We present a study of the spatial distribution and kinematics of star-forming galaxies in 30 massive clusters at 0.15

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 Mr=−18.0, we find that the Hα 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(Hα) = 2Å. 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 Mr≲−21 to zero by Mr∼−18. Indeed for the lowest luminosity range covered (−18 < Mr < −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 Hα emission from dwarf (−19 < Mr < −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 Mr∼−21 to ∼0 per cent by Mr∼−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.

We study the mid-infrared properties of galaxies in 30 massive galaxy clusters at 0.02≤z≤0.40, using panoramic Spitzer/MIPS 24µm 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 mid-infrared data covering not only the cluster cores, but extending into the infall regions. We use these data to revisit the so-called Butcher-Oemler effect, measuring the fraction of massive infrared luminous galaxies (K5×10 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 clustercentric radii, specifically r 500

We present an analysis of the levels and evolution of star formation activity in a representative sample of 30 massive galaxy clusters at 0.153 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 intra-cluster 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 susceptability 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 star-burst episodes.

We present a study of the distribution of X-ray AGN in a representative sample of 26 massive clusters at 0.1510 42 erg s −1 . In the stacked caustic diagram that shows (v los −)/σ v versus r pro j /r 500 , the X-ray AGN appear to preferentially lie along the caustics, suggestive of an infalling population. They also appear to avoid the region with lowest cluster-centric radii and relative velocities (r pro j <0.4r 500 ; |v−|/σ v <0.8), which is dominated by the virialized population of galaxies accreted earliest into the clusters. The line-of-sight velocity histogram of the X-ray AGN shows a relatively flat distribution, and is inconsistent with the Gaussian distribution expected for a virialized population at 98.9% confidence. Moreover the velocity dispersion of the 48 X-ray AGN is 1.51× that of the overall cluster population, which is consistent with the √ 2 ratio expected by simple energetic arguments when comparing infalling versus virialized populations. This kinematic segregation is significant at the 4.66-σ level. When splitting the X-ray AGN sample into two according to X-ray or infrared (IR) luminosity, both X-ray bright (L X >10 42 ) and IR-bright (L T IR >2×10 10 L ⊙ ) sub-samples show higher velocity dispersions than their X-ray dim and IR-dim counterparts at >2σ significance. This is consistent with the nuclear activity responsible for the X-ray and IR emission being slowly shut down as the host galaxies are accreted into the cluster. Overall our results provide the strongest observational evidence to date that X-ray AGN found in massive clusters are an infalling population, and that the cluster environment very effectively suppresses radiatively-efficient nuclear activity in its member galaxies. These results are consistent with the view that for galaxies to host an X-ray AGN they should be the central galaxy within their dark matter halo and have a ready supply of cold gas.

We present far-infrared (FIR) analysis of 68 brightest cluster galaxies (BCGs) at 0.08 < z < 1.0. Deriving total infrared luminosities directly from Spitzer and Herschel photometry spanning the peak of the dust component (24-500 μm), we calculate the obscured star formation rate (SFR). 22 +6.2 −5.3 % of the BCGs are detected in the farinfrared, with SFR = 1-150 M yr −1 . The infrared luminosity is highly correlated with cluster X-ray gas cooling times for cool-core clusters (gas cooling time <1 Gyr), strongly suggesting that the star formation in these BCGs is influenced by the cluster-scale cooling process. The occurrence of the molecular gas tracing Hα emission is also correlated with obscured star formation. For all but the most luminous BCGs (L TIR > 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.

We study the case of a bright (L>L ⋆ ) barred spiral galaxy from the rich cluster A 3558 in the Shapley supercluster core (z=0.05) undergoing ram-pressure stripping. Integral-field spectroscopy with WiFeS at the 2.3m ANU, complemented by imaging in ultra violet (GALEX), B and R (ESO 2.2m WFI), Hα (Magellan), K (UKIRT), 24µm and 70µm (Spitzer), allows us to reveal the impact of ram pressure on the interstellar medium. With these data 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, extending ∼13 kpc in projection from it. Narrow-band Hα imaging resolves this outflow into a complex of knots and filaments, similar to those seen in other cluster galaxies undergoing ram-pressure stripping. The gas velocity field is complex with the extraplanar gas showing signature of rotation, while the stellar velocity field is regular and the K-band image shows a symmetric stellar distribution. We use line-ratio diagnostics to ascertain the origin of the observed emission. 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 (A v =1.5-2.3 mag). 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. These two regions account for half of the total star formation in the galaxy, which overall amounts to 7.2±2.2 M ⊙ yr −1 . In the SW region we identify a starburst characterized by a ∼ 5× 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 • . The ram pressure is therefore acting at an intermediate angle between face-on and edge-on. 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, as also recently observed in the Virgo cluster.

We present an analysis of optical spectroscopically-identified AGN down to a cluster magnitude of M ⋆ + 1 in a sample of 6 self-similar SDSS galaxy clusters at z ∼ 0.07. These clusters are specifically selected to lack significant substructure at bright limits in their central regions so that we are largely able to eliminate the local action of merging clusters on the frequency of AGN. We demonstrate that the AGN fraction increases significantly from the cluster centre to 1.5R virial , but tails off at larger radii. If only comparing the cluster core region to regions at ∼ 2R virial , no significant variation would be found. We compute the AGN fraction by mass and show that massive galaxies (log(stellar mass) > 10.7) are host to a systematically higher fraction of AGN than lower mass galaxies at all radii from the cluster centre. We attribute this deficit of AGN in the cluster centre to the changing mix of galaxy types with radius. We use the WHAN diagnostic to separate weak AGN from 'retired' galaxies in which the main ionization mechanism comes from old stellar populations. These retired AGN are found at all radii, while the mass effect is much more pronounced: we find that massive galaxies are more likely to be in the retired class. Further, we show that our AGN have no special position inside galaxy clusters -they are neither preferentially located in the infall regions, nor situated at local maxima of galaxy density as measured with Σ 5 . However, we find that the most powerful AGN (with [OIII] equivalent widths < −10Å) reside at significant velocity offsets in the cluster, and this brings our analysis into agreement with previous work on X-ray selected AGN. Our results suggest that if interactions with other galaxies are responsible for triggering AGN activity, the timelag between trigger and AGN enhancement must be sufficiently long to obfuscate the encounter site and wipe out the local galaxy density signal.