Lecture 22; Nov 13, 2017

High-Redshift Galaxies III- Galaxy Evolution- Galaxy “Main Sequence”: Relevance- Star Formation Law, Gas Fractions- First Light and Reionization

Reading: Chapter 9 of textbook

Continue working on your final project (presentations due Nov 15/20).

• We are happy to meet with you to discuss remaining issues, questions etc.• You have 12-15 minutes (practice!)

• An electronic version of your final paper is due by Dec 6, 4:30 pm

In class presentationsNov 15/20: Final presentation (15 minutes incl. questions)

Wed/Mon: Final Presentations (aim for min. “1 min per slide”, so <~10 slides)try to prepare to speak freely, use your summary to memorize key issues

Email us a copy by morning (make sure we get it!) or bring your own laptop to avoid unexpected technical issues

I use a Mac, so if you use another OS to prepare, a pdf may be safer in case

2015151515202020

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First galaxies

“Epoch of galaxy assembly”

Present day

CIB vs. Star Formation HistoryThe dust-obscured fraction of star formation in the universe is significant!UV/optical studies miss substantial fraction of cosmic star formationÞNew phenomenon: distant, very IR-luminous (observed-frame submm) galaxies

The (sub)mm is a key wavelength regime to understand galaxy evolution!

ALMA Deep Fields

ALMA Deep Fields

• The “K-correction” is the correction we apply to the observed flux of an object of a given SED that accounts for its redshift.

• The K correction at the mm and submm at λ > 250 µm, which yields a flux density that is almost independent of redshift.

“Negative” K-correction

• At high z the SFR was >10x higher than today (Lilly et al. 1996, Madau et al. 1996)

• Did it drop because there were fewer merger-driven starbursts?

Þ insufficient, only ~20% of galaxies at z~0.75 are visibly interacting (but: lower limit due to morphological classification)

• Did it drop because “quiescent” disk galaxies today form fewer stars?

Þ yes, but why? (fgas, discuss later)

Why did the global SFR decrease towards the present epoch?

Star formation => stellar mass• The rate at which stars

form over time must correspond to the buildup of the stellar masses of galaxies

• Individual galaxies can grow in mass by

- forming new stars

- coalescence (merging) of pre-existing bits

• Merger rate?

- of order 1 major merger since z~1 for massive galaxies

• At present: galaxy mass function is– “Schechter function”– most stars in Mgal=1010.5Msun

• At earlier epochs:– Define M*-limited sample, independent of SFR

(which brightens galaxies)à near-IR selection is needed

• Results:– Galaxy mass function looks similar 0<z<4– “characteristic mass” was only slightly lower

at high-z– Co-moving density was considerably lower

• Most stars were always in the most massive galaxies!– At least for z<4, since when 95% of all stars

formed

e.g. Marchesini et al. 2009

Galaxy Mass Function at Earlier Epochs

Connecting the BzK, LBG and DRG PopulationsSF Density Contributions

LBG

Distribution of M>1011M� galaxies

LBG DRG

• Reddy et al. (2005) - sBzK and LBGs are (largely) identical populations

• Kong et al. (2006) - Clustering of sBzK and pBzK galaxies is identical, suggesting same population and SF is simply transient

• van Dokkum et al. (2006) - LBGs constitute only 17% of massive galaxies!

The star formation “main sequence” of galaxies

Daddi et al. 2007, Noeske et al. 2007

The general high-redshift galaxy population:BzK, BX/BM, LBG-selected galaxies (“typical”/“normal”), SMGs (“starbursts”)

- There appears to be a relation between SFR and M* for actively star-forming galaxies, a “main sequence” (MS) of star formation

- “passive” galaxies fill the triangular region below- Merger-driven “starbursts” deviate from the MS (few times higher SFR)- The normalization appears to evolve with redshift towards higher SFR

(caution: there are some mergers/starbursts on the MS, but they are a minority)

Physical relevance of the star formation “main sequence” of galaxies

- where disk galaxies are located at high z

- 90% of cosmic star formation out to z~2 occurs on main sequence

- MS galaxies have typically high duty cycles, are mostly not starbursts (but they can have high SFRs)

e.g., Adelberger et al. 2004, Noeske et al. 2007, Daddi et al. 2007, Foerster Schreiber et al. 2009, 2012, Rodhigiero et al. 2011, Wuyts et al. 2012

Herschel/PACS

Evolution of the Main Sequence

Comparison of 25 studies in the literature (Speagle et al. 2014 - undergrad student):- MS is constrained out to z~6- When putting all on a common calibration scale, remarkable agreement- Width of the MS: remarkably narrow, 0.2 dex- Slope and normalization of the MS: both are likely time-dependent:

SFR(M*, tcos) = (0.84 ± 0.02 - 0.026 ± 0.003 × tcos)log M* - (6.51 ± 0.24 - 0.11 ± 0.03 × tcos), where tcos is the age of the universe in Gyr

Specific SFR:sSFR = SFR/M*

Galactic star formation in equilibrium with cosmic accretion & outflows

e.g., Keres et al. 2005, 2009, Guo et al. 2009, Oppenheimer & Dave 2006, Dekel et al. 2009, Dave et al. 2010

Star Formation Law at High Redshift

- High redshift galaxies follow the same SF law as nearby galaxies

- SF law may have two sequences, “quiescent”/disk vs “starburst”/merger-driven

- difference in SF efficiencies is reflected in different gas depletion times (<100 Myr vs. 1 Gyr)

- physical mechanisms are the same, but high-z galaxies are high on both axes

- main complication: conversion factor aco to obtain Mgas is difficult to measure at high z

Daddi et al. 2010, Genzel et al. 2010

Carilli & Walter 2013 ARAA; after Magdis et al. 2012

const.?

Even “main sequence” galaxies(defined as typical SFR/M*(z))

show 10-30x higher gas fractionsat z=1-3 compared to present day

Ø Increased SF history driven by high gas fractions of galaxies(not by extreme merger rates)

Ø Star formation is elevated, but underlying physics are similar

Ø Evolution at z>3 poorly known

Metallicity and Conversion Factor aCO

- Local universe: dwarfs are high on SF law due to metallicity effects, resulting in high aCO

- High redshift: metallicity evolution, even massive galaxies can have sub-solar metallicity

Þ can mimick different SF efficiency. In principle, running aCO could unify SF “sequences”

Þ likely to contribute to the scatter in the SF law, but unlikely to result in single tdepletion

Genzel et al. 2012

cold gas history of the universe

connection:

star formation law (Mgas vs. SF rate)

Star formation law: SF history of the universe is a reflection of the cold gas history of the universe (gas supply)

Ø Studies of galaxy evolution are shifting focus to cold gas (source vs. sink)

WSFR WM(gas)

Problem: populations at high-z so far are highly selected (IR, radio, UV/optical luminosity)Ø may miss cold gas rich, quiescent galaxy populations

Solution: complete census of molecular gas, the fuel for star formationi.e. a molecular deep field (at the same time: continuum deep field)

H2 density

SFR density

COLDz Molecular Deep Field: A JanskyVLA Large Program

CO luminosity function is only measured well at z=0 to date.

Ø Require very deep, wide-band line survey over substantial cosmic volume to measure CO luminosity function at high redshift

Ø CO à H2 mass: Obtain “Cold Gas History of the Universe”

à observe CO J=1-0

Only possible with the fully upgraded Karl G. JanskyVLA

Ø VLA Large Program (PI: Riechers)

R. Pavesi, PhD Thesis project

Big Bang f(HI) ~ 0

f(HI) ~ 1

f(HI) ~ 10-5

History of Normal Matter (IGM ~ H)

0.4 Myr

13.8 Gyr

Recombination

Reionization

z = 1100

z = 0

z ~ 6 to 120.4 – 1.0 Gyr

Djorgovski/Caltech

• Once the H-atoms form (380,000 yrs), the universe is filled with neutral gas which should emit in the 21 cm line (ground state, spin-flip transition)

• The universe continues to expand and cool; the gas remains neutral in the absence of any source of ionizing radiation.

• “Dark Ages”: there are no sources of “light”.

• Once the first objects formed, the hydrogen was reionized!ÞThe “Epoch of Reionization”

10cMpc

F(HI) from z=20 to 5

Numerical simulation of the evolution of the IGM

Three phases

• Dark Ages

• Isolated bubbles (slow)

• Percolation (bubble overlap, fast): ‘cosmic phase transition’

Mean free path of ionizing photons depends on IGM density structure

Note: galaxy clustering drives the evolution

http://home.fnal.gov/~gnedin/GALLERY/rei_p.html(Gnedin & Fan 2006)

Sources of re-ionization• Population III stars:

• Need to seed heavy elements even in first galaxies and oldest Pop II stars

• Star formation/nuclear fusion in primordial abundance material different

• Very high T: good for re-ionization!• Not yet observed

• QSOs:• How do SMBHs form? (One of biggest mysteries is how first

QSOs formed)• Are there enough? Perhaps not.• But, good source of high E photons

• First galaxies:• Likely the main source: faint, but numerous

Did not yet find enough sources to explain re-ionization, there must be more, faint sources at very high z than we know thus far

Evolution of the IGM neutral fraction: Robertson et al. 2013

FHI_vol

Gunn-Peterson

Quasar Near-zones

Ly-a-galaxies

1 Gyr 0.5 Gyr

Large scale polarization of the CMB•Temperature fluctuations = density inhomogeneities at the surface of last scattering

• Polarized = Thomson scattering local quadrupol CMB

WMAP Hinshaw et al. 2008

Large scale polarization of the CMB (WMAP)

• Angular power spectrum (~ rmsfluctuations vs. scale)

• Large scale polarization

Ø Integral measure of te back to recombination

Ø Earlier => higher τeτe ~ σTρL ~ (1+z)3/(1+z) ~ (1+z)2

Ø Large scale ~ horizon at zreionl < 10 or angles > 10o

Ø Weak: µK rms~ 1% total intensity

Jarosik et al. 2011, ApJS 192, 14

Baryon Acoustic Oscillations: Sound horizon at recombination

te = 0.087 +/- 0.015

Sachs-Wolfe

CMB large scale polarization: constraints on F(HI)

§ Rules out high ionization fraction at z > 15

§Allows for small (≤ 0.2) ionization to high z

§ Most ‘action’ at z ~ 8 – 13

Two-step reionization: 7 + zr

Dunkley ea 2009, ApJ 180, 306

1-F(HI)

FHI_vol

Ø Systematics in extracting large scale signal

Ø Highly model dependent: Integral measure of te

CMB large scale polarization: constraints on F(HI)

Lyman-a Forest&Gunn-PetersonEffect

Atz=1100(CMBdecoupling),theuniverseis(mostly)neutralHI,andopaquetoLy-a photons.

Atsomepoint,thefirststarsandgalaxiesformandbegintoionizethegas:HI->HII.

Atz<5,theuniverseismostlyionized(HII),withsomedense,neutralpocketsalongtheway.

Þ Ly-a forest

Atz>6,thereisstillsufficientneutralgastoabsorbvirtuallyalllightshortward ofLy-aÞ Gunn-Petersoneffect

However,evensmallamountsofHIaresufficienttoabsorbalmost100%ofUVphotons.

Þ Thiseffect,onitsown,onlysetsalowerlimitontheneutralfraction.

Quasars

Gunn-Peterson Effect

Ly-a resonant scattering by neutral gas in IGM clouds

• Linear density inhomogeneities, δ ~ 10

• N(HI) = 1013 – 1015 cm-2

• F(HI) ~ 10-5

tGP » 6x105 xHI

1+ z10

æ è ç

ö ø ÷ 3 / 2

Gunn-Peterson effect

5.7

6.4

SDSS z~6 quasars

Opaque (τ > 5) at z>6

Þpushing into reionization?

Gunn-Peterson constraintson F(HI)

• Diffuse IGM:

tGP = 2.6e4 F(HI) (1+z)3/2

• Clumping: tGP dominated by higher density regions => need models of ρ, T, UVBGto derive F(HI)

Becker et al. 2011

τeff

• z<4: F(HI)v ~ 10-5

• z~6: F(HI)v ≥ 10-4

• GP => systematic (~10x) rise of F(HI) to z ~ 5.5 to 6.5

• Challenge: GP saturates at very low neutral fraction (10-4)

FHI_vol

Gunn-Peterson constraints on F(HI)

• J1148+5251: Host galaxy redshift: z=6.419 (CO + [CII])

• Quasar spectrum => photons leaking down to z=6.32

• Time bounded Stromgren sphere (ionized by quasar?)

• cf. ‘proximity zone’ interpretation, Bolton & Haehnelt 2007

White et al. 2003

zhost – zGP => RNZ = 4.7Mpc ~ [Lγ tQ/FHI]1/3 (1+z)-1

Quasar Near Zones

HI

HII

Quasar Near-Zones: 28 GP quasars at z=5.7 to 6.5

ü No correlation of UV luminosity with redshiftü Correlation of RNZ with UV luminosity

Note: significant intrinsic scatter due to local environ., tQ

R Lγ1/3

LUV

LUV

Carilli et al. 2010

Quasar Near-Zones: RNZ vs. redshift[normalized to M1450 = -27]

<RNZ> decreases by ~10x from z=5.7 to 7.1

z ≤ 6.4

z=7.1

• <RNZ> decreases by factor ~ 10 from z=5.7 to 7.1• If CSS => F(HI) ≥ 0.1 by z ~ 7.1

5Mpc 0MpcCarilli et al. 2010

• Damped Ly-a profile: N(HI) ~ 4 x 1020 cm-2

• Substantially neutral IGM: F(HI) > 0.1 at 2 Mpc distance[or galaxy at 2.6 Mpc along LOS; probability ~ 5%]

Simcoe et al. 2012(Bolton et al.; Mortlock et al. 2011)

Highest redshift quasar (z=7.1)

Highly Heterogeneous metallicities: galaxy vs. IGM

Simcoe et al.

Venemans et al. 2012• [CII] + Dust detection of host galaxy => enriched ISM, but,

• Very low metallicity of IGM just 2 Mpc away

Intermittency: Large variations expected during epoch of first galaxy formation

Z/H < -4

[CII] 158 µm

• QNZ + DLA => rapid rise in F(HI) z~6 to 7 (10-4 to > 0.1)

• Challenge: based (mostly) on one z>7 quasar

FHI_vol

Quasar near zone constraints on F(HI)

z=7.1 quasar

• Neutral IGM attenuates Ly-a emission from early galaxies• Search for decrease in:

Ø Number of Ly-a emitting galaxies at z>6

Ø Equiv. Width of Ly-a for LBG candidates at z > 6

Galaxy demographics: effects of IGM on apparent galaxy counts

Ly-a Typical z~5 to 6 galaxy(Stark et al.)

Ly-a

Konno et al. 2014

zLy-a = 7.3

Galaxy demographics: Ly-α emitters

• NB survey COSMOS + GOODS-North

• Space density of LAEs decreases faster from z=6 to 7 than expected from galaxy evolution

• Expected 65, detected 7 at z=7.3!

• Modeling attenuation by partially neutral IGM => F(HI) ~ 0.5 at z ~ 7

• LBGs: dramatic drop in Ly-a EW at z > 6

• F(HI) > 0.3 at z~8

Galaxy demographics: effects of IGM on apparent galaxy countsStrength of Ly-a from LBGs

Tilvi et al. 2014; Treu et al. 2013

• Galaxy demographics suggests possibly 50% neutral fraction at z~7!• Challenge: separating galaxy evolution from IGM effects

LBGs

LAEs

FHI_vol

Galaxy constraints on F(HI)

• Amazing progress (paradigm shift): rapid increase in neutral fraction from z~6 to 7 (10-4 to 0.5) = ‘cosmic phase transition’?

•All values have systematic uncertainties: suggestive but not compelling=> Need new means to probe neutral IGM

1Gyr 0.5Gyr

Robertson et al. 2013

FHI_vol

F(HI): Synergy

Summary of Reionization ConstraintsN

eutr

al fr

actio

n

Redshift

QuasarsGRBsLy-alpha

CMB

Zahn et al. 2012

Did Galaxies Re-Ionize the Universe?

Freee- scatteringopt.depth

(causeCMBpolarization)

Robertson et al. 2010

Galaxies seem to have a tough time w/ taking responsibility for the observed ionization state of the universe…

Planck 2015

HI 21cm line: Most direct probe of the neutral IGM

• Low frequencies: zreion ~ 6 to 12 => νobs = 1420MHz/(1+z)

= 110MHz to 200MHz

Advantages of the 21cm line

• Direct probe of neutral IGM

• Spectral line signal => full three dimensional image of structure formation (freq = z = depth)

• Low freq => very (very) large volume surveys (1sr, z=7 to 11)

• Hyperfine transition = weak => avoid saturation (translucent)

• Ly-a lifetime ~ 1 nanosec; 21cm lifetime ~ 107 yrs

Seeing Through the “Fog”: HI 21cm cosmology @z=20 to 5 (67 to 240 MHz)

Gnedin et al. (2006)

ABCDE

‘Richest of cosmological data sets’

TKTSTcmb

First stars

Pritchard & Loeb 2010; Loeb & Furlanetto textbook ch. 12

τ = 0.008(TCMBTS

)(1+ z10

)1/2 fHI (1+δ)

A. z>200: TCMB = TK= TS by residual e-, photon, and gas collisions. No signal.

B. z�30 to 200: gas cools as Tk ≈ (1+z)2

vs. TCMB ≈ (1 + z), but TS = TK via collisions => absorption, until density drops and TS àTCMB

C. z�20 to 30: first stars => Ly-α photons couples TK and TS

=> 21-cm absorption

D. z�6 to 20: IGM warmed by hard X-rays => TK > TCMB. TS coupled to TKby Ly-a. Reionization is proceeding => bubble dominated

E. IGM reionized

Signal: HI 21cm Tomography of IGM

z=12 9

7.6

§ DTB(2’) ~ 20 mK

§ SKA rms ~4 mK

§ Pathfinders: rms ~ 80 mK

ÞRequires SKA (>2023-30)

Furlanetto, Zaldarriaga et al. 2004

Reionization• After recombination, the universe was neutral• At z~20 – 30, the first generation of galaxies and mini quasars formed• At z~6 – 15, the UV radiation from the first generation objects ionized

most of the HI in the universe– The neutral fraction of the universe changed from 1 to 10-5 (phase

transition in ionization state)– The temperature of the IGM electrons changed from CMB

temperature to 104 K (phase transition accompanied by temperature change)

– IGM becomes transparent to UV radiation, the universe is like a giant HII region (temperature change accompanied by opacity change)

Riechers 2013