The influence of environment on galaxy populations Michael Balogh University of Waterloo, Canada

Outline Low redshift –Simple trends encompass most of what we know of as environmental influences Models: what works and what doesn’t Redshift evolution The future: what’s next?

The influence of environment on galaxy populations Populations Current star formation rate Recent star formation Stellar mass (average SFR) Morphology (of stars, neutral gas, ionized gas) AGN Gas content Environment Mass of dark matter halo Position within halo Local density Large-scale density

The influence of environment on galaxy populations Nature vs. nurture? Entangled in current models –Gas accretion, merger, and feedback history scale with halo mass. –No longer the right question? A better question: what physics operates in haloes of a given mass, at a given epoch? –Today’s population is the result of different environments at different epochs: cannot try to isolate one mechanism as responsible for the observed trends.

The local Universe

Colour-magnitude distribution Nearby galaxies seem to fall into two surprisingly well- defined, smoothly varying distributions. Colour, luminosity, concentration, star formation rate Blanton et al. 2004

Colour-magnitude distribution Colour distribution in 0.5 mag bins can be fit with two Gaussians Mean and dispersion of each distribution depends strongly on luminosity Dispersion includes variation in dust, metallicity, SF history, and photometric errors At bright magnitudes, significant fraction of “blue” population “contaminates” red: c.f. talk by Wolf. (u-r) Bright Faint Baldry et al. 2003

Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution. Mean colours depend weakly on environment: transitions between two populations must be rapid (or rare at the present day) Balogh et al. 2004

Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution. Mean colours depend weakly on environment: transitions between two populations must be rapid (or rare at the present day) Trend is not completely absent for fainter galaxies; but never dominant Balogh et al. 2004

The star-forming population Rines et al. 2005: H  distribution in virial, infall and field regions nearly identical. Carter et al. (2001) –3150 nearby galaxies H  for SF galaxies does not depend on environment –Triggering of SF occurs on small spatial scales Hard to explain with simple, slow-decay models (e.g. Balogh et al. 2000)

Colour and environment Bright, red galaxies: luminosity strongly correlated with environment Remainder: average density increases with colour. –Trend driven by galaxies between the two peaks – consistent with statement that blue peak colour is independent of environment, while red fraction varies strongly. Blanton et al Contours: Galaxy numbers Contours: Local density

SFR-colour Recent SDSS analysis: split by colour and SFR Environment: halo mass –Use luminosity as tracer of mass. Compare with theoretical mass function Weinmann et al Log (SFR/M) (yr -1 ) 0.1 (g-r)

Halo mass dependence Environment: halo mass –Use luminosity as tracer of mass. Compare with theoretical mass function At fixed mass the late-fraction depends weakly on luminosity Late-type fraction depends most strongly on halo mass Weinmann et al [-21,-22] [-22,-23] [-20,-21] [-19,-20] [-18,-19] R luminosity

Halo mass dependence colour SFR concentration Average properties of galaxies in either peak is independent of halo mass –But depends on luminosity [-21,-22] [-22,-23] [-20,-21] [-19,-20] [-18,-19] R luminosity Weinmann et al. 2005

Local effects? Still a (weak) trend with radius in haloes of fixed mass Dependence on luminosity (surprisingly?) weak Weinmann et al <M< <M<10 14

Conformity Properties of “satellite” galaxies appear to be connected with properties of “central” (actually brightest) galaxy Weinmann et al Similar to effect seen in 2PIGG groups? See Vince Eke’s talk. Definition of central?

Implications Simple dependence of “late-type” fraction on environment characterizes much of observed trends (e.g. SFR-density, morphology-density, colour-density etc.). Interpretation? 1.Two modes of formation. Within each peak is variance due to dust, metallicity (second-order effects). 2.Transitions: Where do S0, E+A fit in? 3.Burst vs. continuous SFR (Kauffmann et al. 2005)

Signs of Nurture: Virgo spirals Kenney et al Vollmer et al Ram-pressure stripping in Virgo H  for Virgo galaxy H  for normal galaxy Truncated H  disks in clusters Koopmann & Kenney 2004 also: Vogt et al. 2004

Signs of Nurture: morphology and SFR Passive Spirals E+A galaxies? S0, dSph, UCDs Wolf’s dusty spirals? Peak in infall region? e.g. Christlein & Zabludoff (2005) –Residual [OII] after subtracting expectation for given B/T, D4000 and Mstar. SFR gradient is not entirely: –Consequence of MDR –Consequence of change in mass function –Effect of initial conditions

HI gas Springob et al. 2005: HIMF in dense regions flatter? May suggest smaller galaxies more HI deficient HI deficiency in 18 nearby clusters: Solanes et al High-density Low-density

AGN AGN fraction independent of density –Surprising? Carter et al. (2001) Miller et al. (2003)

Models

Semi-analytic approach Trace merger histories with N-body simulations (cannot use Press-Schechter because you need to know where the galaxies are) More massive haloes form earlier: longer merger history. –There is also a larger-scale bias: haloes of a given mass form earlier in denser environments (Sheth & Tormen 2004; Abbas & Sheth 2005; Harker et al. 2005) Make simple assumptions about gas accretion (e.g. no accretion onto satellites) and feedback (supernova, AGN)

General trends: successes Springel et al. 2001: morphology-density relation Okamoto & Nagashima (2003) SFR-radius Diaferio et al. (2001) colour- radius R/R200

Bimodality? Springel et al. 2001; Diaferio et al –Bimodality in field not clear –All cluster galaxies are red Okamoto & Nagashima 2003 –SFR is suppressed in all galaxies: blue peak is distorted M V -logh M V -logh Cole et al Supernova feedback prescription does not produce bimodal colour distribution at faint magnitudes. Spirals Ellipticals All cluster Data Model

SPH simulations Keres et al. (2005): SPH simulations reproduce trend of decreasing SFR with increasing density (see also Berlind et al. 2004). Confirm this is due to reduced accretion of hot gas But colour-distribution of galaxies doesn’t look quite right… SFR Hot accretion Cold accretion SPH Observe d

Contours: Galaxy numbers Contours: Local density SPH simulations Berlind et al. –Qualitative agreement of environment-age (colour?) trends –Central galaxy mass (luminosity?) correlates with halo mass –Satellite galaxies: age (colour) associated with mean accretion time But colour distribution is still wrong (unimodal) Observations: Blanton et al. (2004)

Improving the colour distribution Springel, Di Matteo & Hernquist (2005) –Including black hole feedback terminates star formation more quickly. Leads to rapid reddening of merger remnants Sijacki & Springel 2005 AGN feedback removes young population in cD galaxies No feedback Magorrian-AGN BH accretion rate

Improving the colour distribution Croton et al. (2005) Radio-feedback most efficient in large groups. Proportional to M gas ×M BH Cooling rate (Msun/yr)

Models: summary When feedback parameters are tuned to reproduce the field luminosity function and colour distribution, what will we find as a function of environment? –General trends will be reproduced. But will it be for the right reasons? –Any differences in detail: will they signify “nurture” processes? Or just that feedback parameters need further tuning?

Back to observations: Evolution

Evolution: clusters (briefly) Morphology-density relation (see talks by Postman, Dressler) –Fewer S0 in z=1 clusters, but non-zero –Little evolution in MDR z=1 to z=0.5 –Suggests high-z MDR is primordial, with z<0.5 environment-driven evolution SFR and colour gradients –Radial gradients steeper in the past (Ellingson et al. 2001; Kodama & Bower 2001) –Can be related to truncation of star formation in an infalling field population

Clusters Tanaka et al (see poster) – tight CMR in place in clusters to z=0.8 –Faint end of CMR in groups formed z~0.5 –No CMR in field at z=0.8 –Also De Lucia (2004): faint end of red sequence disappears at z>0.5

Clusters Field 2dF Nakata et al Postman, Lubin & Oke 2001 van Dokkum et al Fisher et al Czoske et al Cluster galaxy evolution Supported by observed evolution in [OII]- emission fraction (Nakata et al. 2005) –Field evolves much more strongly than clusters (for bright galaxies)

Cluster galaxy evolution Complete H  studies: emission line fraction depends more strongly on cluster mass than on redshift. Finn et al

Evolution: photo-z surveys Similar rate of increase in red fraction in the field and clusters –average field red sequence galaxy came into the sample later All galaxies CFHTLS: Nuijten et al. (2005) Red galaxy fraction M V < -20 High density Low density Redshift Red galaxy fraction COMBO-17: E. Bell et al.

Luminosity, density and redshift dependence of red fraction RCS z>0: Yee et al. (2005)SDSS z=0: Balogh et al. (2004)

Luminosity, density and redshift dependence of colour RCS z>0: Yee et al. (2005) SDSS z=0: Balogh et al. (2004)

Luminosity, density and redshift dependence of colour Little evolution in red peak colour RCS z>0: Yee et al. (2005) SDSS z=0: Balogh et al. (2004)

Luminosity, density and redshift dependence of colour Little evolution in red peak colour Colours of bright blue galaxies evolve strongly RCS z>0: Yee et al. (2005) SDSS z=0: Balogh et al. (2004)

Galaxy groups at z=0.4 Selected from CNOC2 survey >30 nights Magellan spectroscopy (better completeness, depth) ACS image of ~30 groups GALEX data rolling in slowly Spitzer (IRAC and shallow MIPS) data from GTO programs Collaborators: Dave Wilman (MPE), Richard Bower (Durham), Gus Oemler, John Mulchaey (Carnegie), Ray Carlberg (Toronto)

Groups at z=0.4: Morphologies Spiral-dominated group  =270 km/s E/S0-dominated group  =226 km/s

Morphologies: early results There are fewer spiral galaxies in groups than in the field, at the same redshift. No evidence for more disturbance/irregularities in group galaxies Field Spiral fraction E/S0 fraction Groups Field Spiral fraction Vel. Dispersion (km/s)

The connection between star formation rate, morphology and environment Like clusters, groups contain passive spirals: disk morphology but low star formation rates Field Groups Elliptical Early spiral Late spiral S0 Distributions are corrected for differences in luminosity function between group and field

Stellar mass-SFR Stellar masses from archival Spitzer (IRAC) data Significant star formation seen in more massive galaxies than locally: downsizing? No significant difference between group and field for this subsample. Rosati? z=1 SDSS (Kauffmann et al.)

Evolution in groups Wilman et al. (2004) Fraction of non-SF galaxies Use [OII] equivalent width to find fraction of galaxies without significant star formation most galaxies in groups at z~0.4 have significant star formation – in contrast with local groups cf. Gonzalez talk: supergroup

Wilman et al Fraction of non-SF galaxies increases with redshift for both groups and field Insensitive to aperture effects Evolution cannot be account for by passive-evolution models. Require truncation of star formation (both groups and field) Fraction of non-SF galaxies Groups Group SFR evolution Fraction of non-SF galaxies Field

Clusters Field 2dF Nakata et al Postman, Lubin & Oke 2001 van Dokkum et al Fisher et al Czoske et al Group Evolution Groups: Wilman et al. (2005)

Group SFR evolution Wilman et al shape of [OII] distribution evolves with redshift but does not depend on environment Result sensitive to aperture effects

High redshift Spectroscopic survey: ~100 redshifts 1.48<z<2.89 Overdense region has more massive, older galaxies Consistent with expectations for earlier formation time (1600 Myr vs 800 Myr) Steidel et al. (2005)

High redshift UV-selected LBG survey No environmental dependence of SFR Can be consistent: cluster galaxies get head start, but instantaneous SFR the same Even at z=0 it seems star-forming galaxies have a distribution independent of environment Bouché & Lowenthal (2005)

The future Theory: still has a lot of catching up to do –Thus we are in discovery mode rather than testing mode Observations: –Dust-obscured SF (Spitzer, Herschel) –AGN/SF connection at z>0 –Lower luminosities –Spatial dependence of SFR (i.e. IFU spectroscopy) –Transitional galaxies