HWRPrinceton, 2005

Observing the Assembly of Galaxies

Hans-Walter Rix

Max-Planck-Institute for AstronomyHeidelberg

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Overview

I. The Build-Up of the Stellar Mass in Galaxies

II. The Formation and Evolution of Massive GalaxiesThursday May 5, 2:00PM

III. The Evolution of (Internal) Galaxy StructureWednesday May 11, 2:00PM

IV. Archeo-Cosmology in the Local GroupFriday, May 13, 2:00PM

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I. The Build-Up of Stellar Mass

1. Casting the problem into specific questions

2. Diagnostic Tools

3. A brief survey of surveys

4. Estimating the star-formation rate = f(z)

5. Estimating the stellar mass density = f(z)

6. Results

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1. Re-phrasing “the build-up of stellar mass”

• What is <SFR(z)> and <*(z) >?

• What epoch encloses the formation of most stars?

• How to best measure <SFR (z) > and <*(z) > ?

• How much important are mergers in triggering SF and in setting the present-day mass function?

• What are the expectations from models?

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2. Diagnostic tools for star-formation rates and stellar masses

• Star formation rate estimates are based on UV luminosity produced by hot, massive, short-lived stars– Observe the UV – Observe H – Observe absorbed UV flux, re-

radiated by dust in thermal IR ! LIR(re-radiated) >> LUV(escaped) !

– Mtot estimate is based on stars >10Mo, which are small fraction of Mtot

Kroupa 2002

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Starlight and Re-processed Starlight

Devriend et al 2000Single-age, dust-free stellar population

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ground

SED of an ageing stellar population of solar metalicity with dust

Spitzer

Herschel (2007)

Redshift

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f(24m) vs Lbol

Papovich and Bell 2003

Given that Spitzer can only observe well at 24 m, what are the bolometric corrections?

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Mass measurements in cosmologically distant galaxies

• Dynamics:– OK to z~1, but quite expensive.– Very limited spatial resolution conceptually

problematic– Currently not feasible for most galaxies z>1.5

• Clustering:– Measures halo mass, not stellar mass

• M* = L x (M/L)* with M/L from SEDs

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Stellar Masses from Spectral Energy Distributions

Optical/near-IR spectra of galaxies are a nearly 1D sequence

Near-degeneracy of age, metallicity and dust

Source of despair or opportunity?

tstars = [Gyrs]

Bell and de Jong 2001

BK

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• Mapping one or few integrated galaxy colors to – age– dust extinction– metallicity

is poor!

• Mapping (optical -- across 4000A break) color to M/L should be robust!

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M/L from Colors? Compare to dyn! Van der Wel, Franx, can Dokkum and Rix, 2004

at z~1

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Look-back Galaxy Surveys: Desiderata

• Select SFR surveys by SFR, and mass surveys by stellar mass– SFR: assure most of the intense star-burst are not missing

due to dust– Stellar mass: select galaxies obs > (1+z) 4000A break

• Number of galaxies as a function of – Epoch redshift (few %)– Luminosity/stellar mass– Color/stellar age 1,000 – 10,000 galaxies

• Measure galaxy sizes/internal structure ~0.3” resolution

• Either Nfield >> 1 or field > 2xcorrelation length ~10’

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A Survey SurveyName Nfield Field

sizeHST imaging

# of bands

Depth Nredshift

HDFs/UDF 3 2.5’ + 7 R=29 700GOODS 2 12’ + 10 i=27.5 400

3000 (5%)FIRES 2 5’ + 10 KAB=26 600

(5%)COMBO-17 GEMS

3 30’ + 22 R=24 30,000(1%)

MUNICS 3 30’ - 7 K=19.5 20.000 (5%)

GDDS/LCIRS 2 30’ - 7 H=21.5 500(2000)SUBARU Rest-

UVSteidel et al Rest-

UV

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COMBO-17Wolf, Meisenheimer, Rix et al. 01/03Heidelberg, Oxford,Potsdam,Edinburgh

• 3 fields @ 30’x30’• 17 filters to mr~23.6• ~10.000 redshifts (1.5%)+ SEDs per

field

Wavelength [nm]MB

Z

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Comparison of COMBO-17 with VIMOS Spectra(data from Le Fevre et al 2004)

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A quick Tour through Redshift Space

GEMS(CDFS)

Abell 901 S11 (random)

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Stellar Masses from the COMBO-17 SurveyBorch, Rix, Meisenheimer et al 2005

• Stellar masses to z~1 can be estimated for 10.000s of galaxies

• Flux limit (R-band) is VERY different from mass limits.

0.65<z<0.75

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FIRES: FaintInfra-Red-Extragalactic-Survey ultra-deep VLT survey

*HDF-south100 hours in JHK

FWHM=0.45”

*MS1054:5xlarger area25 hours in JHK per pointing

Franx, Rix, Rudnick, Labbe, van Dokkum, Foerster-Schreiber, Trujillo, Moorwood, et al.2001-2005

Selecting and studying galaxies z>2 in their rest-frame optical bands

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Not a Ly-break!!Just a red SED

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What kind of galaxies are found in such a search?

• Galaxies without many (really) young stars won’t be found by their Ly-break or their sub-mm dust emission.

• Ditto for galaxies with significant dust extinction that are not powerful enough for a sub-mm detection.

• Remember: both UV searches (dust) and sub-mm searches (fainter galaxies) have ~10 corrections to get total SFR

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SED fits for DRGs

Near-IR selected

UV selected

Förster-Schreiber, Franx, Rix et al; FIRES

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Improving Mass, SFR and Av Estimates at z~2.5 through IRAC (3.6m-8m) data

Labbe, Franx, Rix et al 2005

Förster-Schreiber, Rix et al 2005; FIRES

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Comparing dynamical (?) with SED masses

Van Dokkum, Franx, Rix, et al. 2004

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Results I: Cosmic Star-Formation Rate

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SFR’s from thermal-IR flux 0<z<1Zheng, Rix, Rieke, Bell et al 2004

Stacking galaxy classes (z,L) from COMBO-17 and measuring the 24m flux

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SFR’s from thermal-IR flux 0<z<1Zheng, Rix, Rieke, Bell et al 2004

LIR/LUV = f(SFR) @ all z,Lopt

Local relation

• Through stacking, Spitzer’s (single source) confusion limit can be beat by >10 to <10Jy

• IR flux dominates in all galaxies (to 3% of L*) to z~1.2;– large majority of UV photons absorbed.

• Mean LIR/LUV drops with galaxy luminosity faint galaxies contribute hardly to SF integral

• “Correction” seems to be a function of (absolute) SFR only– Insensitive to stellar luminosity, redshift

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State of Affairs: Star-fomration rate

Borch, Rix, Meisenheimer et al 2005

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Why the drop of the SFR since z~1?or

In what type of galaxies did stars form back then?

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Whence the UV flux at z~0.7?j280 (z~0.7) ~ 4 x j280nm (now)

Pick f(2800A) as a proxy for young stars (t<tdyn)

[not necessarily true in massive, old systems]

Explore “morphology” of galaxies that give rise to these photons

Subjective – use 6 eyes

[Morphologies from GEMS, see Thursday]

UV

-to-o

ptic

al fl

ux (M

280n

m –

V)

UV luminous

“blu

e”

Wolf, Bell, Rix et al 2004 0.65<z<0.75

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• At MV>-19 and z~0.75– ½ the flux comes from

seemingly normal spirals– 20% from visibly

interacting systems

• only minority of UV flux from manifestly interacting systems at z~0.75

drop in (major) merger rate not cause of SFR drop

z~0.75Normal spiralsInteracting/Peculiar

UV-light contribution by

Galaxy type at z~0.75

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Results II:Evolution of the Stellar Mass Density with

Redshift

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The Evolution of the Stellar Mass Function over the Last 7 Gyrs

Borch, Meisenheimer, Rix, Bell et al 2005, COMBO-17

Present-day stellar mass function

COMBO-17 survey; 30,000 galaxies Mean stellar mass Build-

Up

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Where is the stellar mass at z=2-3.5?

DRGs (“distant red galaxies”) vs Ly-Break Galaxies

Distant red galaxies likely dominate the mass budget

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<*(z)>: State of Affairs

Borch, Meisenheimer, Rix et al 2005

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…half the mass since z~1.5…

Borch et al 2005

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Putting it together

Borch, Meisenheimer, Rix et al 2005

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Summary• Waning SFR not a consequence of waning major mergers

– Waning cold gas supply

• SED-based stellar mass estimates now available for 1000’s of galaxies to z~3– Need to observe at least torest>4000A– Available testing against dynamics OK

• “Distant red galaxies”, between Ly-break and sub-mm galaxies, may contain the bulk of stellar mass 2<z<3.5– Found through near-IR surveys– Quite frequent objects with SFR x tSFR ~1010-11M

• <*(z) > can be traced from z~3.5 to 0– enclosing ~90% of all stars formed

• Integral over SFR estimate agrees with <*(z) > to < 2– Assuming diet-Salpeter IMF (e.g. Kroupa 2002)– Leaves not much room for overlooked SFR

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Where do we go from here?

• Role of merging in the build-up of the galaxy mass function is observationally barely constrained

• Comprehensive linkeage of SED-based and dynamical masses

• Beat field-to-field variations at z>2

• Relate stellar masses at different z to halo masses– Lensing, clustering

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Improving Mass, SFR and Av Estimates at z~2.5 through IRAC (3.6m-8m) data

Labbe, Franx, Rix et al 2005

Förster-Schreiber, Rix et al 2005; FIRES

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SED Fitting of FIRES Galaxies

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Where is the stellar masses at z=2-3.5DRGs (“distant red galaxies”) vs Ly-Break

Galaxies

Förster-Schreiber, Franx, Rix et al 2005