2 NOAO Newsletter September 2012

Director’s Corner

Focus on La SerenaDavid Silva

Events in La Serena have captured my attention much of the time since the last Newsletter.

Accidents that led to serious personnel injuries and equipment damage at the Blanco 4-m telescope and one of the CTIO infrastructure improve-ment projects were most unfortunate reminders that safety and risk man-agement must remain the highest priority for all NOAO activities at all times. Both accidents were promptly and thoroughly reviewed by inter-nal and external panels, whose reports were provided to NSF, the Depart-ment of Energy, and the Chilean authorities as applicable. An exter-nal panel also reviewed the safety process and culture within the Blanco enclosure with a particular focus on the Dark Energy Camera (DECam) installation. Many helpful rec-ommendations emerged from these reviews, and we are in the process of applying them in Arizona and Chile. The DECam project, which has had no serious safety issues to date, is also serving as a model for building new safety procedures and an improved culture of safe-ty throughout NOAO. Fortunately, the three injured people have or will recover completely. The process of repairing and returning the Blanco secondary mirror to service is discussed elsewhere in this Newsletter.

Significant personnel matters in Chile also demanded my attention. As in Arizona, reduced funding from NSF regrettably forced NOAO to re-duce the number of Chile-based employees. I am sorry to say that Dr. Eric Mamajek decided to return to the University of Rochester. Else-where in this Newsletter is an article about the NOAO South director transition. Maintaining a strong team with excellent leadership remains a high priority and requires continuous attention.

On a happier note, completion of the Blanco facility improvement proj-ect and installation of the Dark Energy Camera has been proceeding smoothly since work restarted after the Blanco secondary mirror ac-cident. At times, the installation team has included NOAO personnel from both Arizona and Chile, working side-by-side with personnel from Fermilab. Bringing all these people together and managing them in a

coordinated and safe way has been a major challenge, but one that all have overcome jointly. Work on the DECam Community (calibration) Pipeline at the National Center for Supercomputing Applications ac-celerated and delivery of an operational system should be in time for the commissioning and science verification. Above all this hovers the Dark Energy Survey, a major international, multi-agency, multi-organi-zation collaboration. Many interfaces, many meetings, and many hours have gone toward delivering a revolutionary capability to attack major problems on the science frontier. We have learned many technical and

organizational lessons that can be applied to the Big Baryon Oscilla-tion Spectroscopic Survey (BigBOSS) and the Large Synoptic

Survey Telescope (LSST) projects. It is an exciting time for NOAO and its user community.

Speaking of LSST, the recent decision by the Nation-al Science Board to move LSST into the final design phase opens the door to a possible construction start in fiscal year 2014. NOAO remains the lead institution for the Telescope and Site Facilities design, development,

and construction team and is involved in various aspects of LSST data management. Of course, LSST is not just a

telescope, it is an end-to-end system for the production of open-access science data products. NOAO remains excited about

hosting and operating the telescope and data management components to be located in Chile and aspires to play a significant role in supporting the general US research community during LSST science operations.

Other instrument projects progressed as well, including the SOAR Adap-tive Optics Module (SAM, a ground-layer AO system), the CTIO Ohio State Multi-Object Spectrograph (COSMOS), and TripleSpec 4. SAM and COSMOS should be released for science operations during 2013 if all goes well, while TripleSpec is on its way toward delivery in 2014.

CTIO in general, and the Blanco 4-m telescope in particular, have a long history of enabling scientific excellence. On balance, activities in the last six months have laid a strong foundation for continuation of that proud tradition.

(Image credit: Tim Abbott/NOAO/AURA/NSF

Science Highlights

NOAO Newsletter September 2012 3

Gemini Catches a Disappearing Warm Debris DiskCarl Melis (University of California, San Diego)

Carl Melis (UCSD), Ben Zuckerman (University of California, Los Angeles), Joseph Rhee (California Polytechnic), Inseok Song (University of Georgia), and Simon Murphy and Michael Bessell (Australian National University) used Thermal-Region Camera Spec-trograph (T-ReCS) observations at Gemini South to capture the rapid disappearance of a substantial, warm, dusty debris disk orbiting a nearby, young, Sun-like star (Melis et al. 2012a). This system, TYC 8241 2652 1, was originally identified in their search of the Infrared Astronomical Sat-ellite (IRAS), AKARI, and other catalogs for stars hosting mid-infrared emission in excess of what one would expect from the star alone and hence indicative of orbiting circumstellar dust. The rapid disappearance of a debris disk has not been seen or predicted before, and thus is an im-portant test of mechanisms that control their evolution.

Figure 1 shows how the T-ReCS-measured mid-infrared emission of this source evolved from being a factor of ~30 times the stellar photospheric flux before 2009, to being ~13 times the photospheric flux in early 2009, to being barely detectable after 2010. At the time of its discovery, TYC 8241 2652 1 was the dustiest main sequence star known. (Figure 2 shows an artist’s conception of this exceptionally dusty system; it has since been superseded by V488 Per, see Zuckerman et al. 2012.) The copious amounts of dust that were present suggest a system undergoing an active

stage of terrestrial planet formation (Kenyon & Bromley 2005; Melis et al. 2010; see also artist’s conception in Figure 2). Remarkably, two epochs of measurements from the Wide-field Infrared Survey Explorer (WISE) show that the excess mid-infrared emission has all but disappeared leav-ing only a weak (~3 times the stellar photosphere) excess at a wavelength of 22 mm (Figure 1). Measurements made after the WISE epochs us-ing the SpeX spectrograph at the NASA Infrared Telescope Facility, the Photodetector Array Camera and Spectrograph (PACS) for the Herschel Space Observatory, and T-ReCS are consistent with the WISE data (Fig-ure 1: note especially the 2012 T-ReCS data), indicating that the mid-infrared emission from the dust orbiting this star has been consistently depleted to barely detectable levels since at least early 2010. In short, the substantial, dusty debris disk orbiting TYC 8241 2652 1 vanished in less than two years.

TYC 8241 2652 1 itself is a young star in the nearby, southern star-form-ing associations. Optical spectroscopic data confirm the youth of this source through strong lithium absorption, mild Hα emission, and kine-matics consistent with either of the Lower-Centaurus-Crux association (10–20 Myr old: e.g., Torres et al. 2008, Song et al. 2012) or the TW Hydrae association (TWA, ~8 Myr old: e.g., Zuckerman & Song 2004).

The Gemini data alone show that a dramatic event occurred between 2008 and 2010 that emptied the TYC 8241 2652 1 inner planetary system dust reservoir. Such a rapid disk evolution timescale and flux diminish-ment is unheard of (see Meng et al. 2012 for a discussion of weaker mid-infrared variability for warm debris disk stars) and is in contrast to mod-els suggesting very long timescales for the evolution of debris from the

Figure 1: Spectral Energy Distribution of TYC 8241 2652 1. Measurements and the associated epoch (for mid- and far-infrared data) are indicated in the legend. The solid brown curve is a synthetic stellar photosphere for a 4950 K effective temperature star that is fit to the optical and near-infrared data. The dotted line is a blackbody fit to the 12 mm and 25 mm IRAS excess data points—the temperature of this blackbody is 450 K and it suggests that roughly 11% of the optical and near-infrared starlight was being reprocessed into the mid-infrared by orbiting dust. The solid black line is the sum of the photosphere and the 450-K blackbody. Fitting a blackbody to the WISE and Herschel measurements suggests a dust temperature of roughly 200 K and a fractional infrared luminosity of 0.1%. Some vertical error bars, e.g., those of the two earlier epochs of T-ReCS measurements, are smaller than the point sizes on the plot; for these measurements, the uncertainty is comparable to or less than 10% of the correspond-ing measurement. Horizontal lines through each data point represent the filter full-width at half-maximum.

11 0 100Wavelength ( m)

0.01

0.1

Flux

den

sity

(Jy)

BV J HK 12 25 60 160

Tycho-22MASSIRAS (1983)AKARI (2006)T-ReCS (May, 2008)T-ReCS (Jan, 2009)WISE (Jan and Jul, 2010)SpeX guider (Apr, 2011)SpeX spectrum (Apr, 2011)Herschel PACS (Jul, 2011)T-ReCS (May, 2012)

Figure 2: Artist’s conceptualization of the dusty TYC 8241 2652 1 system as it might have ap-peared several years ago when it was emitting large amounts of excess infrared radiation. With a fractional infrared luminosity of 11%, TYC 8241 2652 1 was the dustiest main sequence star known at the time of its discovery. Since the disappearance of its dusty belt, an even dustier system has been identified (Zuckerman et al. 2012). (Image credit: Gemini Observa-tory/AURA, artwork by Lynette Cook.)

continued

4 NOAO Newsletter September 2012

Scien

ce H

ighlig

hts

Gemini Catches a Disappearing Warm Debris Disk continued

Figure 3: All Sky Automated Survey (ASAS) V-band measurements of TYC 8241 2652 1. Data points and associated uncertainties were extracted from the ASAS project (Pojmanski et al. 2002). The abscissa is the heliocentric Julian date (spanning from roughly 2000.9 to 2009.9) while the ordinate is apparent visual magnitude. The horizontal dotted line is the median of all plotted values. The colored vertical dashed lines correspond to the various epochs of mid-infrared measurements (from left to right, respectively): AKARI (green), first and second T-ReCS (red and purple), and first WISE (gold).

2000 2500 3000 3500 4000 4500 5000

11.6

11.4

11.2

11.0

10.8

mV (m

agni

tude

s)

terrestrial planet formation process (Jackson & Wyatt 2012). It is worth noting that the disk is unlikely to be blocked from view as any structure capable of blocking the disk light should also block out stellar light, and the star is photometrically very stable as the disk has faded (Figure 3). Mechanisms that might act to rapidly remove small dust grains orbiting around TYC 8241 2652 1 involve either a collisional avalanche within

the dusty disk or runaway accretion driven by a gas disk component that drags on the dust grains (Melis et al. 2012a). Neither model is without problems, but they are at least capable of getting close to a 1- to 3-year disk removal timescale (more details can be found in Melis et al. 2012a).

This discovery is part of a long-term effort by Carl Melis, Ben Zucker-man, Inseok Song, and Joseph Rhee to discover and characterize some of the dustiest terrestrial-planet-forming star systems currently known (e.g., BD+20 307, Song et al. 2005; EF Cha, Rhee et al. 2007; HD 23514, Rhee et al. 2008; HD 15407, Melis et al. 2010; V488 Per, Zuckerman et al. 2012; HD 131488 and HD 121191, Melis et al. 2012b). The major goal of this research is to study and understand the formation and evolution of terrestrial planets around stars of various masses (e.g., Melis et al. 2010, Melis et al. 2012b). The search continues with the recent release of the WISE database and ongoing monitoring of known, warm excess stars like TYC 8241 2652 1.

ReferencesJackson, A.P. & Wyatt, M.C. 2012, accepted to MNRAS (arXiv1206.4190)Kenyon, S.J. & Bromley, B.C. 2005, AJ, 130, 269Melis, C. et al. 2010, ApJL, 717, 57Melis, C. et al. 2012a, Nature, 487, 74Melis, C. et al. 2012b, ApJ, submittedMeng, H.Y.A. et al. 2012 ApJL, 751, 17Pojmanski, G. et al. 2002, Acta Astronomica, 52, 397Rhee, J. et al. 2007, ApJ, 671, 616Rhee, J. et al. 2008, ApJ, 675, 777Song, I. et al. 2005, Nature, 436, 363Song, I. et al. 2012, AJ, 144, 8Torres, C.A.O. et al. 2008 in Handbook of Star Forming RegionsZuckerman, B. & Song, I. 2004, ARA&A, 42, 685Zuckerman, B. et al. 2012, ApJ, 752, 58

Leo P: A Newly Discovered Local Group CandidateJohn Salzer & Katherine Rhode (Indiana University)

John Salzer, Katherine Rhode, and their students at Indiana University, used the KPNO 4-m, WIYN, and 2.1-m telescopes to carry out the first optical observations of a new, nearby dwarf galaxy. This object is the lowest-mass system known with current star formation. In com-bination with its unusually high gas to stellar-mass ratio and its ultra-low metallicity, this makes it one of the most extreme objects in the local universe. The discovery team has designated it as Leo P, with P standing for “pristine.” It may shed light on the long-standing “missing satellites” problem (e.g., Klypin et al. 1999).

Leo P was first discovered as a low-velocity HI source in the ongoing Arecibo Legacy Fast ALFA (ALFALFA) Survey being carried out with the Arecibo 305-m radio telescope (Giovanelli et al. 2012). The extreme sensitivity of the Arecibo telescope allows it to detect very low-mass HI systems (e.g., Leo P has an HI mass of only 3×105 M

if it lies at 1 Mpc).

Cross-matching of the HI detection with imaging data from the Sloan Digital Sky Survey (SDSS) showed an extended, faint blue source locat-

ed close to the HI coordinates. Leo P has an observed HI velocity of 264 km/s, similar to the well-known Local Group dwarf Leo I (velocity = 285 km/s, distance = 250 kpc) that is located ~7° away on the sky. Sub-sequent observations with the Expanded Very Large Array led by John Cannon (Macalester College) verified the HI detection and revealed an HI size of ~2 arcmin.

ALFALFA team members at Indiana University (IU) were notified quick-ly of the possible discovery of a dwarf galaxy in the HI data. Within a month of the initial discovery, they used scheduled time on three Kitt Peak telescopes to observe the new source. The first observations were obtained in March 2012 on the KPNO 2.1-m telescope using time al-located to the NOAO Survey program called ALFALFA Hα. Salzer and IU graduate student Angela Van Sistine obtained narrowband Hα images of the target and detected an HII region in Leo P. Subsequent analysis suggests that this nebula is ionized by a single late-O or early-B type star.

continued

NL

NOAO Newsletter September 2012 5

Science Highlights

Leo P: A Newly Discovered Local Group Candidate continued

Rhode and IU student Michael Young then imaged Leo P with the Mini-Mosaic camera on the WIYN 3.5-m telescope through optical broad-band (BVR) filters. Excellent image quality (0.6–0.8 arcsec PSF FWHM) resolved the galaxy into stars. Figure 1 shows the color composite image of Leo P. The light from this galaxy is dominated by young, blue stars, par-ticularly in its southern (lower) half. The HII region is the brightest object in the clump of blue stars. The brightest individual stars are V ~ 22; PSF-fitting recovers photometry for stars as faint as V ~ 25. A color-magnitude diagram (CMD) for the brighter stars in Leo P is shown in Figure 2. The CMD reveals a well-defined, upper main sequence, but a weak or under-populated red giant branch.

Finally Salzer, IU graduate student Nathalie Haurberg, and John Cannon utilized part of a scheduled run on the KPNO Mayall 4-m telescope in April 2012 to obtain a spectrum of the HII region in Leo P, which re-veals the very metal-poor nature of this dwarf system (Figure 3). The velocity of the HII region matches that of the HI gas. Preliminary esti-mates based on this spectrum, plus a subsequent deep spectrum obtained with the Large Binocular Telescope by collaborator Evan Skillman (Uni-versity of Minnesota), indicate that Leo P has an oxygen abundance of log(O/H)+12 < 7.2, comparable to the lowest extragalactic sources known (hence the use of the term “pristine”). More comprehensive analysis of the spectroscopic data is underway.

The combination of the upper main-sequence photometry and the pres-ence of a single HII region constrains the distance to between roughly 400–700 kpc, suggesting that Leo P is an outlying member of Local Group. However, at this distance one would expect to see a more ex-tensive red giant branch with a tip at much brighter magnitudes than was observed. Applying the tip of the red giant branch (TRGB) distance method to the CMD results in a distance estimate of 1.0–1.5 Mpc (de-pending on which stars are used to represent the TRGB). The problem with the larger distance is that several of the upper main-sequence stars would then be luminous enough to be hosting HII regions, which is not observed. Hence, the current situation regarding the distance is rather enigmatic: the nearer distance requires that Leo P have a very unusual star-formation history to account for the under-populated RGB, while the greater distance appears to violate basic stellar and nebular astro-physics. This distance ambiguity might be resolved with deeper photo-metric observations.

Regardless of the final distance, Leo P is an amazing object. Adopting a fiducial distance of 1.0 Mpc for the purpose of discussion, Leo P has a vi-sual absolute magnitude of MV = -8.1. The HI-to-stellar mass ratio is 2.6, making Leo P one of the most gas-rich galaxies in the nearby Universe. It is the lowest-mass system known that is actively making stars at the current time. Its ultra-low metal abundance indicates that it is relatively unevolved chemically. The location of Leo P in the periphery (or just outside) the Local Group, coupled with its high gas content, suggests that it has not yet traveled inside the virial radius of either the Milky Way or Andromeda. The emerging evolutionary scenario is one in which Leo P

Figure 1: BVR color composite image of Leo P obtained with the WIYN telescope. The field of view of this image is 2.4 by 2.5 arcmin and the orientation is N-up, E-left. The lower (southern) portion of Leo P is dominated by a clump of blue main-sequence stars, indicating very recent star formation has occurred. The brightest object in Leo P (located within the clump of blue stars) is an HII region that appears to be photo-ionized by a single B-type star. The upper portion of the galaxy has very low surface brightness but includes a number of redder stars, presumably RGB members in Leo P. The total size of the galaxy at this sensitiv-ity level is ~90 arcsec.

Figure 2: Color-magnitude diagram constructed from point spread function photometry of the stars in Leo P measured in the WIYN telescope images (see Figure 1). The plot includes all objects within the galaxy that have photometric uncertainties in B-V less than 0.25 mag. The upper main sequence and red giant branch stars are indicated. The dashed line indi-cates the 50% completeness limit for the data. Note the well-defined upper main sequence and the under-populated red giant branch.

continued

6 NOAO Newsletter September 2012

Scien

ce H

ighlig

hts

has lived on the outskirts for most or all of its existence. Perhaps a re-cent encounter is responsible for the current round of star formation? It would appear that prior to this modest burst of star formation Leo P was a rather inconspicuous low-surface-brightness dwarf galaxy. It is unclear whether it would have been detectable as having any optical counterpart in the SDSS data if it had been observed pre-burst.

The ALFALFA survey has detected dozens of ultra-compact HI clouds (e.g., Giovanelli et al. 2010) with velocities that place them in or near the Local Group. It has been proposed that these objects represent mini-halos (objects with dark matter masses Teff > 4800 K and log L/L

≥ 4) is a short-lived, transitional phase. Stars with

initial masses between ~9 M

and 40 M

briefly pass through this region of the H-R diagram while transitioning either from the main sequence to the red supergiant (RSG) phase or, in some cases, from the red back

to the blue. The lifetime of the YSG phase is only on the order of tens of thousands of years, thus these stars are very rare.

The incredibly short lifetime of the YSG phase is the main reason this portion of the H-R diagram is ideal for observational tests of evolutionary codes. The YSG lifetimes predicted by the models for stars of various masses can be used to predict the relative number of stars that should appear in various luminosity bins. However, these predicted lifetimes are highly sensitive to uncertain model parameters (mass loss, overshooting, and rotationally induced mixing), and because of their short duration, even small variations can dramatically change the predicted relative number of stars. Indeed, previous studies (Drout et al. 2009, Neugent et al. 2010) found large discrepancies between the relative number of yellow supergiants observed as a function of mass and those predicted by many, single-star evolutionary models, with the models over-predicting the number of high luminosity YSGs by a factor of 100 or more. It thus seemed prudent to characterize the YSG population of additional galaxies (at various metallicities) in the hopes of shedding additional light on these discrepancies.

continued

NOAO Newsletter September 2012 7

Science Highlights

The Yellow Supergiants in the Local Group continued

Identifying the SupergiantsOne large hurdle that must be overcome is, ironically, identifying the supergiants. When one looks toward a local group galaxy at the colors and magnitudes of YSGs, a majority of stars observed will not be bona-fide supergiants, but rather foreground yellow dwarfs. In these studies, the modest radial velocities of M33 and the LMC were used to distinguish extragalactic supergiants based on their radial velocities and the luminosity-dependent OI λ7774 triplet (see Figure 1). Here, the Hydra and Hectospec multifiber spectrographs on the Blanco 4-m telescope and the MMT, respectively, were invaluable. They provided spectra of ~3000 YSG candidates in only a handful of nights. In the end, the program identified 121 probable YSGs in M33 and 317 in the LMC. This corresponds to a foreground contamination of ~80% in the directions of both galaxies.

Testing the Evolutionary ModelsAfter placing the newly identified YSG populations on the H-R diagram (see Figure 2), it is apparent that the latest generation of Geneva evolutionary tracks (Ekstrom et al. 2012, Chomienne et al. in

-1 -0.5 0 0.5 1-200

-100

0

100

200

300

X/R

Figure 1: Observed radial velocity minus expected M33 velocity (based on M33’s rotation curve and the location of the object in the disk) versus X/R (proxy for location within M33 disk) for the YSG candidates. Green points represent stars whose spectra show a strong OI λ7774 feature. Bona-fide supergiants should lie along zero on the vertical axis while foreground dwarfs make up the strong diagonal band. (Drout, M.R. et al. 2012, ApJ, 750, 97. Reproduced by permission of the AAS.)

preparation) show excellent agreement with the observed locations of our YSGs, as well as the relative number of YSGs at various luminosities. These models therefore represent a drastic improvement over previous generations. Unfortunately, it is not possible to identify one physical cause for this improved behavior; rather, it is likely due to a combined effect of modified initial compositions, opacities, rotation prescriptions, and RSG mass-loss rates. Clearly, there are still unanswered questions, such as the role of binarity and variability. However, the identification of representative populations of these stars serves as a necessary first step for future studies of massive star evolution in this portion of the H-R diagram.

ReferencesDrout, M.R., Massey, P., & Meynet, G. 2012, ApJ, 750, 97Neugent, K.F., Massey, P., Skiff, B., & Meynet, G. 2012, ApJ, 749, 177Drout, M.R., Massey, P., Meynet, G., Tokarz, S., & Caldwell, N. 2009, ApJ, 703, 441Ekstrom, S., et al. 2012, A&A, 537, A146Neugent, K.F., et al. 2010, ApJ, 719, 1784

Figure 2: The population of newly identified M33 YSGs, as well as several newly identi-fied M33 RSGs, plotted with the newest generation of rotating Geneva evolutionary models (z = 0.014 are shown as solid lines while z = 0.006 are shown as dashed lines). The vertical black lines designate the YSG region. (Drout, M.R. et al. 2012, ApJ, 750, 97. Reproduced by permission of the AAS.)

NL

8 NOAO Newsletter September 2012

Scien

ce H

ighlig

hts

Needles in a Haystack: Studying Andromeda Stellar Populations through Those of the Milky WayRachael L. Beaton, Steven R. Majewski & Richard J. Patterson (University of Virginia)

The Spectroscopic and Photometric Landscape of Andromeda’s Stellar Halo (SPLASH) is systematically exploring the system of dwarf spheroidal satellites (dSph) in the Andromeda galaxy (M31) through a unique two-phase approach: (1) deep imaging with the Mayall 4-m + Mosaic led by graduate student Rachael Beaton (U. of Virginia) and (2) highly efficient spectroscopic follow-up with Keck II + DEIMOS led by Erik Tollerud (Yale). Dwarf spheroidal satellites within the Local Group include the least luminous galaxies known and provide critical tests of theories of galaxy formation and evolution. Studies of the Milky Way (MW) satellites have revealed a surprising absence of a trend between galaxy luminosity and total mass spanning five orders of magnitude of luminosity, suggesting that galaxy formation is effectively stochastic at these mass scales (~107 M

). These conclusions, however,

are drawn from only a single population of dSph galaxies. The M31 dSph population remains unexplored and becomes a key test bed to understanding the MW observations.

To date, the team has complementary imaging and spectroscopic datasets for 16 of the 30 known M31 dSph galaxies. Studying the M31 stellar populations in color-magnitude space (CMD) is a challenge due to the superposition of the dominant foreground MW dwarfs over the “needle in the haystack” M31 red giant branch (RGB) stars. The SPLASH survey uniquely identifies the target RGB stars using the Washington+DDO51 filter system.

Figure 1: The Washington+DDO51 filter system permits separation of stars of the same temperature into their respective luminosity classes using the DDO51 filter. The top figure demonstrates the sensitivity of the Mgb feature to the surface gravity of the star. The bot-tom figure illustrates giant selection in the color-color diagram. The MW dwarfs form a characteristic “swoosh” and the giants are generally above it.

Figure 2: Demonstration of the Washington+DDO51 giant selection technique on the M31 dSph, Andromeda V. The top row illustrates all of the stars detected in the Mosaic field-of-view (CMD, left; Spatial, right). In the CMD, we have labeled some of the major features: orange = TriAnd MW substructure, blue = MW dwarf sequence, and teal = AndV RGB. In the middle row, we apply an RGB selection indicated in blue. In the bottom row, we add the Wash+D51 selection using the color-color diagram. The combination of RGB and color-color selection greatly enhances the contrast between the dSph and the underlying M31 stellar halo background.

continued

NOAO Newsletter September 2012 9

Science Highlights

and Beaton). Nearly 50% of the Mayall 4-m fields have complementary Keck + DEIMOS follow-up spectroscopy that permits careful identifica-tion of RGB stars in the underlying M31 stellar halo. The SPLASH sur-vey will begin its next phase of implementation this fall with a recently accepted NOAO Survey program to obtain J and Ks imaging using the NEWFIRM wide-field infrared imager in each of the 80 SPLASH survey fields (Co-PIs Guhathakurta and Beaton). Combined with the optical Washington+DDO51 photometry, this will permit identification of in-termediate stellar populations both in the dSphs and in the M31 stellar halo, which provides constraints on the merger history of the M31 halo and the star formation histories of the dSphs.

AcknowledgmentsThis work benefits tremendously from the help of the night assistants at the Mayall 4-m telescope, including, but not limited to: Hal, George, Jenny, Karen, and Ed. With the NEWFIRM runs approaching, we look forward to many more productive nights at the Mayall.

ReferencesMcConnachie, A.W., & Irwin, M.J. 2006, MNRAS, 365, 1263 Majewski, S.R., Ostheimer, J.C., Kunkel, W.E., & Patterson, R.J. 2000, AJ, 120, 2550Ostheimer, J.C., Jr. 2003, PhD ThesisTollerud, E.J., et al. 2012, ApJ, 752, 45

The DDO51 filter is centered on the surface-gravity-sensitive Mgb triplet, which for the same temperature will separate dwarf and giant stars, providing a drastically cleaner sample of candidate dSph mem-ber stars than a selection in color-magnitude space alone. Figure 1 demonstrates the Washington+DDO51 method by comparing the stel-lar spectrum of a dwarf and a giant star of the same spectral type (top panel). Using the photometric method, the spectral difference results in a fainter DDO51 magnitude in a dwarf star than that of a giant. This difference is illustrated in the bottom panel of Figure 1, which displays the color-color diagram. The MW dwarf stars form a characteristic “swoosh” as a function of temperature, whereas the giants form a swath above the “swoosh.”

An application of the method is illustrated in the panels of Figure 2 for Andromeda V (AndV), which is a dSph representative of the median properties (size, luminosity, mass) seen in our sample. The top row of Figure 2 displays the full field CMD (left) and the RA-Dec spatial dis-tribution (right). AndV is easily identified as the overdensity in the lower right of the spatial distribution. The middle row demonstrates selection of the RGB in the CMD (left) and the resulting spatial distri-bution (right), which shows a stronger contrast between the dSph and the underlying background (a combination of the smooth M31 stellar halo and foreground MW dwarfs). In the bottom row of Figure 2, an additional selection in color-color space is applied to select high-like-lihood giant stars (left), showing the resulting stellar distribution made by applying both sets of selection criteria (right). The resulting contrast between AndV and background is again increased. This increase in contrast permits far more precise measurement of the size and shape of the dSph than with RGB selection alone.

The resulting best fit King profile for AndV is shown in the left panel of Figure 3 and, in the left panel, is compared to the corresponding fit by McConnachie & Irwin (2006). Figure 3 illustrates how reducing the MW foreground substantially reduces the noise in the most distant radial bins. In the process of fitting a radial profile to a dSph, the outer-most bins set the background stellar density, which in turn dramatically affects the resulting best-fit profile. Thus, the SPLASH method provides unparalleled ability to study the M31 dSphs on a par with those of the MW. Beaton is currently in the process of finalizing radial profile fits for the 16 dSphs that have both photometry and spectroscopy. The fit-ting process, developed in the PhD thesis of Ostheimer (2003), takes great care to understand all of the errors inherent in the fitting process.

The observational effort required to study the dSphs is enormous. Each Mayall 4-m + Mosaic field requires three hours of on-sky time (including calibration overheads). Despite its observational expense, the method is invaluable for a detailed analysis of the M31 dSphs. The M31 dSph sam-ple, however, represents only 20% of the larger SPLASH M31 halo survey that uses a pencil beam sampling approach to study the overall halo out to 165 kpc (projected). This represents seven individual Mayall 4-m ob-serving runs over five years led by Beaton, first as an undergraduate and now as a graduate student at the University of Virginia (PIs Majewski

Figure 3: (Left) Results of fitting a King radial profile to the resulting stellar distribution of AndV selected in Figure 2 (highlighted in orange). (Right) We compare the SPLASH-derived profile (orange) to a comparable profile by McConnachie & Irwin (2006, teal). This com-parison emphasizes the improvements enabled by the Washington+DDO51 method. We emphasize that the increased contrast between the dSph and the backgound substantially improves the tracing of the dwarf into larger radial bins.

Needles in a Haystack continued

NL