GALAXY MASSES…AND BACK TO DOWNSIZING!

Motivation Great progress in tracking star formation history but: - SF density averages over different physical situations (e.g. bursts, quiescent phases) - hard to link to theory (witness how CDM is always able to match the data!) Stellar mass assembly is a fundamental measurement - here is where CDM is in deep trouble - measurements of galaxy masses better link progenitors and descendants - most galaxy properties depend on galaxy mass… Importance of developing mass diagnostics stressed in several early papers: Broadhurst et al, Kauffmann et al, Cohen et al, Brinchmann & Ellis

“spiral” “elliptical”” 1990s N-body, CDM “con gas!” “No model has succeeded so well -- structure formation using dark matter halos, and failed so miserably -- making galaxies, as CDM with baryons.” -- Me. Cold Dark Matter!

Stellar Mass Assembly History (  CDM) CDM predicts recent growth in assembly of spheroids, slower growth in disks z=5,3,2,1,0.5,0 z=1 Evolution of stellar mass function time z=5,3,2,1,0.5,0 Merger trees time

Dry Mergers? Caveat: Most observations measure the ages of the stars in galaxies of different masses. Young ages are seen for stars in low mass galaxies and old ages for stars in massive galaxies..seemingly in contrast to hierarchical predictions. van Dokkum (2005) argues high preponderance of red tidal features & red mergers in local samples, coupled with a postulated increase in merger rate (1+z) m implies significant mass evolution is still possible in large galaxies: i.e. stars could be old but assembled mass could be younger via self-similar merging of red sub-units (so-called `dry mergers’)

Dry Mergers at High Redshift Clusters: Tran et al (astro-ph/ ) Field: Bell et al (astro- ph/ ) No good statistics yet on how prevalent this process is

“Dry mergers” -- the latest thing! Bell et al ApJ, 640, 241. “Dry Mergers in GEMS: The Dynamical Evolution of Massive Early-Type Galaxies” By analyzing the images in the Combo-17 survey, Bell et al. conclude that the typical massive galaxy could have undergone ~1 dry merger since z ~ 1, more consistent with the hierarchical picture (at least from the halo standpoint)).

“ Spectroscopic Confirmation of Multiple Red Galaxy- Galaxy Mergers at z < 1” -- Tran et al ApJ627,L25 The apparent mergers are real…R<14 kpc, Δ V < 165 km s -1. “…these bound pairs must evolve into E/S0 members by z = 0.7…most if not all, of its early- type members evolved from (passive) galaxy-galaxy mergers at z < 1” Identified as a special epoch in the cluster’s life, or significant subcluster merging (relative velocities still low), before virialization

Dry mergers? Jeltema, Muchaey and collaborators -- study of x-ray groups

environment redshift galaxy mass WHY ?

the best predictor of galaxy type is mass! Look at these luminosity functions for elliptical, S0, and spiral galaxies: Luminosity! Mass-to-light ratio of (star forming) spirals are ~3-10 x less than those of E and S0 galaxies

And now we also know that these other properties (all really a function of mass?) go along the Hubble Sequence of “bulge-to-disk: Environment: “early” types = dense environments Color & star formation: late-types are blue, star forming Mass-to-light ratios: star forming systems produce a lot more light per unit mass Stellar Age: early galaxies are early, at least their stars are. Late types are slow developers with younger stars Mass of central black hole: scales with the spheroid Mass of the dark matter halo: scales with (drives?) all of the above.

Kauffmann et al 2003 Blue, star-forming = spirals, irregulars Massive-Passive = E & S0 galaxies 3 x M sun Blue Red Moving through time Sloan Digital Sky Survey

Baldry et al Color distribution versus galaxy magnitude in the Sloan

Kauffmann et al Sloan Current star formation rate per unit galaxy stellar mass (M * ) vs M * But depending on environment …

Measuring galaxy masses: what are the options? Dynamics: rotation & dispersions (only for restricted populations) Gravitational lensing (limited z ranges) IR-based stellar masses (universally effective 0<z<6) K

Dynamical methods Rotation curves for disk systems (e.g. Vogt et al. 1996,1997) Stellar velocity dispersion for pressure-supported spheroidals (e.g. van Dokkum & Ellis 2003, Treu et al. 2005, Rettura et al. 2006) Issue of preferential selection of systems with “regular” appearance

The Fundamental Plane: Empirical correlation between size, μ and  * Dressler et al. 1987; Djorgovski & Davis 1987; Bender Burstein & Faber 1992; Jorgensen et al Considerably superior as a tracer of evolving mass/light ratio and assembly history: Dynamical mass: - no IMF dependence - Closer proxy for halo mass Tough to measure: -  demands high s/n spectra - large samples difficult M = K σ 2 R/G (e.g. Bertin et al. 2002)

Stellar Masses from Multicolor Photometry (especially near-infrared) K-band luminosity less affected by recent star formation than optical Spectral energy distribution  (M/L) K Redshift  L K hence stellar mass M log mass spectral energy distribution Mass likelihood function e.g. Kodama & Bower 2003, or Bundy et al. 2005,2006

What if you don’t know the redshift? Catastrophic errors securing photo-z & masses from same photometry Bundy et al 2006 z spec  logM Expected scatter based on photo-z error distribution

What if you only have optical photometry? Bundy 2006 Ph.D. thesis z spec A key ingredient in the mass determination is infrared photometry which is sensitive to the older, lower mass stars; important z > 0.7 log M opt - M IR BRI vs BRIK log  (M opt )log  (M IR )

Einstein Rings For a compact strong lens aligned with a background source, a ring of light is seen at a radius depending on the geometry and the lens mass, i.e. this allows us to measure the mass of the lens lensing galaxy ring arising from single background source

DOWNSIZING EFFECT IN STAR FORMATION AND MASS

4 clusters at z= EDisCS collaboration De Lucia et al ApJL, 610, L77 Data from Terlevich et al. (2001) Smail et al. 1998; Kajisawa et al. 2000, Nakata et al. 2001, Kodama et al. 2004

De Lucia et al The effect is seen also in the single-cluster distributions, despite of the variety of cluster properties: such a deficit may be a universal phenomenon in clusters at these redshifts A deficiency of red galaxies at faint magnitudes compared to Coma -- A synchronous formation of stars in all red sequence galaxies is ruled out, and the comparison with Coma quantifies the effect as a function of galaxy magnitude

Observing late star-forming faint galaxies becoming “dwarf ellipticals” About 10% of the dwarf cluster population in the Coma cluster (see also Tran et al. 2003, Caldwell et al.’s works, De Propris et al.) Poggianti et al. 2004

Downsizing-effect Going to lower redshifts, the maximum luminosity/mass of galaxies with significant SF activity progressively decreases. Active star formation in low mass galaxies seems to be (on average) more protracted than in massive galaxies. IN ALL ENVIRONMENTS.

Mass downsizing: Fundamental Plane (Treu et al 2005) 142 spheroidals: HST-derived scale lengths, Keck dispersions Increased scatter/deviant trends for lower mass systems? If log R E = a log s + b SB E +  Effective mass M E   2 R E / G So for fixed slope, change in FP intercept  i   log (M/L) i

Evolution of the Intercept  of the FP Strong trend: lower mass systems more scatter/recent assembly 1-3% of mass growth in massive(>10^11.5) galaxies since z=1.2 – 20-40% at lower masses

Stellar Mass Functions by Type in GOODS N/S Bundy et al (2005) Ap J 634,977 No significant evolution in massive galaxies since z~1 In fact, almost no change in total mass function with time above 5 X10^10, indicating little mass growth at the high mass end Bulk of evolution is in massive Irrs/spirals 2dF ( h = 1 )

Cimatti et al and Brown et al emphasize that, if only a factor of two in mass is added to the red sequence since z~1, and it is mainly in lower luminosity (< M sun ) galaxies, then simple “running down” of star formation in disk galaxies, turning them red, can account for the growth. A key point to be resolved, and one that may be telling as to how much the hierarchical picture is in trouble.

Redshift >1.5 – How many massive galaxies at z=2? Pioneering study: N=737, H<26.5, z photo <3, 5 arcmin 2 H=26.5 incomplete Significant uncertainty estimating contribution of H-faint low mass galaxies z>1.5 50% of the assembled mass is only in place at a surprisingly low redshift z~1 Integrated SFH underestimates mass assembly: dust, cosmic variance? Similar HDF-S analysis by Rudnick et al 2003 Ap J 599, 847 2dF Dickinson et al. 2003

Gemini Deep Deep Survey: Stellar Masses Glazebrook et al Nature 430, 181 (2004) Color pre-selected spectroscopic sample K<20.6, I<24.5 N=240 in 4  30 arcmin 2 fields 0.5<z<2 Surprising abundance of massive galaxies at z>1.5 Many are `red and dead’

Glazebrook et al Nature 430, 181 (2004) Gemini Deep Deep Survey: Slow Mass Assembly Growth rate slower than semi-analytic models (without AGN feedback) Rate ~independent of mass so problem for M > M  10.5 particularly acute

Census of Stellar Mass 2<z<3 van Dokkum et al 2006 Most M>10 11 M  galaxies are DRGs(77%) - LBGs constitute only 17% No single technique complete in estimating assembly history DRG LBG

Bower et al 2006, MNRAS 370, 645, “Breaking the hierarchy of galaxy formation” + Springel et al. 2005, Croton et al. 2006, Granato and collaborators Works out a model of ending star formation early by AGN heating, claiming to restore  CDM hierarchical clustering to good working order.

Sijacki & Springel, 2006 MNRAS, 366, 397

Summary Techniques are now well-established for estimating the stellar masses of galaxies to high redshift; reliability depends on having spectroscopic redshifts and long wavelength data It is now clear that mass assembly since z~2 does not proceed hierarchically; growth is suppressed in high mass systems at early times continuing in low mass systems to z~0 (`downsizing’) The mass downsizing parallels the star formation downsizing: SF is quenched above a certain threshold mass whose value declines with time AGN feedback may be able to reproduce this behavior in  CDM models, but further work is needed to understand environmental dependence of this process: are downsizing trends occurring at a different rate in clusters vs `field’? Massive galaxies are now being found at z>2 in surprising numbers; many are already passively evolving. This implies much SF activity at higher redshift