Interpreting stellar populations in a cosmological context

rachel somerville

MPIA

with thanks to the GOODS & GEMS teams, S. Faber, B. Allgood, J. Primack, A. Dekel, & R. Wechsler

Stellar populations can be used to ‘weigh’ galaxies

Bell et al. 2003 Papovich et al. 2002

Dickinson et al. 2003 (HDFN)

Fontana et al. 2004 (K20)Glazebrook et al. 2004 (GDDS); Brinchmann & Ellis 2000; Cohen et al. 2000; Rudnick et al. 2004 (FIRES); Drory et al. 2004 (MUNICS); van Dokkum, et al. 2004

stel

lar

mas

s

massive galaxies (both old/evolved and dusty/star forming) are being discovered in significant numbers at redshifts as high as z=2…

local galaxiesm*>2.5E10 Msun

m*>1.0E11 Msun

EROs

sub-mm

K20

SDSS QSOs

LBGs

Do massive galaxies at high redshift pose a crisis for CDM?

these kinds of observations couldrefute CDM, butso far they do notpose a problem.n.b. all theoristsagree on this

the overcooling problemhalo mass functioncooling+SF…+squelching…+SN FB…+ merging suppressed

in clusters

need to suppress coolingand/or star formation inmassive halos to fit z=0 stellar mass functionand luminosity functions

Glazebrook et al. 2004 Fontana et al. 2004

Stellar mass assembly history: comparison with

LCDM models

stellar mass assembly history

good agreement withobservational estimates

Glazebrook et al. (GDDS)Rudnick et al. (FIRES)Dickinson et al. (HDFN)Fontana et al. (K20)Borch et al. (COMBO-17)Somerville et al. (GOODS)

IMF=Kroupa

Tecza et al. 2003(SMG’s)

why do galaxies come in two basic types?

thin diskdynamically cold supported by rotation blue colorsstrong emission linesbroad range of stellar ages,ongoing star formation

spheroidal, dynamically hotred colorsstrong absorption lines predominantly old stars little recent star formation

Baldry et al. 2003

colo

r

blue

red

luminosity

bright faint

SDSS

galaxy colors (and manyother properties) are strongly bimodal

Baldry et al. 2003

colo

r

blue

red

luminosity

bright faint

SDSS

old, no recent star formation, high concentration/surface brightness

The two types are divided by a critical mass

young, recent star formation, low concentration/surface brightness

~3x1010 Msun

old

young

Kauffmann et al. 2003

Balogh et al. 2004

increasing density-->

decreasing luminosity-->

u-r

what is the role of environment?

(u-r)

the color of the redsequence is almostindependent ofenvironment…but the fraction ofgalaxies in the redsequence vs. theblue cloud is a strong function oflocal density

rest

U-V

col

or

rest V magnitude (luminosity)

the red sequence & color bimodality seen at z=1!

Bell et al. 2003

also seen in the DEEP2redshift survey(Willmer et al.in prep)

cluster of galaxies

‘Milky Way’ galaxy

in hierarchical models, merger history determines galaxy morphology

Color-magnitude distribution

SDSS SAM

predicted color distributions are not bimodal

-22.5 -21.5 -20.5

-19.5 -18.5black: SDSSpurple: SAM

rest

U-V

col

or

rest V magnitude (luminosity)

model prediction: color-magnitude relation at high redshift

colored pointsmeet R<24 COMBO-17selection criterion

rest

U-V

col

or

rest V magnitude (luminosity)Bell et al. 2003

red: B/T>0.5blue: B/T<0.5cyan: tmrg < 0.5 Gyr

red: E/S0blue: S/Irrcyan: merger

GEMS

models produce enough bright/massive/bulgedominated galaxies -- but they are too blue

KAB<22

13.55.83.21.00.50.1

rss et al. 2004 GOODS ApJLGOODS

not enough EROs

Bell et al 2003Results from state-of-the-artnumerical hydrodynamic simulations are very similar

Dave et al., see also Nagamine et al.

Why are red galaxies red?o CDM models produce enough old,

massive galaxies. the problem is a continuous ‘trickle’ of star formation

o there must be some process that shuts off star formation after galaxies have become massive

o this process must be rapid, and seems to be connected with the presence of a spheroid

o must work in all environments, but happen to a larger fraction of galaxies in dense places

toy models

1. remove all remaining gas after major mergers

2. shut off cooling/SF when Mh>Mcrit

3. shut off star formation when M*>Mcrit

4. shut off star formation when M*,bulge>Mcrit

toy models

1. remove all remaining gas after major mergers

– has almost no effect (fresh gas gets accreted)

2. shut off cooling/SF when Mh>Mcrit

– kills massive galaxies entirely; does not produce bimodality

o shut off star formation when M*>Mcrit

– kills massive galaxies entirely; does not produce bimodality

1. shut off star formation when M*,bulge>Mcrit

Color-magnitude distribution

SDSS SAM: SF shut off when Mh>Mcrit

Color-magnitude distribution

SDSS SAM: SF shut off when Mbulge>Mcrit

Metallicity normalization increased

by a factor of 2SDSS SAM: SF shut off when Mbulge>Mcrit

SF quenched whenMbulge>Mcrit Mr<-22.75

-21.75

-20.75

-19.75

-18.75

(purple=SAMblack=SDSS)

when do galaxies become ‘quenched’?

SF quenched when Mbulge>Mcrit

Mbulge quenched model

GEMS

dry mergers?

AGN: the missing link?

o tight observed relation between Mbulge and MBH

o energy emitted expected to be proportional to MBH

Di Matteo, Springel & Hernquist 2005

AGN feedback by momentum-driven winds

€

Lcrit =4 fgc

Gσ 4

€

M•,crit / Msun = 0.12η Edd−1 fg

0.1

⎛

⎝ ⎜

⎞

⎠ ⎟

σ

km /s

⎛

⎝ ⎜

⎞

⎠ ⎟

4

Murray, Quataert & Thompson 2004

€

L• = η Edd LEdd BH

bulge

SDSS ‘transition mass’

fg=0.1fg=0.05

observed MBH- rln

‘momentum wind’ modelcold gas ejected (and never re-accreted) if Mbulge>Mcrit()

still have a ‘cooling flow’problem!

AGN‘momentum wind’

model

-22.75

-18.75

red sequence improved, and bimodality appears in the right place, but too many intermediate luminosity blues…still have a ‘cooling flow’ problem

AGN-feedback model

too much scatter in red sequence at highredshift…formation time too late or toospread out

AGN feedback model

too much scatter in red sequence at highredshift…formation time too late or toospread out

‘Effervescent’ heating by giant radio jets

o recent work suggests even columnated jets can heat a large filling factor of ICM

o resulting bubbles look similar to those seen in Chandra images of some clusters

o Effective in cluster or perhaps group environments

Bruggen, Ruszkowski & Hallen 2005

Stellar Populations as fossil relics of star

formation10 realizations of a ‘Coma’ cluster

actual light-weighted age actual metallicity

age

from

gri

ds

Z f

rom

gri

ds

‘real’ vs. ‘grid-derived’ age and metallicity

SAM Coma

Trager et al.Coma data

Dry mergers: simulations

Bell, Naab, McIntosh, rss et al.

Dry mergers: GEMS

Dry mergers visible for ~250 Myr

every luminous E has had ~0.5-1 dry merger since z~1

in good agreement with expectations from hierarchical models

Summaryo CDM-based models of galaxy formation that produce

reasonable agreement with the z=0 stellar mass function form enough massive galaxies at high z<2

o But default models do not produce enough massive red galaxies, especially at high redshift, because of continuous low level star formation. need a new process that quenches star formation in massive, bulge-dominated galaxies

o momentum-driven winds powered by AGN a promising mechanism…another process needed to solve ‘cooling flow’ problem -- but must make enough massive galaxies at high redshift!