Galaxy Formation and Evolution _in Clusters_ _2

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Galaxy Formation and Evolution _in Clusters_ _2 Powered By Docstoc
					Galaxy Formation and Evolution (in Clusters) #2

Alice Shapley (Princeton) June 14, 15, 16th, 2006

Overview and Motivation
0. I. Introductory remarks about galaxy formation & evolution and why clusters are useful for this Galaxy evolution in clusters from z~0-1 (emphasis on early-type)

II. Galaxy evolution in general from z~0-1 III. Direct observations of cluster galaxy progenitors forming at high redshift (z ≥ 2) IV. Protoclusters at high redshift (z ≥2)

Recap…..

Different Types of Galaxies
• Galaxies observed in
different forms

• Divide by morphology, color, spectra
• E.g., morphological type: E/S0 vs. spiral • ~80% of galaxies in cores of nearby clusters are E/S0 (Dressler 1980)

Strateva et al. (2001)

• Galaxies of different types have different formation mechanisms

History of Galaxy Evolution
Initially used cluster elliptical galaxies as standard candles

• Traditionally, galaxies used to constrain cosmological model (Sandage 1961) • Cosmological tests compare measure of distance and redshift: e.g., apparent magnitude, m, of standard candle (with known M) vs. redshift

History of Galaxy Evolution
Don’t use elliptical galaxies to measure cosmic deceleration!!!!

• Not only were galaxies brighter in the past (i.e., at higher z, M was brighter), but, Tinsley (1976) pointed out uncertainties in dM/dt translate into unacceptable uncertainties in q0 (form of IMF, metallicity, starformation history) • In order to constrain q0, need to know evolutionary correction to high precision

Models of Galaxy Evolution
Of course, there is mutual uncertainty: uncertainty in evolution of galaxies hinders interpretation of cosmological tests BUT uncertainty in cosmological model hinders interpretation of galaxy evolution data
Population synthesis models tell how SED evolves with TIME -- but we observe galaxy mags, colors, spectra at different REDSHIFTS Growth of structure (e.g., the halo mass function and its evolution) depends on cosmological parameters

Models of Galaxy Evolution
Of course, there is mutual uncertainty: uncertainty in evolution of galaxies hinders interpretation of cosmological tests BUT uncertainty in cosmological model hinders interpretation of galaxy evolution data
Population synthesis models tell how SED evolves with TIME -- but we observe galaxy mags, colors, spectra at different REDSHIFTS Growth of structure (e.g., the halo mass function and its evolution) depends on cosmological parameters

Models of Galaxy Evolution
• Late 1970s, motivation for studying distant galaxies became not only for cosmological probes, but rather for understanding their history and formation • Two basic paradigms for understanding galaxy formation:

 Monolithic collapse
 Hierarchical structure formation

Monolithic Collapse
Color (bluer, metal poorer)

• Eggen, Lynden-Bell & Sandage (1962) observed that metal-poor halo stars in the Milky Way have highly elliptical orbits characteristic of system in free-fall • Metal-rich stores have more disk-like distribution and kinematics

Monolithic Collapse
• Interpretation: ~1010 years ago protogalaxy collapsed from intergalactic material, collapse was rapid (~108 years for equilibrium to be reached), big burst of star-formation, formed stars with eccentric orbits during collapse, disk stars formed later
Color (bluer, metal poorer)

Monolithic collapse “classical” formation mechanism for ellipticals and bulges, which are collections of old stars

Hierarchical Stucture Formation
• Whereas monolithic collapse works backwards from present using understanding of stellar evolution and stellar dynamics (what’s the cosmological model?), hierarchical structure formation works from within the CDM cosmological framework, provides ab initio model for galaxy formation, motivated by CMB and large-scale structure • Galaxy formation and evolution is a natural consequence of the growth of the power spectrum of fluctuations by gravitational instability, in a universe dominated by dark matter

• Model predicts evolution of dark matter halo mass function through merging and accretion
(Springel et al. 2005)

Hierarchical Stucture Formation
• While the evolution of the dark matter is now fairly well understood (gravity, cosmological model), tracing the evolution of the baryons is complicated! • Gas cooling and other hydrodynamical effects, star formation and IMF, feedback (from AGN and supernovae) • Unfortunately, it is all these messy baryonic processes that translate the population of dark matter halos into the galaxies that we observe over a range of cosmic epochs. • Big question: what’s the best way to constrain the baryonic physics of galaxy formation, now that there appears to be agreement on underlying cosmological model?

Hierarchical Stucture Formation:
A comment on the meaning of “formation”
• Possible difference between redshift at which XX% of stars formed vs. redshift at which XX% of stars were assembled into one unit
Star-formation Mass Assembly From de Lucia et al. (2005), on formation of elliptical galaxies Semi-analytic model (w/AGN feedback) grafted onto Millennium DM Simultion

Why Clusters are Useful
• Clusters useful for galaxy evolution studies because (based on
various identification techniques: X-ray, optical/red-sequence, lensing, SZ) a cluster provides a large samples of galaxies at the same redshift and relatively compact field, now to z=1.45 (Stanford et al. 2006) • Also, close proximity of galaxies with each other and ICM allows for study of environmental effects in high density environments (gravitational and hydrodynamical) • Complexities: to join in timeline, need to understand how clusters at high redshift relate to clusters at lower redshift (e.g., in terms of mass) -- also need to understand variation in cluster populations at each redshift before connecting cluster galaxies at different redshifts

I. Galaxy Evolution in Clusters from z~0-1

Evolution of E/S0 galaxies
• Collectively refer to E/S0 as early-type galaxies (may be some ambiguity in classifying each type), the type that make up ~80% of galaxy population in cores of nearby clusters. HST important for morph. Classification at higher redshift. • Evidence that stars in these galaxies formed at z>2 (evidence for passive evolution? monolithic collapse?)  Evolution of colors

 Evolution of Color-Magnitude (CM) relation
 M/LB from evolution in Fundamental Plane

Evolution of E/S0 galaxies
• Early-type galaxies in local clusters form a homogeneous class
• Color-magnitude diagram in Virgo/Coma; scatter is 0.05 mag, of which ~0.03 mag is observational error

• Physical sequence is increasing metallicity at increasing mass
• Small scatter around relation implies that stars (galaxies) formed at z>2 (Bower et al. 1992)

Evolution of E/S0 galaxies
• Early-type galaxies in local clusters form a homogeneous class
• Color-magnitude diagram in Virgo/Coma; scatter is 0.05 mag, of which ~0.03 mag is observational error

• Physical sequence is increasing metallicity at increasing mass
• Small scatter around relation implies that stars (galaxies) formed at z>2 (Bower et al. 1998)

Evolution of E/S0 galaxies
• Early-type galaxies in local clusters form a homogeneous class
• z~0 Fundamental Plane • Relationship among velocity dispersion, surface brightness, and effective radius • Implies M/LM0.24 with small scatter (20%), which also implies small age scatter at fixed mass

Re  I

1.24 0.82 e

(Jorgensen et al. 1996)

Evolution of E/S0 galaxies
• Evolution in E/S0 colors vs. z can give us some clues about early-type galaxy formation
Central ~1 Mpc •Ellis et al. (1997) look at CM relation in clusters at z~0.5 (use HST for morphological separation)

•CM-relation has same slope at z~0.5 as z~0, small scatter, which does not increase at fainter magnitudes
z=0.56 •Tight scatter at z~0.5 can be understood if bulk of sf occurred 5-6 Gyr ago z>2

Evolution of E/S0 galaxies
• Evolution in E/S0 colors vs. z can give us some clues about early-type galaxy formation
•Ellis et al. (1997) look at CM relation in clusters at z~0.5 (use HST for morphological separation)

•CM-relation has same slope at z~0.5 as z~0, small scatter, which does not increase at fainter magnitudes
(images are 10”x10” or 60x60 kpc) •Tight scatter at z~0.5 can be understood if bulk of sf occurred 5-6 Gyr ago z>2

Evolution of E/S0 galaxies
• Evolution in E/S0 colors vs. z can give us some clues about early-type galaxy formation
Coma CMD w/0 color evolution

•Ellis et al. (1997) look at CM relation in clusters at z~0.5 (use HST for morphological separation)

z=0.56

•CM-relation has same slope at z~0.5 as z~0, small scatter, which does not increase at fainter magnitudes
•Tight scatter at z~0.5 can be understood if bulk of sf occurred 5-6 Gyr ago z>2

• Evolution in E/S0 colors vs. z can give us some clues about early-type galaxy formation
slope •Stanford et al. (1998) look at clusters at 0.3<z<0.9, (again use HST for morph. •Colors get bluer consistent w/ expectations from passive evolution, roughly independent of cluster props., CMD slope does not evolve (CMD is M-Z), nor does scatter •Again, consistent with stars being formed in single episode at high redshift, relative age spread low

Evolution of E/S0 galaxies

scatter

• Evolution in E/S0 M/LBvs. z can give us some clues about early-type galaxy formation
From Treu et al. (2005)

Evolution of E/S0 galaxies

• Offset in FP 0-pt indicates difference in M/LB (see problem)

• Evolution in E/S0 M/LBvs. z can give us some clues about early-type galaxy formation
Statistics at z~1 not great!

Evolution of E/S0 galaxies

• van Dokkum & Stanford (2003) look at cluster at z=1.27, spectra for 3 galaxies, see how they relate to local fundamental plane. Offset in FP 0-pt indicates difference in M/LB • Mean star-formation age higher than z~2

• Some unresolved questions at z~1: •De Lucia et al. (2004)
construct CM relations for 4 z~0.7-0.8 EDisCS clusters, find deficit of faint red galaxies, relative to Coma, important implications for formation of fainter red galaxies • BUT Andreon et al. (2005) analyze MS 1054083 at z=0.83 and find no deficit •Interloper corrections!

Evolution of E/S0 galaxies

• Some unresolved questions at z~1: • Homeier et al. (2006)
measure CM-relation in clusters at z~0.9 (part of supercluster), and find evidence for scatter increasing at fainter magnitudes, consistent with younger ages

Evolution of E/S0 galaxies

• Some unresolved questions at z~1:

Evolution of E/S0 galaxies
• What about field E/S0 galaxies? Treu et al. (2005) find difference in M/LB evolution stronger for less massive morphologicallyselected E/S0 galaxies over redshift range z~0.3-1.2 • Note: evidence at z~0.4 that field E/S0 are younger by 20% than cluster E/S0, zform>1.5 (van Dokkum et al. 2001)

Evolution of Galaxy Mix
• While cluster E/S0 appear homogeneous, with stars formed at high redshift and passively evolving, there is evidence that cluster galaxy population mix is evolving • Multiple ways to consider this, historically, which are all correlated:

• Evolution in morphological mix (morph-dens relation)
• Evolution in mix of red/blue galaxies (Butcher/Oemler) • Evolution in spectral types of galaxies (em/abs/E+A)

Different Types of Galaxies
• Galaxies observed in
different forms

• Divide by morphology, color, spectra
• E.g., morphological type: E/S0 vs. spiral • ~80% of galaxies in cores of nearby clusters are E/S0 (Dressler 1980)

Strateva et al. (2001)

• Galaxies of different types have different formation mechanisms

Evolution of Galaxy Mix: MD
• Morphology-Density Relation: in the local universe, the fraction of E/S0 galaxies is higher in clusters than in less dense environments (Dressler 1980)

X-axis is #/Mpc2, field is <10/Mpc2

Evolution of Galaxy Mix: MD
(Dressler et al. 1997)

z~0

z~0.5

•MD relation is present at z~0.5, but is different from z~0 relation

•z~0.5 S0 fraction is lower than in clusters at z~0, while proportion of spirals is higher
•Suggests S0 galaxies in clusters may have evolved from spirals

Evolution of Galaxy Mix: MD

• Out to z~1, Smith et al. (2005) find that the E/S0 fraction increases from 0.7 to 0.9 in highest-density regions • At lowest densities (field), E/S0 fraction is constant at 0.4

• Model: early-type population at z~1 made up of E, subsequent evolution is from transformation of infalling spirals into S0 (what about low-density environments?)

Evolution of Galaxy Mix: BO
Coma z=0.02
Solid: E Hatch: S0 Clear: Sp

Cl 0024+1624 z=0.4

• Butcher & Oemler (1978) looked at two rich clusters at z~0.4. • Color distributions are strikingly different from that in Coma, in that 1/3-1/2 of galaxies in these clusters have colors of spiral galaxies

Evolution of Galaxy Mix: BO
• What is nature of blue-type galaxies? Spirals and irregulars and post-starburst galaxies (strong Balmer absorption lines) • Infall of blue, latetype galaxies from the field, which subsequently lose fuel in denser environment

Compilation by van Dokkum (2001), trend with lots of scatter

Evolution of Galaxy Mix: Spectra
z~0.5

z~0.0

• Composite spectra of five clusters at z~0.5 and z~0.0, see [OII] in z~0.5 spectra. For example in MORPHS sample of 10 z~0.4-0.5 clusters, 30% of bright cluster galaxies have emission lines (Dressler et al. 1999), BUT varies from cluster to cluster (IMPORTANT ISSUE!)

Evolution of Galaxy Mix: Spectra
• The spectrum on top is a “post-starburst” or E+A, or K+A spectrum. • The most striking feature is strong Balmer absorption lines. What does that mean?

• This spectrum is viewed as a sign that star-formation recently stopped

(Poggianti 2004)

Evolution of Galaxy Mix: Spectra

• Difficult to quantify fraction of emission-line galaxies vs. redshift and cluster properties, need larger sample of clusters • SFR above estimated from H NB imaging (Finn et al. 2004)

Selection Effects/Biases
• How were clusters selected? (optical, X-ray, SZ) Is the measured property correlated at all with the cluster selection (B-O in optically-selected clusters) • Morphological classification? How robust is this? Especially at high-z. HST is required.

• If galaxy properties depend on cluster properties, must understand scatter from cluster to cluster at any given redshift • How do you deal with interlopers?
• Dust???

Transformation mechanisms
• Consider both gravitational and hydrodynamical effects:

• Mergers and strong galaxy/galaxy interactions. Most efficient when relative galaxy velocities are lower than seen in big clusters: in groups • “Harassment” Tidal forces from high-speed close (~50kpc) encounters, effects especially important on smaller galaxies (in encounters with larger galaxies) and in clusters (1/Gyr), lead to disturbed spirals and starbursts, prolate morphology with no further star

formation (Moore et al. 1996)

Transformation mechanisms
• Consider both gravitational and hydrodynamical effects:

• Ram-pressure stripping. Interaction between galaxy and ICM. ISM of galaxy can be stripped, depends on ICM density and speed of galaxy (P~ICMv2), so only important for galaxies passing through cluster core (short timescale 107 yr)
• “Strangulation”: Removal of reservoir of gas that can cool and become available for star formation, once the galaxy enters more massive DM halo, (timescale longer timescale 109 yr), incorporated in SAM • How important is each of these? (e.g. RPS)

Transformation mechanisms
• Ram-pressure stripping. Interaction between galaxy and ICM. ISM of galaxy can be stripped, depends on ICM density and speed of galaxy (P~ICMv2), so only important for galaxies passing through cluster core, disk gas gets stripped (simulation by Quilis et al. 2000)

Progenitor Bias
• Contradictory information? Cluster E/S0 consistent with passive evolution from z~1-0. Yet, we know there is morphological/color/spectral transformation (MD, B-O effect) • “Progenitor Bias” states that progenitors of youngest E/S0 at low redshift are not classified as E/S0 at higher redshift, e.g. z~1, leads to artificially slow evolution in colors and low color scatter at high redshift, and causes an overestimate of redshift of when bulk of stars formed

• High redshift sample not fair comparison w/ low redshift

Progenitor Bias
(van Dokkum & Franx 2001)

E/S0 progenitors All progenitors

(Left) Lines shows evolution of M/LB for galaxies. Only classified as E/S0 when solid.

(Right) Solid line shows evolution in mean M/LB of E/S0, comparable to single galaxy formed at very high redshift (dashed), much faster evolution for all progenitors of z~0 E/S0 (long-dashed)

Progenitor Bias
(van Dokkum & Franx 2001)

• As M/LB compared for E/S0 at different redshifts, different sets of galaxies are included, evolution can be misinterpreted (i.e. simple passive evolution, too high redshift for star-formation stopping) • Simple, analytic model. Needs to be put in cosmological context (i.e. do numbers work out?)

Concluding Philosophical Comments
• Given many different ways of selecting clusters, must understand how observed evolution depends on cluster properties, other selection effects • Many observational questions not resolved yet (i.e. is faint end of CM-relation populated in clusters at z~0.8? How effective are various classification schemes vs. z? How does CM-scatter evolve to z~1? Need statistical sample of FP measurements at z~1.) • Many theoretical questions not resolved yet: in order to gauge importance of transformation processes, simulations must be form a realistic spiral disk ab initio, and model star-formation and feedback correctly in full cosmological context -- a tall order

Concluding Philosophical Comments
• But, now that we have cosmological framework, we can understand how mass builds up in clusters as a function of redshift, and interactions among dark matter halos • Figure out which are robust predictions for galaxy evolution from cosmological simulation, and which are more uncertain (i.e. baryons, star-formation, feedback), and what is the best way to test them • Also: place these results in a more general observational context

II. Galaxy Evolution in General from z~0-1

Global Galaxy Evolution from z~1
(averaged over all environments) • How do we understand z~0 galaxy population as the descendants of objects at z~1? (new z~1 surveys, e.g. COMBO-17, DEEP2) • Bimodality in the z~0 population • Evolution in the luminosity function/density, for red/blue, or E/S0 • Perhaps more fundamental: evolution in the stellar mass function/density, and number density of galaxies vs. mass

• Other ways of estimating the importance of mergers for the evolution in mass functions: pair counts->red-galaxy mergers

Evolution of sfr density
• One thing agreed on: the sfr density in the universe has significantly declined since z~1 (many references)

(from Bouwens et al. 2005)

Galaxy population at z~0: Bimodality
• SDSS sample of ~183,000 galaxies at z~0.0-0.2 • Blanton et al. (2003) show distributions in abs. mag, colors, surface brightness, light profile

• Bimodality in G-R color (blue/red galaxies)

Galaxy population at z~0: Bimodality

• Contours indicate densities of ~150,000 SDSS galaxies in colormagnitude space from Strateva et al. (2001)

• (Left) Spectroscopic classification: Triangles are early-type galaxies; open squares are late-type
• (Right) Morphological classification: Triangles are early-type; open squares are late-type

Galaxy population at z~0: Luminosity and Mass Functions
• Polo reviewed LF • Cole et al. (2001) construct stellar mass function from K-band luminosity function of 2dF/2MASS galaxies and population synthesis models of opt/IR colors, >1000 sq degrees

Find: stars=6x108M/Mpc3
Local benchmark stars=0.004

Galaxy population at z~1
• Bell et al. (2004) use ~25,000 COMBO-17 galaxies out to z~1, covering 0.8 sq. degrees, and photometric redshifts • high redshift galaxies show CMD • look at evolution of B-band LF of redsequence galaxies (i.e. early-type, E/S0)

Galaxy population at z~1
• Find B-band luminosity density of red-sequence galaxies is constant out to z~0.8 • If z~0 red-sequence galaxies had all formed at higher redshift and evolved passively to z~0, expected luminosity would have been factor of ~2 higher • M/L lower at z~1, so same luminosity means less mass

Galaxy population at z~1
• Where does this increase in stellar mass for luminous red galaxies increase come from? • Dissipationless merging, truncation of SF in some fraction of blue population • Note: there aren’t enough luminous blue galaxies that can fade to become luminous red galaxies • Big uncertainty: COSMIC VARIANCE!

Galaxy population at z~1
• DEEP2 redshift survey finds the same thing (Faber et al. 2006) • Cimatti et al. (2006) find different result: less evolution in mass density of massive red galaxies (mass dependent evolution)

Galaxy population at z~1
DEEP2 SDSS shifted to z~1

• Another way of looking at it: Blanton (2005) takes the SDSS sample at z~0.1 and “observes” it at z~1, assuming no evolution, just k-corrections. Then he compares the colors and mags with those of the DEEP2 z~1 survey.

Galaxy population at z~1

• In 28’x28’ GEMS area (deep HST imaging), Bell et al. (2005) find pairs
of red galaxies (criteria for calling it a red merger), luminosity ratios <4:1, estimate timescale over which it would be identified as such and translate counts-> merger rate, and # of major mergers experienced by red galaxy since z~0.7

Galaxy population at z~1

• Conclusion: luminous (MV<-20.5) present-day early-type galaxy
experiences 0.5 to 2.0 such major mergers since z~0.7, van Dokkum (2005) finds similar result
• Is this consistent with other observations? Models? Uncertainty in merger timescale important!

Evolution of stellar masses
• Luminosity density and pair counts are both methods of looking at evolution, but there is a more direct method • Distribution of stellar masses (of different types of galaxies) as a function of redshift; total stellar mass density as a function of redshift; abundance of objects of a given stellar mass • (stellar mass of an object can only increase, unlike luminosity in a given band) • With the advent of wide-field and deep K-band imaging, estimates of stellar masses for high redshift galaxies becomes possible

Evolution of stellar masses
• Drory et al. (2005) find that ~50% of z~0 stellar mass density in place at z~1, ~25% at z~2 • They estimate stellar mass density by constructing stellar mass function • Compare stellar mass evolution with integral of sfr(z)

Evolution of stellar masses
• Drory et al. (2005) find that ~50% of z~0 stellar mass density in place at z~1, ~25% at z~2 • How does it compare wth integral of sfrhistory? • How do you make that plot? How might points at high-z need to be adjusted?

Evolution of stellar masses
• Bundy et al. (2006)
analyze >8000 DEEP2 data with K-band observations over 1.5 sq. degrees, model stellar mass functions of red/blue galaxies vs. redshift • Largest set of z~1 galaxies with stellar masses and redshifts, so they can divide up sample (larger area and fainter limit than Drory et al.)

Evolution of stellar masses
• Similar result, but
emphasize lack of significant evolution of total mass function • Also, favor truncation of star-formation in blue galaxies, rather than dry mergers, for explaining evolution of red galaxy mass function •Furthermore, find more massive red galaxies assembled first

Hierarchical Stucture Formation
• Prediction by de Lucia model: more massive elliptical galaxies assembled later than less massive ones
From de Lucia et al. (2005), on formation of elliptical galaxies Semi-analytic model (w/AGN feedback) grafted onto Millennium DM Simultion

Concluding Philosophical Comments
• There is not consensus about the evolution of stellar mass density and number of red (E/S0) galaxies as a function of mass: the importance of dissipationless (stellar only, “dry”) mergers vs. truncation of starformation in blue galaxies -- even for people analyzing the same datasets • These are important quantities to pin down observationally if we are going to constrain theories of galaxy formation • Note: we didn’t talk about the evolution of blue/disk galaxies, or about metals, or about clustering