C. P. Haines

The different physical mechanisms that drive the star formation histories of giant and dwarf galaxies

We present an analysis of star formation and nuclear activity in galaxies as a function of both luminosity and environment in the fourth data release of the Sloan Digital Sky Survey. Using a sample of 27 753 galaxies in the redshift range 0.005 < z < 0.037 that is ≥90 per cent complete to M r = -18.0, we find that the Ha equivalent width, EW(Hα), distribution is strongly bimodal, allowing galaxies to be robustly separated into passively evolving and star-forming populations about a value EW(Ha) = 2 A. In high-density regions ∼70 per cent of galaxies are passively evolving independent of luminosity. In the rarefied field, however, the fraction of passively evolving galaxies is a strong function of luminosity, dropping from 50 per cent for M r ≤ -21 to zero by M r ∼ -18. Indeed for the lowest luminosity range covered (-18 < M r < -16) none of the ∼600 galaxies in the lowest-density quartile is passively evolving. The few passively evolving dwarf galaxies in field regions appear as satellites to bright (≥ L*) galaxies. We find a systematic reduction of ∼30 per cent in the Ha emission from dwarf (-19 < M r < - 18) star-forming galaxies in high-density regions with respect to field values, implying that the bulk of star-forming dwarf galaxies in groups and clusters are currently in the process of being slowly transformed into passive galaxies. The fraction of galaxies with the optical signatures of an active galactic nucleus (AGN) decreases steadily from ∼50 per cent at M r ∼ -21 to ∼0 per cent by M r ∼ -18 closely mirroring the luminosity dependence of the passive galaxy fraction in low-density environments. This result reflects the increasing importance of AGN feedback with galaxy mass for their evolution, such that the star formation histories of massive galaxies are primarily determined by their past merger history. In contrast, the complete absence of passively evolving dwarf galaxies more than ∼2 virial radii from the nearest massive halo (i.e. cluster, group or massive galaxy) indicates that internal processes, such as merging, AGN feedback or gas consumption through star formation, are not responsible for terminating star formation in dwarf galaxies. Instead the evolution of dwarf galaxies is primarily driven by the mass of their host halo, probably through the combined effects of tidal forces and ram-pressure stripping.

Shapley Optical Survey – II. The effect of environment on the colour–magnitude relation and galaxy colours

We present an analysis of the effects of environment on the photometric properties of galaxies in the core of the Shapley supercluster at z = 0.05, one of the most massive structures in the local universe. The Shapley Optical Survey (SOS) comprises archive Wide Field Imager (WFI) optical imaging of a 2.0deg 2 region containing the rich clusters A3556, A3558 and A3562 which demonstrate a highly complex dynamical situation including ongoing cluster mergers. The B - R/R colour-magnitude relation has an intrinsic dispersion of 0.045 mag and is 0.015 ± 0.005 mag redder in the highest-density regions, indicative of the red sequence galaxy population being 500-Myr older in the cluster cores than towards the virial radius. The B - R colours of galaxies are dependent on their environment, whereas their luminosities are independent of the local density, except for the very brightest galaxies (M R < -22). The global colours of faint (?M* + 2) galaxies change from the cluster cores where ∼90 per cent of galaxies lie along the cluster red sequence to the virial radius, where the fraction has dropped to just ∼20 per cent. This suggests that processes directly related to the supercluster environment are responsible for transforming faint galaxies, rather than galaxy merging, which should be infrequent in any of the regions studied here. The largest concentrations of faint blue galaxies are found between the clusters, coincident with regions containing high fractions of ∼L* galaxies with radio emission indicating starbursts. Their location suggests star formation triggered by cluster mergers, in particular the merger of A3562 and the poor cluster SC 1329-313, although they may also represent recent arrivals in the supercluster core complex. The effect of the A3562-SC 1329-313 merger is also apparent as a displacement in the spatial distribution of the faint galaxy population from both the centres of X-ray emission and the brightest cluster galaxies for both systems. The cores of each of the clusters/groups are marked by regions that have the lowest blue galaxy fractions and reddest mean galaxy colours over the whole supercluster region, confirming that star formation rates are lowest in the cluster cores. In the cases of A3562 and SC 1329-313, these regions coincide with the centres of X-ray emission rather than the peaks in the local surface density, indicating that ram-pressure stripping may have an important role in terminating any remnant star formation in galaxies that encounter the dense intracluster medium (ICM) of the cluster cores.

LOCUSS: THE MID-INFRARED BUTCHER-OEMLER EFFECT

We study the mid-infrared (MIR) properties of galaxies in 30 massive galaxy clusters at 0.02 ≤ z ≤ 0.40, using panoramic Spitzer/MIPS 24 μm and near-infrared data, including 27 new observations from the LoCuSS and ACCESS surveys. This is the largest sample of clusters to date with such high-quality and uniform MIR data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the so-called Butcher-Oemler (BO) effect, measuring the fraction of massive infrared luminous galaxies (K 5 × 10^(10) L_☉) within r_(200), finding a steady increase in the fraction with redshift from ~3% at z = 0.02 to ~10% by z = 0.30, and an rms cluster-to-cluster scatter about this trend of 0.03. The best-fit redshift evolution model of the form f_(SF) ∝ (1 + z)^n has n = 5.7^(+2.1)_(–1.8), which is stronger redshift evolution than that of L*_(IR) in both clusters and the field. We find that, statistically, this excess is associated with galaxies found at large cluster-centric radii, specifically r_(500) < r < r_(200), implying that the MIR BO effect can be explained by a combination of both the global decline in star formation in the universe since z ~ 1 and enhanced star formation in the infall regions of clusters at intermediate redshifts. This picture is supported by a simple infall model based on the Millennium Simulation semianalytic galaxy catalogs, whereby star formation in infalling galaxies is instantaneously quenched upon their first passage through the cluster, in that the observed radial trends of f_(SF) trace those inferred from the simulations. The observed f SF values, however, lie systematically above the predictions, suggesting an overall excess of star formation, either due to triggering by environmental processes, or a gradual quenching. We also find that f SF does not depend on simple indicators of the dynamical state of clusters, including the offset between the brightest cluster galaxy and the peak of the X-ray emission. This is consistent with the picture described above in that most new star formation in clusters occurs in the infall regions, and is thus not sensitive to the details of cluster-cluster mergers in the core regions.

LoCuSS: Connecting the Dominance and Shape of Brightest Cluster Galaxies with the Assembly History of Massive Clusters

We study the luminosity gap, ∆m_(12), between the first- and second-ranked galaxies in a sample of 59 massive (~10^(15)M_⊙) galaxy clusters, using data from the Hale Telescope, the Hubble Space Telescope, Chandra and Spitzer. We find that the ∆m_(12) distribution, p(∆m_(12)), is a declining function of ∆m_(12) to which we fitted a straight line: p(∆m_(12))∝−(0.13 ± 0.02)∆m_(12). The fraction of clusters with ‘large’ luminosity gaps is p(∆m_(12) ≥ 1) = 0.37 ± 0.08, which represents a 3σ excess over that obtained from MonteCarlo simulations of a Schechter function that matches the mean cluster galaxy luminosity function. We also identify four clusters with ‘extreme’ luminosity gaps, ∆m_(12) ≥ 2, giving a fraction of p(∆m_(12) ≥ 2) = 0.07^(+0.05)_(−0.03). More generally, large luminosity gap clusters are relatively homogeneous, with elliptical/discy brightest cluster galaxies (BCGs), cuspy gas density profiles (i.e. strong cool cores), high concentrations and low substructure fractions. In contrast, small luminosity gap clusters are heterogeneous, spanning the full range of boxy/elliptical/discy BCG morphologies, the full range of cool core strengths and dark matter concentrations, and have large substructure fractions. Taken together, these results imply that the amplitude of the luminosity gap is a function of both the formation epoch and the recent infall history of the cluster. ‘BCG dominance’ is therefore a phase that a cluster may evolve through and is not an evolutionary ‘cul-de-sac’. We also compare our results with semi-analytic model predictions based on the Millennium Simulation. None of the models is able to reproduce all of the observational results on ∆m_(12), underlining the inability of the current generation of models to match the empirical properties of BCGs. We identify the strength of active galactic nucleus feedback and the efficiency with which cluster galaxies are replenished after they merge with the BCG in each model as possible causes of these discrepancies.

LoCuSS: The slow quenching of star formation in cluster galaxies and the need for pre-processing

C.P.H. was funded by CONICYT Anillo project ACT-1122. G.P.S. acknowledges support from the Royal Society. F.Z. and G.P.S. acknowledge support from the Science and Technology Facilities Council. We acknowledge NASA funding for this project under the Spitzer program GO:40872. This work was supported in part by the National Science Foundation under grant No. AST-1211349. The Millennium simulation databases used in this paper and the web application providing online access to them were constructed as part of the activities of the German Astrophysical Virtual Observatory.

LoCuSS: The steady decline and slow quenching of star formation in cluster galaxies over the last four billion years

We present an analysis of the levels and evolution of star formation activity in a representative sample of 30 massive galaxy clusters at 0.15 3 M ☉ yr–1, of the form f SF∝(1 + z)7.6 ± 1.1. We dissect the origins of the Butcher-Oemler effect, revealing it to be due to the combination of a ~3 × decline in the mean specific SFRs of star-forming cluster galaxies since z ~ 0.3 with a ~1.5 × decrease in number density. Two-thirds of this reduction in the specific SFRs of star-forming cluster galaxies is due to the steady cosmic decline in the specific SFRs among those field galaxies accreted into the clusters. The remaining one-third reflects an accelerated decline in the star formation activity of galaxies within clusters. The slow quenching of star formation in cluster galaxies is consistent with a gradual shut down of star formation in infalling spiral galaxies as they interact with the intracluster medium via ram-pressure stripping or starvation mechanisms. The observed sharp decline in star formation activity among cluster galaxies since z ~ 0.4 likely reflects the increased susceptibility of low-redshift spiral galaxies to gas removal mechanisms as their gas surface densities decrease with time. We find no evidence for the build-up of cluster S0 bulges via major nuclear starburst episodes.

The Relation between Cool Cluster Cores and Herschel-detected Star Formation in Brightest Cluster Galaxies

We present far-infrared (FIR) analysis of 68 brightest cluster galaxies (BCGs) at 0.08 2 × 10^(11) L_☉), only a small (≾0.4 mag) reddening correction is required for SFR(Hα) to agree with SFR_(FIR). The relatively low Hα extinction (dust obscuration), compared to values reported for the general star-forming population, lends further weight to an alternate (external) origin for the cold gas. Finally, we use a stacking analysis of non-cool-core clusters to show that the majority of the fuel for star formation in the FIR-bright BCGs is unlikely to originate from normal stellar mass loss.

The SDSS-GALEX viewpoint of the truncated red sequence in field environments at z∼ 0

We combine GALEX near-UV photometry with a volume-limited sample of local (0.005 2u This contamination is greatest at faint magnitudes (Mr> 19) and in field regions where as many as three-quarters of red sequence galaxies are star-forming, and as such has important consequences for following the build-up of the red sequence. We find that the NUV r colour instead allows a much more robust separation of passively-evolving and star-forming galaxies, which allows the build-up of the UV-selected red sequence with redshift and environment to be directly interpreted in terms of the assembly of stellar mass in passively-evolving galaxies. We find that in isolated field regions the number density of UV-optical red sequence galaxies declines rapidly at magnitudes fainter than Mr� 19 and appears completely truncated at Mr� 18. This confirms the findings of Haines et al. (2007) that no passively-evolving dwarf galaxies are found more than two virial radii from a massive halo, whether that be a group, cluster or massive galaxy. These results support the downsizing paradigm whereby the red sequence is assembled from the top down, being already largely in place at the bright end by z�1, and the faint end filled in at later epochs in clusters and groups through environment-related processes such as ram-pressure stripping or galaxy harassment.

The Different Environmental Dependencies of Star Formation for Giant and Dwarf Galaxies

We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3 × 1010 M☉ by comparing the environmental dependencies of star formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic data set is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z = 0.03 that is 90% complete to M + 3.3. From this we measure global trends with environment for both giant (Mr 7 Gyr) or passive (EW [Hα] ≤ 4 A) falls gradually from 80% in the cluster cores to ~40% in field regions beyond 3-4Rvir, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ~1Rvir, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences in the evolution of giant and dwarf galaxies: whereas the star formation histories of giant galaxies are determined primarily by their merger history, star formation in dwarf galaxies is much more resilient to the effects of major mergers. Instead, dwarf galaxies become passive only once they become satellites within a more massive halo either by losing their halo gas reservoir to the host halo or through other environment-related processes such as galaxy harassment and ram pressure stripping.

ACCESS- V. dissecting ram-pressure stripping through integral-field spectroscopy and multiband imaging

We study the case of a bright (L>L*) barred spiral galaxy from the rich cluster A3558 in the Shapley supercluster core (z=0.05) undergoing ram-pressure stripping. Integral-field spectroscopy, complemented by multi-band imaging, allows us to reveal the impact of ram pressure on the interstellar medium. We study in detail the kinematics and the physical conditions of the ionized gas and the properties of the stellar populations. We observe one-sided extraplanar ionized gas along the full extent of the galaxy disc. Narrow-band Halpha imaging resolves this outflow into a complex of knots and filaments. The gas velocity field is complex with the extraplanar gas showing signature of rotation. In all parts of the galaxy, we find a significant contribution from shock excitation, as well as emission powered by star formation. Shock-ionized gas is associated with the turbulent gas outflow and highly attenuated by dust. All these findings cover the whole phenomenology of early-stage ram-pressure stripping. Intense, highly obscured star formation is taking place in the nucleus, probably related to the bar, and in a region 12 kpc South-West from the centre. In the SW region we identify a starburst characterized by a 5x increase in the star-formation rate over the last ~100 Myr, possibly related to the compression of the interstellar gas by the ram pressure. The scenario suggested by the observations is supported and refined by ad hoc N-body/hydrodynamical simulations which identify a rather narrow temporal range for the onset of ram-pressure stripping around t~60 Myr ago, and an angle between the galaxy rotation axis and the intra-cluster medium wind of ~45 deg. Taking into account that the galaxy is found ~1 Mpc from the cluster centre in a relatively low-density region, this study shows that ram-pressure stripping still acts efficiently on massive galaxies well outside the cluster cores.