Galaxies in X-ray Groups. III. Satellite Color and Morphology Transformations

While the star formation rates and morphologies of galaxies have long been known to correlate with their local environment, the process by which these correlations are generated is not well understood. Galaxy groups are thought to play an important role in shaping the physical properties of galaxies before entering massive clusters at low redshift, and transformations of satellite galaxies likely dominate the buildup of local environmental correlations. To illuminate the physical processes that shape galaxy evolution in dense environments, we study a sample of 116 X-ray selected galaxy groups at z=0.2-1 with halo masses of 10^13-10^14 M_sun and centroids determined with weak lensing. We analyze morphologies based on HST imaging and colors determined from 31 photometric bands for a stellar mass-limited population of 923 satellite galaxies and a comparison sample of 16644 field galaxies. Controlling for variations in stellar mass across environments, we find significant trends in the colors and morphologies of satellite galaxies with group-centric distance and across cosmic time. Specifically at low stellar mass (log(M_stellar/M_sun) = 9.8-10.3), the fraction of disk-dominated star-forming galaxies declines from >50% among field galaxies to <20% among satellites near the centers of groups. This decline is accompanied by a rise in quenched galaxies with intermediate bulge+disk morphologies, and only a weak increase in red bulge-dominated systems. These results show that both color and morphology are influenced by a galaxys location within a group halo. We suggest that strangulation and disk fading alone are insufficient to explain the observed morphological dependence on environment, and that galaxy mergers or close tidal encounters must play a role in building up the population of quenched galaxies with bulges seen in dense environments at low redshift.

The Correlated Formation Histories of Massive Galaxies and Their Dark Matter Halos

Using observations in the COSMOS field, we report an intriguing correlation between the star formation activity of massive (~10^{11.4}msol) central galaxies, their stellar masses, and the large-scale (~10 Mpc) environments of their group-mass (~10^{13.6}msol) dark matter halos. Probing the redshift range z=[0.2,1.0], our measurements come from two independent sources: an X-ray detected group catalog and constraints on the stellar-to-halo mass relation derived from a combination of clustering and weak lensing statistics. At z=1, we find that the stellar mass in star-forming centrals is a factor of two less than in passive centrals at the same halo mass. This implies that the presence or lack of star formation in group-scale centrals cannot be a stochastic process. By z=0, the offset reverses, probably as a result of the different growth rates of these objects. A similar but weaker trend is observed when dividing the sample by morphology rather than star formation. Remarkably, we find that star-forming centrals at z~1 live in groups that are significantly more clustered on 10 Mpc scales than similar mass groups hosting passive centrals. We discuss this signal in the context of halo assembly and recent simulations, suggesting that star-forming centrals prefer halos with higher angular momentum and/or formation histories with more recent growth; such halos are known to evolve in denser large-scale environments. If confirmed, this would be evidence of an early established link between the assembly history of halos on large scales and the future properties of the galaxies that form inside them.

The integrated stellar content of dark matter halos

Measurements of the total amount of stars locked up in galaxies as a function of host halo mass contain key clues about the efficiency of processes that regulate star formation. We derive the total stellar mass fraction f_star as a function of halo mass M500c from z=0.2 to z=1 using two complementary methods. First, we derive f_star using a statistical Halo Occupation Distribution model jointly constrained by data from lensing, clustering, and the stellar mass function. This method enables us to probe f_star over a much wider halo mass range than with group or cluster catalogs. Second, we derive f_star at group scales using a COSMOS X-ray group catalog and we show that the two methods agree to within 30%. We quantify the systematic uncertainty on f_star using abundance matching methods and we show that the statistical uncertainty on f_star (~10%) is dwarfed by systematic uncertainties associated with stellar mass measurements (~45% excluding IMF uncertainties). Assuming a Chabrier IMF, we find 0.012<f_star<0.025 at M500c=10^13 Msun and 0.0057<f_star<0.015 at M500c=10^14 Msun. These values are significantly lower than previously published estimates. We investigate the cause of this difference and find that previous work has overestimated f_star due to a combination of inaccurate stellar mass estimators and/or because they have assumed that all galaxies in groups are early type galaxies with a constant mass-to-light ratio. Contrary to previous claims, our results suggest that the mean value of f_star is always significantly lower than f_gas for halos above 10^13 Msun. Combining our results with recently published gas mas fractions, we find a shortfall in f_star+f_gas at R500c compared to the cosmic mean. This shortfall varies with halo mass and becomes larger towards lower halos masses.

On Active Galactic Nuclei as Sources of Ultra-High Energy Cosmic Rays

We measure the correlation between sky coordinates of the Swift BAT catalogue of active galactic nuclei with the arrival directions of the highest energy cosmic rays detected by the Auger Observatory. The statistically complete, hard X-ray catalogue helps to distinguish between AGN and other source candidates that follow the distribution of local large-scale structure. The positions of the full catalogue are marginally uncorrelated with the cosmic ray arrival directions, but when weighted by their hard X-ray flux, AGN within 100 Mpc are correlated at a significance level of 98 per cent. This correlation sharply decreases for sources beyond ~100 Mpc, suggestive of a GZK suppression. We discuss the implications for determining the mechanism that accelerates particles to these extreme energies in excess of 10^19 eV.