[en] We present a joint gravitational lensing and near-infrared study of the galaxy cluster Abell 3192 (A3192) that has been associated both with galaxies at z = 0.168 and with the X-ray luminous cluster RXC J0358.8–2955 (RXC J0358) at z = 0.425. Weak-lensing analysis of our Hubble Space Telescope snapshot observation with the Advanced Camera for Surveys detects two mass over-densities separated by ∼2 arcmin, one adjacent to the optical position of A3192 (4.4σ significance) and the other adjacent to the X-ray position of RXC J0358 (6.2σ significance). These mass peaks coincide with peaks in the K-band luminosity density of galaxies with near-infrared colors consistent with the red sequence at z = 0.168 and z 0.425, respectively. Moreover, the Bayesian evidence of parameterized mass models that include two cluster/group-scale halos centered on the respective mass peaks exceeds that of single-halo models by a factor of ≥10. The total projected mass of each galaxy system within 250 kpc of the respective peaks is M_WL(z = 0.168) ≅ 3 × 10¹³ M_☉ and M_WL(z = 0.425) ≅ 1.2 × 10¹⁴ M_☉, both with total mass-to-light ratios of M_WL/L_K ≅ 20 M_☉/L_☉. The original Abell cluster therefore comprises two independent galaxy systems—a foreground group at z = 0.168 and RXC J0358 at z = 0.425. Our results demonstrate the power of combining X-ray, near-infrared, and weak-lensing observations to select massive clusters, place those clusters and interloper galaxy systems along the line of sight, and measure their masses. This combination will be invaluable to robust interpretation of future high-redshift cluster surveys, including eROSITA.

[en] 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 < K* + 1.5, L _IR > 5 x 10¹⁰ L _sun) 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.

[en] 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 < z < 0.30 from the Local Cluster Substructure Survey, combining wide-field Spitzer/MIPS 24 μm data with extensive spectroscopy of cluster members. The specific SFRs of massive (M > or approx. 10¹⁰ M_☉) star-forming cluster galaxies within r₂₀₀ are found to be systematically ∼28% lower than their counterparts in the field at fixed stellar mass and redshift, a difference significant at the 8.7σ level. This is the unambiguous signature of star formation in most (and possibly all) massive star-forming galaxies being slowly quenched upon accretion into massive clusters, their star formation rates (SFRs) declining exponentially on quenching timescales in the range 0.7-2.0 Gyr. We measure the mid-infrared Butcher-Oemler effect over the redshift range 0.0-0.4, finding rapid evolution in the fraction (f_SF) of massive (M_K < – 23.1) cluster galaxies within r₂₀₀ with SFRs > 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

[en] We present a study of the distribution of X-ray active galactic nuclei (AGNs) in a representative sample of 26 massive clusters at 0.15 < z < 0.30, combining Chandra observations sensitive to X-ray point sources of luminosity L_X ∼ 10⁴² erg s^–1 at the cluster redshift with extensive and highly complete spectroscopy of cluster members down to ∼M*_K + 2. In total we identify 48 X-ray AGNs among the cluster members, with luminosities 2 × 10⁴¹-1 × 10⁴⁴ erg s^–1. Based on these identifications, we estimate that 0.73% ± 0.14% of cluster galaxies brighter than M_K = –23.1 (M*_K + 1.5) host an X-ray AGN with L_X > 10⁴² erg s^–1. In the stacked caustic diagram that shows (v_los – (v))/σ_v versus r_proj/r₅₀₀, 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_proj < 0.4r₅₀₀; |v – (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 AGNs is 1.51 times 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 4.66σ level. When splitting the X-ray AGN sample into two according to X-ray or infrared (IR) luminosity, both X-ray bright (L_X > 10⁴²) and IR-bright (L_TIR > 2 × 10¹⁰ L_☉) subsamples 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 AGNs 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.