that would allow accurate localization of these sources. While the origin of long-duration GRBs in the death of massive stars is definitively established by their association with core-collapse SNe, searches for supernova events associated with short bursts have yielded null results (Fox et al., 200575; Hjorth et al., 200576) and some of the few localized short-duration GRBs are found near old stellar populations (e.g., Bloom et al., 2006)77.

To explain the short burst duration and lack of supernova association, a leading model of short GRB progenitors is binary merger, either neutron-star/neutron-star or neutron-star/black hole. This model can be tested by combining gravitational wave observations and afterglow follow-up. In a binary coalescence, the energy and angular momentum are carried away by gravitational waves. A direct association of gravitational wave detections and afterglow radiation of short GRBs therefore provides a critical test for the binary merger progenitor model (e.g., Lee & Ramirez-Ruiz, 2007)78. Spectroscopic follow-up of faint afterglows associated with short GRBs is also necessary to unambiguously establish the distances of these sources and to study the progenitor environment.

Finally, a large fraction of galaxies are believed to host supermassive black holes at their centers. Mass deposits onto the central black hole due to tidal disruption of surrounding gaseous clouds and stars give rise to near-infrared flares near galactic centers. Observations of these flares provide a unique window for studying the size and spin of supermassive black holes outside of our own Milky Way.

2.1.3 Scientific Synergies with Other Major Facilities Astronomical facilities rarely work in isolation and most problems in contemporary astronomy and astrophysics are approached with a variety of observational and theoretical tools. The GMT and other ELTs will have unique capabilities, but input from other sources will maximize their impact. Similarly, a number of exciting new ground- and space-based facilities are on the horizon and these too will benefit from spectroscopic follow-up with large apertures and ELTs in particular.

First light on the GMT and other ELTs is 8-10 years distant. For the present discussion, we limit our time horizon to the next decade or so. Some of the missions and facilities currently under consideration will be completed within that time frame, while others will be pushed further into the future or may be abandoned or evolve into something else.

The top-level science drivers for most of the large missions or facilities under consideration are fairly similar. The US Decadal Survey79 and similar planning exercises in other commun

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High Level Science Goals, Key Science Requirements, Operational Concept · 2014. 7. 16. · argument arise from increased collecting area; the other two arise from improved image