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High Redshift Galaxies. Scott Chapman (Cambridge, IoA) Aug22, 2007. Overview and Motivation. 0. Introductory remarks about galaxy formation & evolution Direct observations of galaxy progenitors forming at high redshift (z ≥ 1.5) Protoclusters at high redshift (z ≥1.5).

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high redshift galaxies

High Redshift Galaxies

Scott Chapman (Cambridge, IoA)

Aug22, 2007

overview and motivation
Overview and Motivation

0. Introductory remarks about galaxy formation & evolution

Direct observations of galaxy progenitors forming at high redshift (z ≥ 1.5)

Protoclusters at high redshift (z ≥1.5)

context hierarchical galaxy formation how when are the galaxy components assembled
Context: Hierarchical Galaxy Formation(How/when are the galaxy components assembled?)

Big Bang …Cosmic Microwave Background …

… Galaxy Formation and Evolution … Fossil Records today!

Submm/Radio is a superb probe of obscured forming galaxies; well defined SEDs; unimpeded by dust obscuration; “bolometric selection”

Andromeda (M31) is ideal laboratory to study L* galaxy components.



Dark matter

Bright galaxies

N-body + semi-analytic

-gas cooling

-star formation

-SNe feedback

-galaxy mergers

within halos

  • Success reproducing
  • observed parameters
  • at z=0.
  • High-z objects of various classes continue to be problematic -- SMGs, DRGs,
  • (e.g., Sommerville et al. 2000; Baugh et al. 2005)


High SFR

(Kauffmann et al. 1998)

progenitors of massive galaxies
Progenitors of Massive Galaxies
  • Until the mid-1990’s the only z>2 objects known were QSOs, radio galaxies, and QS0 absorbers (DLA/LLS)
  • How can we go about isolating more normal galaxies during the epoch of star/galaxy formation?
  • The study of high-redshift (let’s say z>1.5) galaxies has exploded in the last ~10 years, with multiple techniques for isolating high redshift galaxies, making use of multi-wavelength data spanning from the radio to the X-ray
  • As opposed to traditional magnitude-limited surveys, down to specific flux limit, new results utilize several complementary selection techniques for finding high-z galaxies, selecting overlapping yet complementary populations --> must determine how they overlap/complement each other to describe entire galaxy population at a given epoch.
foreground z 2 galaxies
“Foreground (z<2) Galaxies”

NB: IAB=25 

1 photon/year/cm2/A

From: LeFevre, Vettolani et al 2003

 “Brute force” spectroscopy is inefficient for z>2!

the laboratory for studying high z galaxies






Palomar 5-m


The Laboratory for studying high-z galaxies



ground based vs space based

.........................opt..NIR.................radio window.......

0.1nm 1 10 100 1mm 10 100 1cm 10 1m 10 100m


Ground-based vs. space-based?

NB: in addition to transmissivity, the emissivity of the Earth’s atmosphere is a big problem: sky at 2mm is 10,000 brighter than at 0.5mm

Some sub-mm windows from good sites


(Spitzer Space


10-m mirror

0.85-m mirror

2-m astronomer

2.4-m mirror

(Hubble Space


(Keck Telescope)

local galaxies e g luminous elliptical galaxies
Local Galaxies - e.g. Luminous Elliptical Galaxies
  • … the most massive galaxies in the local Universe
    • Pressure-supported, metal-rich stellar systems. “Simple” galactic-scale stellar systems
    • Extremely homogeneous and old (implies z>2 formation)
    • Live in the highest density regions (clusters)
        • Largest stellar mass of any galaxies ( up to 10 M*)
        • Host the most massive Black Holes in local Universe, >109Mo
    • -observations of the high-z Universe
formation mechanisms
Formation Mechanisms
  • Wide range of proposed mechanisms for forming big galaxies:
    • (pseudo-)monolithic collapse (Tmerge<TSF)
    • major merger of two existing galaxies (Tmerge~TSF)
    • an extended series of minor mergers (Tmerge>TSF)
  • Can we observationally distinguish between these scenarios?
dust obscured merging galaxies e g the antennae
Dust obscured, merging galaxies: e.g.,The Antennae


ISO 15um

  • Distinct opt/UV and far-IR luminosity
  • 90% emitted at far-IR
  • Dust obscures UV;

absorbs and re-radiates at longer wavelengths

(~100-200 microns)


CSO/SHARC-2: 350um

the extragalactic background



The Extragalactic Background


  • FIRB = opt/UV EBL -> half of the energy production (from SF or AGN) over history of the Universe arises in highly obscured regions
  • Much of the star formation in the Universe might be obscured: resolve FIRB into constituent components
components of firb to be studied directly and indirectly
Components of FIRB (to be studied directly and indirectly)



Dole et al. 2005

Submm is almost all high-z galaxies (80% z<4)

MidIR and FarIR is 80% z<1.7

Radio is extremely useful, BUT …

submm best (only?) way to probe the high-z components of FIRB

studying the obscured activity



Herschel (2007)


Studying the obscured activity

SED of an ageing stellar population of solar metalicity with dust

f 24 m m vs l bol
f(24mm) vs Lbol

Papovich and Bell 2003

At “far-IR” wavelengths, Spitzer can only observe well at 24 mm … what are the bolometric corrections?

… they’re large!

And especially bad for the most luminous, dust-obscured populations.

(leave aside for now)

progenitors of massive galaxies1
Progenitors of Massive Galaxies
  • Review of techniques
    • (focus on submm/radio selection, UV, & nearIR)
  • Some key questions and results
z 1 5 submm selection 100 z s
z>1.5 Submm Selection (~100 z’s)

(courtesy I. Smail)

(Smail 2005)

  • Submm galaxies: dusty, most of luminosity comes out in submm waveband
  • First detected by SCUBA/JCMT in 1997 at 850 m (Smail+1997)
  • Counts: ~1000/sq. degree at 5 mJy (limit of bright SCUBA survey); a few 100 submm galaxies ID’d
  • In principle, SCUBA sensitive to dusty galaxies to z>6 (negative K-correction) … but we now know that almost no SCUBA galaxies are that distant!
submm radio bolometric selection of galaxies
Submm/Radio: bolometric selection of galaxies?

Identify SMGs: (although Spitzer proving very useful -- Pope+06)

-Known tight correlation with FIR

-Good spatial resolution (MERLIN 0.3”, VLA 1.4”)

FarIR/radio correlation roughly holds at z=2.5

(Kovacs et al. 2006)

Far-IR correlates with

synchrotron radio

(SNe: shock accelerates electrons)


(Ivison et al. 2002)

scuba galaxies smgs




SCUBA galaxies (SMGs)

High resolution RADIO and HST reveal large, merging galaxies.

Smail+1999,2004; Chapman+2003,2004)





scuba galaxies smgs1
SCUBA galaxies (SMGs)

Redshift surveys have given us tremendous insight into a

hyper-luminous population of cool dust SB-dominated galaxies.



Chapman+03,05, Swinbank+04


Neri+03, Greve+05

Genzel+02, Tacconi+05


Keck 10m

z 1 5 submm selection
z>1.5 Submm Selection

(courtesy I. Smail)

(Chapman et al. 2005)

  • Breakthrough: using radio (1.4 GHz) positions for optical spectroscopy (ameliorates ambiguity from 15” SCUBA beam), 50% of sources have z’s
  • Redshifts of ~100 SMGs enable study of physical properties
  • Typical LIR=8x1012L and dust temperature Td~38K (confirmed by 350 m observations)
  • Less than ~10% of submm galaxies are at z>>3
z 1 5 submm selection1
z>1.5 Submm Selection

(courtesy I. Smail)

(Chapman et al. 2005)

  • IR luminosities correspond to sfr >1000 M/yr (Salpeter) if bulk of FIR is powered by star formation. If SF lasts for ~108 yr, significant stellar mass formed
  • Much rarer than other samples, but higher inferred SFR, likely contribute significantly to SFR density at high z
z 1 5 submm selection2
z>1.5 Submm Selection

(Chapman et al. 2003)

  • Rest-frame UV spectra obtained with Keck/LRIS-B (like UV-selected samples)
  • Spectra show features of star-formation and AGN (Ly, NV, CIV, SIV), rest-frame optical spectra sometimes [NII]/Hor broad H
  • Raises question of AGN contribution to bolometric luminosity, Deep (Chandra 2 Msec image) X-ray emission indicates presence of Fe Kline, and absorbed non-thermal continuum slope
  • BUT: AGN appears not to be energetically important -> submm emission dominated by star formation
z 1 5 submm selection3
z>1.5 Submm Selection

Note: galaxies w/ smaller star formation rates not detected yet in CO

(Tacconi et al. 2006)

  • Masses:stellar masses estimated from SED-fitting (how does AGN affect stellar mass estimates?); dynamical masses estimated from H, CO-linewidths; cold molecular gas masses estimated from CO line luminosities
  • Dynamical masses w/in 10kpc, 2x1011 M(Swinbank et al. 2004, 2006), molecular gas masses 5x1010 M (Tacconi et al. 2006)
z 1 5 submm selection4
z>1.5 Submm Selection
  • Submm galaxies appear to be massive systems with prodigious star-formation rates, may also be strongly clustered (but uncertain because of small number of redshifts)
  • Could be progenitors of QSOs, and massive galaxies at lower redshift
  • Interesting note:several years ago, a lot was made of the fact that there was so little overlap between submm galaxies and z~3 UV-selected galaxies. But now we know that most submm galaxies are at z<3, and have similar redshift distribution to z~2 UV-selected galaxies. >~50% of submm galaxies have colours of UV-selected galaxies (but bigger SFRs)
photometric pre selection uv
Photometric Pre-selection: UV
  • ~50 objects/square arcmin down to R=25. How do you pick out the high-redshift galaxies?
  • Lyman discontinuity at rest-frame 912 A gives z~3 galaxies very distinctive observed UGR colours

(Steidel et al. 1992, 1993, 1995, 1996, 2003)

z 1 5 rest uv color selection
z>1.5 Rest-UV Color Selection
  • z~3 UGR Lyman Break criteria, adjusted for z~2 (Adelberger et al. 2004)
  • Spectroscopic follow-up with optimized UV-sensitive setup (Keck I/LRIS-B)
  • ~1000 galaxies at z~3, >750 galaxies with spectroscopic redshifts at z=1.4-2.5, in what was previously called the Redshift Desert
measuring redshifts z 3
Measuring Redshifts: z~3
  • Meaure: Lya em/abs, IS abs at z>2.5
  • At z =1.4-2.5, these features are in the near UV … bonus: strong rest-frame optical emission lines have shifted into the near-IR:
  • formerly called THE REDSHIFT DESERT
redshift desert
Redshift Desert
  • Low redshift
  • Emission-line z
  • [OII], [OIII], Hb, Ha
  • At z > 1.4, [OII] moves past 9000 AA, while Lya below 4000 AA at z<2.3: no strong features in the optical

SDSS galaxy at z=0.09

redshift desert1
Redshift Desert
  • Low redshift
  • Abs-line z
  • 4000 AA break, Ca H&K, Mg
  • At z > 1.4, 4000AA break moves past 9000 AA

SDSS galaxy at z=0.38

keck lris b efficiency
Keck/LRIS-B Efficiency
  • LRIS-B + 400/3400 grism --> 40% efficiency from 3800-5000 AA
  • Low night-sky background in near-UV (~3 mag fainter than at 9000 AA)
  • (Steidel et al. 2004)


measuring redshifts z 2
Measuring Redshifts: z~2
  • Low- and high-ionization outflow lines, Ly
  • He II emission, CIII] emission
  • Fewer galaxies have Lya emission (57% have no Lya) than in z~3 sample (cf. SMG!)
  • (Steidel et al. 2004)
outflow kinematics z 3 vs z 2
Outflow Kinematics: z~3 vs. z~2
  • Kinematic evidence for large-scale outflows is a generic feature of UV and submm-selected galaxies at z>2
  • Signature of “feedback”, perhaps fundamental to understanding galaxy formation
  • (Steidel et al. 2004)




z 1 5 rest uv color selection1
z>1.5 Rest-UV Color Selection
  • z~3 UGR criteria (Lyman Break), adjusted for z~2 (Adelberger et al. 2004)
  • Spectroscopic follow-up with optimized UV-sensitive setup (Keck I/LRIS-B)
  • ~1000 galaxies at z~3, >750 galaxies with spectroscopic redshifts at z=1.4-2.5, in what was previously called the Redshift Desert
evolution of clustering to z 1 0
Evolution of Clustering to z~1, 0

(Adelberger et al. 2004)

(Blain et al. 2004)

  • Follow evolution of DM halo clustering in simulation
  • Matches early-type absorption line DEEP2 galaxies at z~1 (Coil et al. 2003) & SDSS E’s at z=0.2
  • Typical UV-selected galaxy at z=2-3 will evolve into an elliptical by z=0
summary z 1 5 rest uv colour selection
Summary:z>1.5 Rest-UV Colour Selection
  • (typical sfr~50-100M/yr)
  • (typical stellar mass ~few x 1010M, large range)
  • What does UV-selection mean in terms of physical properties?
  • Star-formation that’s only moderately (factor of few-100) extinguished by dust, but a large range of stellar masses (smaller range of baryonic masses), clustering implies that these will correspond to early-type galaxies at z~0.
z 1 5 near ir selection
… z>1.5 Near-IR selection
  • Extension of K20 survey group (i.e., get z’s for everything with K<20), use B-z, z-K color criteria to select both star-forming galaxies and passive galaxies at z>1.4
  • Incomplete for fainter objects with small Balmer Breaks, weighted more towards fairly massive objects
  • Significant overlap of BzK/SF with UV-selected samples

(Daddi et al. 2004)

z 1 5 fires j k selection 20 zs
z>1.5 FIRES/J-K selection (~20 zs)

(Franx et al. 2003)

(Reddy et al. 2005)

  • J-K>2.3 criteria meant to select massive evolved galaxies with significant Balmer/4000 Å breaks at z>2; turns out selection also yields massive dusty starbursts
  • ~25% appear to contain AGN (much higher than fraction of UV-selected population)
  • Only limited number of spectroscopic redshifts
z 1 5 summary
z>1.5 Summary
  • In addition to UV-selected, BzK, J-K, submm, there are other techniques, such as the K-band/photo-z technique of the Gemini Deep Deep Survey (GDDS), and new Spitzer capabilities: IRAC (mass-selected), MIPS/24 micron (sfr-selected, analogous to SCUBA)
  • Now that there are several groups using different selection techniques to find galaxies at z~2, we need to understand how the samples relate to each other (each sample has certain benefits but is incomplete; e.g., UV-selected sample has largest set of redshifts and spectra)
  • Reddy et al. (2005) considered the overlap among different samples, and contribution of each to the sfr density at z~2-2.5
at the highest redshifts
At the highest redshifts?

Hard stuff to study! (e.g., contamination)

Controversy at z~ 6?

Did the star formation rate change from z~ 6 to z~ 3? If so, did it

increase or decrease?

Each possibility (increase, decrease, stay the same) had

been claimed by various groups studying z~ 6 galaxies.

For z~ 6, Lyαis between I &z bands, so we are looking for I-drops

R. J. Bouwens, G.D. Illingworth, J. P. Blakeslee, M. Franx set

out to do the most careful analysis possible and thereby come

to some firm conclusions about such galaxies.

the star formation history of the universe
The Star Formation History of the Universe
  • Millimeter/Radio galaxies form most of the stars we see today in big bursts
  • 2-5 billion years after the big bang.
    • LESSONS:
    • we still don’t even know about all the galaxies in the Universe!
    • Local galaxies (M31) show unexpected components!
    • Simulations of how galaxies form are not always very predictive!



(Chapman et al. 2005)

key questions
Key Questions
  • What is the evolution in global sfr and stellar mass density vs. z?
  • What is the evolution in number density of galaxies as a function of (stellar) mass and star-formation rate?
  • What are the star-formation histories of galaxies (burst/episodic, continuous), and how do they accumulate their stellar mass?
  • What are the origins of different morphological types?
  • What is the chemical enrichment in galaxies vs. z, and by how much do they enrich their surroundings (vs. mass)?
  • What is the effect of supernovae/AGN feedback on gas in galaxies and the surrounding IGM?
  • How do we make a continuous timeline of galaxies from high redshift to z~0 (map one sample to another)?
key techniques goals
Key Techniques/Goals
  • New multi-wavelength technologies are helping us address these questions, beyond ground-based optical imaging and spectroscopy
  • Wide-field near-IR imaging (stellar masses) and near-IR spectroscopy (dynamical masses, sfr, chemical abundances)
  • Chandra X-ray observations (sfr and AGN)
  • Spitzer/IRAC (stellar masses) and MIPS (dust luminosity, sfr)
  • HST ACS/NICMOS (morphologies)
  • Full understanding of energetics and stellar and metal content is a multi-wavelength endeavor
  • Detailed comparison with numerical simulations and semi-analytic models
concluding philosophical comment
Concluding Philosophical Comment
  • Rich set of observations of galaxies in the early universe, for statistical samples of galaxies selected with different techniques (though it’s still a challenge to get spectra)
  • There’s much more that I could have presented
  • Field of z>1.5 galaxy evolution is completely different since just 10 years ago, when UV-selection technique was first implemented
  • What’s happening in the next 10 years? (feedback, connecting to samples at other redshift, building mass/sfr-limited samples, disk galaxies)