The role of environment on galaxy evolution Michael Balogh University of Durham University of Waterloo (Canada)
Download ReportTranscript The role of environment on galaxy evolution Michael Balogh University of Durham University of Waterloo (Canada)
The role of environment on galaxy evolution Michael Balogh University of Durham University of Waterloo (Canada) Collaborators Richard Bower , Simon Morris, Dave Wilman No picture: Vince Eke, Cedric Lacey, Fumiaki Nakata Durham John Mulchaey & Gus Oemler OCIW Bob Nichol, Chris Miller & Alex Gray Carnegie Mellon Baugh, Cole, Frenk (Durham) Ivan Baldry & Karl Glazebrook Johns Hopkins No picture: Taddy Kodama Ian Lewis (Oxford) and the 2dFGRS team Ray Carlberg Toronto Outline 1. Background 2. Low redshift: SDSS and 2dFGRS 3. GALFORM predictions 4. Groups and clusters at z~0.5 5. Conclusions Outline 1. Background 2. Low redshift: SDSS and 2dFGRS 3. GALFORM predictions 4. Groups and clusters at z~0.5 5. Conclusions Morphology-Density Relation Field Clusters E S0 Spirals Dressler 1980 Morphology-Density Relation The “Outskirts” of clusters Clusters Field Where does the transition begin, and what causes it? E S0 Spirals Dressler 1980 Nature or Nurture? • Nature? Elliptical galaxies only form in protoclusters at high redshift. Rest of population is due to infall. • or Nurture? Galaxy evolution proceeds along a different path within dense environments. Theoretical expectations (?) • Isolated galaxies have (invisible) halo of hot gas that can cool and replenish the disk – allows star formation to continue for longer • Cluster galaxies lose this gas, so their SFR declines more quickly. Also cluster galaxies form earlier. – therefore SFRs should be lower in clusters • no ram-pressure stripping, harassment needed to achieve reasonable match to observed clusters (Diaferio et al. 2001; Okamoto et al. 2003). Butcher-Oemler effect • Concentrated clusters at high redshift have more blue galaxies than concentrated clusters at low redshift Butcher & Oemler (1984) Butcher-Oemler effect • A lot of scatter – appears to be mostly due to correlation with cluster richness • still room to worry about cluster selection? Margoniner et al. (2001) Butcher-Oemler effect SDSS: Goto et al. (2003) • Many of blue galaxies turned out to have poststarburst spectra (Dressler & Gunn 1992; Couch & Sharples 1987) • Suggested nurture: – ram-pressure stripping (Gunn & Gott 1972) – tidal effects (Byrd & Valtonen 1990) – harassment? (Moore et al. 1999) But: Field galaxy evolution • But field population also evolves strongly (Lilly et al. 1996) • Post-starburst galaxies equally abundant in the field (Zabludoff et al. 1996; Goto et al. 2003) • So: does BO effect really point to cluster-specific physics, or just the evolving field and infall rate (Ellingson et al. 2001)? Steidel et al. (1999) Observations: z~0.3 • Strangulation model: – infall rate + assumed decay rate of star formation => radial gradient in SFR Balogh, Navarro & Morris (2000) • Radial gradients in CNOC clusters suggest t ~2 Gyr • higher rate of infall at high redshift leads to steeper gradients Ellingson et al. (2001) Some remaining questions • Where does the morphology-density relation start? i.e. what environment drives transformations? • What is the timescale for this transformation? e.g. Strangulation (slow) or harassment (fast) • How does SF evolve with time in different environments? Can environment-related transformations drive the Madau plot? Outline 1. Background 2. Low redshift: SDSS and 2dFGRS 3. GALFORM predictions 4. Groups and clusters at z~0.5 5. Conclusions 2dFGRS and SDSS • 2dFGRS: 250k redshifts, photographic plates • SDSS (DR1): 200k redshifts, digital ugriz imaging • morphology is difficult to quantify – Especially to distinguish E from S0 – use colours and Ha equivalent widths as tracer of SFR • density: – projected distance to 5th nearest neighbour – 3D density based on convolution with Gaussian kernel – cluster velocity dispersion Median SFR-Density relation R>2R200 Clusters Field Field critical density? 2dFGRS: Lewis et al. 2002 SDSS: Gomez et al. 2003 Ha distribution • Ha distribution shows a bimodality: mean/median of whole distribution can be misleading Balogh et al. 2004 Ha distribution • Ha distribution shows a bimodality: mean/median of whole distribution can be misleading • Isolate star-forming with W(Ha)>4 Å galaxies Balogh et al. 2004 The star-forming population • Amongst the starforming population, there is no trend in Ha distribution with density • Hard to explain with simple, slow-decay models (e.g. Balogh et al. 2000) Correlation with density • The fraction of starforming galaxies varies strongly with density 2dFGRS Correlation with density 2dFGRS • The fraction of starforming galaxies varies strongly with density • Correlation at all densities; still a flattening near the critical value • Fraction never reaches 100%, even at lowest densities Isolated Galaxies All galaxies Bright galaxies • Selection of isolated galaxies: – non-group members, with low densities on 1 and 5.5 Mpc scales • ~30% of isolated galaxies show negligible SF – environment must not be only driver of evolution. Large scale structure Contours are lines of constant emission-line fraction r5.5 (Mpc-3) 0.050 • Emission-line fraction appears to depend on 1 Mpc scales and on 5.5 Mpc scales. 0.010 0.005 Increasing fraction of Ha emitters r1.1 (Mpc-3) 2dFGRS data. Similar results for SDSS Colours: SDSS Balogh, Baldry, Nichol, Miller, Bower & Glazebrook, submitted to ApJ Letters Colour-magnitude relation Sloan DSS data Baldry et al. 2003 (see also Hogg et al. 2003) Blue Fraction Margoniner et al. 2000 De Propris et al. 2004 (2dFGRS) Analysis of colours in SDSS data: Bright • 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 Faint (u-r) Baldry et al. 2003 • 24346 galaxies from SDSS DR1. magnitude limited with z<0.08 • density estimates based on Mr<-20 Balogh, Baldry, Nichol, Miller, Bower & Glazebrook, submitted to ApJ Letters • Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution. • Density-dependence stronger than luminosity. Bright and faint galaxies show trend with density • Use cluster catalogue of Miller, Nichol et al. (C4 algorithm) • No strong dependence on cluster velocity dispersion observed. Local density is the main driver • Mean colour of distribution is only a weak function of density, but depends strongly on luminosity. Separates internal/external influences • trend may mean galaxies in lowdensity regions have more recent star formation, on average • but not likely related to large population of red galaxies in clusters • How rapid must the bluered transition be? Red Peak • colour evolves rapidly if timescale for star formation to stop is short • if transformations occur uniformly in time: • need t<0.5 Gyr Blue Peak • if transformations are more common in the past, longer timescales permitted Summary: SDSS & 2dFGRS • Colour and Ha distributions suggest any transformations must have a short timescale • SFH depends on environment and galaxy luminosity (mass) in a separable way. Outline 1. Background 2. Low redshift: SDSS and 2dFGRS 3. GALFORM predictions 4. Groups and clusters at z~0.5 5. Conclusions GALFORM model • GALFORM is Durham model of galaxy formation (Cole et al. 2000) – parameters fixed to reproduce global properties of galaxies at z=0 (e.g. luminosity function) and abundance of SCUBA galaxies at high redshift • Use mock catalogues of 2dFGRS which include all selection biasses • Predict Ha from Lyman continuum photons, choose dust model to match observed Ha distribution • Assume hot gas is stripped from galaxies when they merge with larger halo (i.e. groups and clusters) which leads to strangulation of SFR (gradual decline) GALFORM predictions 1. Fraction of SF galaxies declines with increasing density as in data GALFORM predictions • Over most of the density range, correlation between stellar mass and SFR fraction is invariant Therefore SFR-density correlation is due to massdensity correlation • At highest densities, models predict fewer SF galaxies at fixed mass due to strangulation GALFORM predictions S5<0.2 Mpc-2 S5<0.2 Mpc-2 Observed Ha distribution independent of environment at all densities GALFORM predictions 1. 2. Fraction of SF galaxies declines with increasing density as in data At low densities, Ha distribution independent of environment GALFORM predictions 1. 2. Fraction of SF galaxies declines with increasing density as in data At low densities, Ha distribution independent of environment GALFORM predictions 1. 2. 3. Fraction of SF galaxies declines with increasing density as in data At low densities, Ha distribution independent of environment In densest environments, Ha distribution skewed toward low values GALFORM predictions: LSS Data r5.5 (Mpc-3) r5.5 (Mpc-3) Model r1.1 (Mpc-3) GALFORM predictions: LSS Data r5.5 (Mpc-3) r5.5 (Mpc-3) Model r1.1 (Mpc-3) GALFORM predictions • At low densities, models work very well - At higher densities, data require more rapid transition than predicted • Fraction of star-forming galaxies depends primarily on local density, but there is a further weak correlation with large scales - Not expected in CDM models because halo merger history depends only on local environment (Lemson & Kauffmann 1999) - Should be independently confirmed but suggests an important element missing from these models Outline 1. Background 2. Low redshift: SDSS and 2dFGRS 3. GALFORM predictions 4. Groups and clusters at z~0.5 5. Conclusions Evolution - expectations To (over)simplify the issue: • Nature: Evolution in groups, clusters should parallel evolution in the field • Nurture: SFR depends only on environment – so no evolution with redshift Cluster galaxy evolution 2dF CNOC CNOC clusters: Evolution in SFR different from colour? Suggests high blue fraction at high redshift due to increased infall rate Nakata et al. in prep Cluster galaxy evolution z~0.3 z~0.5 Field Field Tresse et al. 2002 Couch et al. 2001 Balogh et al. 2002 Fujita et al. 2003 Complete Ha studies: Even at z=0.5, total SFR in clusters lower than in surrounding field Kodama et al. in prep Cluster galaxy evolution Finn Finn et et al. al. 2003 2003 • Complete Ha based SFR estimates • Evolution in total SFR per cluster not well constrained • considerable scatter of unknown origin • systematic uncertainties in mass estimates make scaling uncertain Kodama et al. in prep Cluster galaxy evolution Finn et al. 2003 • Complete Ha based SFR estimates • Evolution in total SFR per cluster not well constrained • considerable scatter of unknown origin • systematic uncertainties in mass estimates make scaling uncertain Kodama et al. in prep Evolution in groups z~0.05: 2dFGRS (Eke et al. 2004) – Based on friends-of-friends linking algorithm – calibrated with simulations. Reproduces mean characteristics (e.g. velocity dispersion) of parent dark matter haloes z~0.45: CNOC2 (Carlberg et al. 2001) – selected from redshift survey, 0.3<z<0.55 – Cycle 12 HST imaging + deeper spectroscopy with LDSS2-Magellan Fraction of non-SF galaxies Group comparison • 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 Wilman et al. in prep Group Evolution Fraction of non-SF galaxies • Fraction of non-SF galaxies increases with redshift Wilman et al. in prep Field Evolution Fraction of non-SF galaxies • Fraction of non-SF galaxies increases with redshift • for both groups and field Wilman et al. in prep Outline 1. Background 2. Low redshift: SDSS and 2dFGRS 3. GALFORM predictions 4. Groups and clusters at z~0.5 5. Conclusions Conclusions: What have we learned? From the local colour/SFR distribution: – transformations must either be rapid, or occur preferentially at high redshift – simple strangulation model does not work – SFR depends weakly on large-scale structure From comparison with clusters/groups at higher redshift: – evolution may be stronger in groups/field than in clusters The End • shape of [OII] distribution evolves with redshift but does not depend on environment Wilman et al. in prep • CNOC2 field lacks significant population of galaxies without star formation • [OII] distribution in groups looks similar at both redshifts, with some evolution Wilman et al. 2004 GALFORM predictions Kauffmann et al. (2004) work with SDSS suggests correlation between SFR and stellar mass depends on environment. However this is not directly comparable in this form.