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

Observational evidence for the radial alignment of satellites with their dark matter host has been accumulating steadily in the past few years. The effect is seen over a wide range of scales, from massive clusters of galaxies down to galaxy-sized systems, yet the underlying physical mechanism has still not been established. To this end, we have carried out a detailed analysis of the shapes and orientations of dark matter substructures in high-resolution N-body cosmological simulations. We find a strong tendency for radial alignment of the substructure with its host halo: the distribution of halo major axes is very anisotropic, with the majority pointing towards the center of mass of the host. The alignment peaks once the sub-halo has passed the virial radius of the host for the first time, but is not subsequently diluted, even after the halos have gone through as many as four pericentric passages. This evidence points to the existence of a very rapid dynamical mechanism acting on these systems and we argue that tidal torquing throughout their orbits is the most likely candidate.Comment: v2: 13 pages, 10 figures, ApJ in press. Revisions include a new section (4.2) comparing our results with observations, and a few added reference

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.

No abstract

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.