A measurement of large-scale peculiar velocities of clusters of galaxies: results and cosmological implications

Peculiar velocities of clusters of galaxies can be measured by studying the fluctuations in the cosmic microwave background (CMB) generated by the scattering of the microwave photons by the hot X-ray emitting gas inside clusters. While for individual clusters such measurements result in large errors, a large statistical sample of clusters allows one to study cumulative quantities dominated by the overall bulk flow of the sample with the statistical errors integrating down. We present results from such a measurement using the largest all-sky X-ray cluster catalog combined to date and the 3-year WMAP CMB data. We find a strong and coherent bulk flow on scales out to at least > 300 h^{-1} Mpc, the limit of our catalog. This flow is difficult to explain by gravitational evolution within the framework of the concordance LCDM model and may be indicative of the tilt exerted across the entire current horizon by far-away pre-inflationary inhomogeneities.Comment: Ap.J. (Letters), in press. 20 Oct issue (Vol. 686

No Evidence of Quasar-Mode Feedback in a Four-Way Group Merger at z~0.84

We report on the results of a Chandra search for evidence of triggered nuclear activity within the Cl0023+0423 four-way group merger at z ~ 0.84. The system consists of four interacting galaxy groups in the early stages of hierarchical cluster formation and, as such, provides a unique look at the level of processing and evolution already under way in the group environment prior to cluster assembly. We present the number counts of X-ray point sources detected in a field covering the entire Cl0023 structure, as well as a cross-correlation of these sources with our extensive spectroscopic database. Both the redshift distribution and cumulative number counts of X-ray sources reveal little evidence to suggest that the system contains X-ray luminous active galactic nuclei (AGNs) in excess to what is observed in the field population. If preprocessing is under way in the Cl0023 system, our observations suggest that powerful nuclear activity is not the predominant mechanism quenching star formation and driving the evolution of Cl0023 galaxies. We speculate that this is due to a lack of sufficiently massive nuclear black holes required to power such activity, as previous observations have found a high late-type fraction among the Cl0023 population. It may be that disruptive AGN-driven outflows become an important factor in the preprocessing of galaxy populations only during a later stage in the evolution of such groups and structures when sufficiently massive galaxies (and central black holes) have built up, but prior to hydrodynamical processes stripping them of their gas reservoirs.Comment: Published in ApJ

The Observations of Redshift Evolution in Large-Scale Environments (ORELSE) Survey. I. The Survey Design and First Results on CL 0023+0423 at z = 0.84 and RX J1821.6+6827 at z = 0.82

We present the Observations of Redshift Evolution in Large-Scale Environments (ORELSE) Survey, a systematic search for structure on scales greater than 10 h^(–1)_70 Mpc around 20 well-known clusters at redshifts of 0.6 < z < 1.3. The goal of the survey is to examine a statistical sample of dynamically active clusters and large-scale structures in order to quantify galaxy properties over the full range of local and global environments. We describe the survey design, the cluster sample, and our extensive observational data covering at least 25' around each target cluster. We use adaptively smoothed red galaxy density maps from our wide-field optical imaging to identify candidate groups/clusters and intermediate-density large-scale filaments/walls in each cluster field. Because photometric techniques (such as photometric redshifts, statistical overdensities, and richness estimates) can be highly uncertain, the crucial component of this survey is the unprecedented amount of spectroscopic coverage. We are using the wide-field, multiobject spectroscopic capabilities of the Deep Multiobject Imaging Spectrograph to obtain 100-200+ confirmed cluster members in each field. Our survey has already discovered the Cl 1604 supercluster at z ≈ 0.9, a structure which contains at least eight groups and clusters and spans 13 Mpc × 100 Mpc. Here, we present the results on the large-scale environments of two additional clusters, Cl 0023+0423 at z = 0.84 and RX J1821.6+6827 at z = 0.82, which highlight the diversity of global properties at these redshifts. The optically selected Cl 0023+0423 is a four-way group-group merger with constituent groups having measured velocity dispersions between 206 and 479 km s^–1. The galaxy population is dominated by blue, star-forming galaxies, with 80% of the confirmed members showing [O II] emission. The strength of the Hδ line in a composite spectrum of 138 members indicates a substantial contribution from recent starbursts to the overall galaxy population. In contrast, the X-ray-selected RX J1821.6+6827 is a largely isolated, massive cluster with a measured velocity dispersion of 926 ± 77 km s^(–1). The cluster exhibits a well-defined red sequence with a large quiescent galaxy population. The results from these two targets, along with preliminary findings on other ORELSE clusters, suggest that optical selection may be more effective than X-ray surveys at detecting less-evolved, dynamically active systems at these redshifts

Do We Expect Most AGN to Live in Disks?

Recent observations have indicated that a large fraction of the low to intermediate luminosity AGN population lives in disk-dominated hosts, while the more luminous quasars live in bulge-dominated hosts, in conflict with some previous model predictions. We therefore build and compare a semi-empirical model for AGN fueling which accounts for both merger and non-merger 'triggering.' In particular, we show that the 'stochastic accretion' model - in which fueling in disk galaxies is essentially a random process arising whenever dense gas clouds reach the nucleus - provides a good match to the present observations at low/intermediate luminosities. However it falls short of the high-luminosity population. We combine this with models for major merger-induced AGN fueling, which lead to rarer but more luminous events, and predict the resulting abundance of disk-dominated and bulge-dominated AGN host galaxies as a function of luminosity and redshift. We compile and compare observational constraints from z~0-2. The models and observations generically show a transition from disk to bulge dominance in hosts near the Seyfert-quasar transition, at all redshifts. 'Stochastic' fueling dominates AGN by number (dominant at low luminosity), and dominates BH growth below the knee in the present-day BH mass function (<10^7 M_sun). However it accounts for just ~10% of BH mass growth at masses >10^8 M_sun. In total, fueling in disky hosts accounts for ~30% of the total AGN luminosity density/BH mass density. The combined model also accurately predicts the AGN luminosity function and clustering/bias as a function of luminosity and redshift; however, we argue that these are not sensitive probes of BH fueling mechanisms.Comment: 13 pages, 5 figures, PDF updated to match published versio

A measurement of large-scale peculiar velocities of clusters of galaxies: technical details

This paper presents detailed analysis of large-scale peculiar motions derived from a sample of ~ 700 X-ray clusters and cosmic microwave background (CMB) data obtained with WMAP. We use the kinematic Sunyaev-Zeldovich (KSZ) effect combining it into a cumulative statistic which preserves the bulk motion component with the noise integrated down. Such statistic is the dipole of CMB temperature fluctuations evaluated over the pixels of the cluster catalog (Kashlinsky & Atrio-Barandela 2000). To remove the cosmological CMB fluctuations the maps are Wiener-filtered in each of the eight WMAP channels (Q, V, W) which have negligible foreground component. Our findings are as follows: The thermal SZ (TSZ) component of the clusters is described well by the Navarro-Frenk-White profile expected if the hot gas traces the dark matter in the cluster potential wells. Such gas has X-ray temperature decreasing rapidly towards the cluster outskirts, which we demonstrate results in the decrease of the TSZ component as the aperture is increased to encompass the cluster outskirts. We then detect a statistically significant dipole in the CMB pixels at cluster positions. Arising exclusively at the cluster pixels this dipole cannot originate from the foreground or instrument noise emissions and must be produced by the CMB photons which interacted with the hot intracluster gas via the SZ effect. The dipole remains as the monopole component, due to the TSZ effect, vanishes within the small statistical noise out to the maximal aperture where we still detect the TSZ component. We demonstrate with simulations that the mask and cross-talk effects are small for our catalog and contribute negligibly to the measurements. The measured dipole thus arises from the KSZ effect produced by the coherent large scale bulk flow motion.Comment: Minor changes to match the published version - Ap.J., 1 Feb 2009 issu