The Origin of [O II] Emission in Recently Quenched Active Galaxy Nucleus Hosts

We have employed emission-line diagnostics derived from DEIMOS and NIRSPEC spectroscopy to determine the origin of the [O II] emission line observed in six active galactic nucleus (AGN) hosts at z ~ 0.9. These galaxies are a subsample of AGN hosts detected in the Cl1604 supercluster that exhibit strong Balmer absorption lines in their spectra and appear to be in a post-starburst or post-quenched phase, if not for their [O II] emission. Examining the flux ratio of the [N II] to Hα lines, we find that in five of the six hosts the dominant source of ionizing flux is AGN continuum emission. Furthermore, we find that four of the six galaxies have over twice the [O II] line luminosity that could be generated by star formation alone given their Hα line luminosities. This strongly suggests that AGN-excited narrow-line emission is contaminating the [O II] line flux. A comparison of star formation rates calculated from extinction-corrected [O II] and Hα line luminosities indicates that the former yields a five-fold overestimate of the current activity in these galaxies. Our findings reveal the [O II] line to be a poor indicator of star formation activity in a majority of these moderate-luminosity Seyferts. This result bolsters our previous findings that an increased fraction of AGN at high redshifts is hosted by galaxies in a post-starburst phase. The relatively high fraction of AGN hosts in the Cl1604 supercluster that show signs of recently truncated star formation activity may suggest that AGN feedback plays an increasingly important role in suppressing ongoing activity in large-scale structures at high redshift

Mass along the Line of Sight to the Gravitational Lens B1608+656: Galaxy Groups and Implications for H_0

We report the discovery of four groups of galaxies along the line of sight to the B1608+656 gravitational lens system. One group is at the redshift of the primary lensing galaxy (z = 0.631) and appears to have a low mass, with eight spectroscopically confirmed members and an estimated velocity dispersion of 150 ± 60 km s^(-1). The three other groups are in the foreground of the lens. These groups contain ~10 confirmed members each and are located at redshifts of 0.265, 0.426, and 0.52. Two of the three additional groups are centered roughly on the lens system, while the third is centered ~1' south of the lens. We investigate the effect of each of the four groups on the gravitational lensing potential of the B1608+656 system, with a particular focus on the implications for the value of H_0 derived from this system. We find that each group provides an external convergence of ~0.005-0.060, depending on the assumptions made in the calculation. For the B1608+656 system, the stellar velocity dispersion of the lensing galaxy has been measured, thus breaking the mass sheet degeneracy due to the group that is physically associated with the lens. The effect of the other groups along the line of sight can be folded into the overall uncertainties due to large-scale structure (LSS) along the line of sight. Because B1608+656 appears to lie along an overdense line of sight, the LSS will cause the measurement of H_0 to be biased high for this system. This effect could be 5% or greater

Pushing the Boundaries of the Cl 1604 Supercluster at z~0.9

The Cl 1604 supercluster at z~0.9 is known to contain at least four distinct member clusters, separated in both projection and redshift. In this paper we present deep, multicolor wide-field imaging of a region spanning ~45' on a side, corresponding to 21 h^{-1}_{70} Mpc (physical) at the supercluster redshift. We select galaxies whose colors correspond to those of spectroscopically confirmed cluster members in the r' vs. (r'-i') color-magnitude diagram. Using an adaptive kernel, we generate a map of the projected red galaxy density and identify numerous new candidate clusters which are likely supercluster members. Assuming that all of the density peaks are associated with the supercluster, its transverse size is ~10 h^{-1}_{70} Mpc, which is still significantly smaller than the nearly 93 h^{-1}_{70} Mpc depth in redshift space.Comment: 5 pages, 5 figures. Accepted for publication in AJ. Figures and text updated with correction

Star Formation Quenching in High-Redshift Large-Scale Structure: Post-Starburst Galaxies in the CI 1604 Supercluster at \u3cem\u3ez\u3c/em\u3e ~ 0.9

The Cl 1604 supercluster at z ~ 0.9 is one of the most extensively studied high-redshift large-scale structures, with more than 500 spectroscopically confirmed members. It consists of eight clusters and groups, with members numbering from a dozen to nearly a hundred, providing a broad range of environments for investigating the large-scale environmental effects on galaxy evolution. Here we examine the properties of 48 post-starburst galaxies in Cl 1604, comparing them to other galaxy populations in the same supercluster. Incorporating photometry from ground-based optical and near-infrared imaging, along with Spitzer mid-infrared observations, we derive stellar masses for all Cl 1604 members. The colors and stellar masses of the K+A galaxies support the idea that they are progenitors of red sequence galaxies. Their morphologies, residual star formation rates, and spatial distributions suggest that galaxy mergers may be the principal mechanism producing post-starburst galaxies. Interaction between galaxies and the dense intracluster medium (ICM) is also effective, but only in the cores of dynamically evolved clusters. The prevalence of post-starburst galaxies in clusters correlates with the dynamical state of the host cluster, as both galaxy mergers and the dense ICM produce post-starburst galaxies. We also investigate the incompleteness and contamination of K+A samples selected by means of Hδ and [O II] equivalent widths. K+A samples may be up to ~50% incomplete due to the presence of LINERs/Seyferts, and up to ~30% of K+A galaxies could have substantial star formation activity

Star Formation Quenching in High-redshift Large-scale Structure: Post-starburst Galaxies in the Cl1604 Supercluster at z∼0.9z \sim 0.9

The Cl1604 supercluster at $z \sim 0.9$ is one of the most extensively studied high redshift large scale structures, with more than 500 spectroscopically confirmed members. It consists of 8 clusters and groups, with members numbering from a dozen to nearly a hundred, providing a broad range of environments for investigating the large scale environmental effects on galaxy evolution. Here we examine the properties of 48 post-starburst galaxies in Cl1604, comparing them to other galaxy populations in the same supercluster. Incorporating photometry from ground-based optical and near-infrared imaging, along with $Spitzer$ mid-infrared observations, we derive stellar masses for all Cl1604 members. The colors and stellar masses of the K+A galaxies support the idea that they are progenitors of red sequence galaxies. Their morphologies, residual star-formation rates, and spatial distributions suggest galaxy mergers may be the principal mechanism producing post-starburst galaxies. Interaction between galaxies and the dense intra-cluster medium is also effective, but only in the cores of dynamically evolved clusters. The prevalence of post-starburst galaxies in clusters correlates with the dynamical state of the host cluster, as both galaxy mergers and the dense intra-cluster medium produce post-starburst galaxies. We also investigate the incompleteness and contamination of K+A samples selected by means of H$\delta$ and [OII] equivalent widths. K+A samples may be up to $\sim50\%$ incomplete due to the presence of LINER/Seyferts and up to $\sim30\%$ of K+A galaxies could have substantial star formation activity.Comment: 19 pages, 13 figures, accepted by Ap

Joint X-Ray and Optical Measurements of the Mass Distribution of the Distant Galaxy Cluster CLJ 0152.7-1357

We present joint X-ray and optical observations of the high redshift (z~0.83) lensing cluster CLJ0152.7-1357 made with the Chandra X-ray Observatory and the Keck telescope. We confirm the existence of significant substructure at both X-ray and optical wavelengths in the form of two distinct clumps, whose temperatures are 6.6(-1.5,+2.4) keV and 5.7(-1.6,+2.9) keV, respectively. The X-ray surface brightness profiles of the two clumps can be fitted by either a single beta-model or an NFW-like profile; the latter giving better fits to the central regions. We find that the X-ray derived mass of this cluster is in good agreement with independent lensing measurements. While its appearance indicates that the cluster has not reached a dynamical equilibrium state, its X-ray luminosity L_x, temperature T and dynamical mass M are consistent with the well-defined L_x-T and M-T relations for low-redshift galaxy clusters, which suggests that the dynamical properties of the clusters have remained almost unchanged since z~0.8.Comment: 32 pages, 14 figures, accepted for publication in A

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 Evolution and Environments of X-ray Emitting Active Galactic Nuclei in High-Redshift Large-Scale Structures

We use deep Chandra imaging and an extensive optical spectroscopy campaign on the Keck 10-m telescopes to study the properties of X-ray point sources in five large-scale structures at redshifts of z ~ 0.7-0.9. We first study X-ray point sources using the statistical measure of cumulative source counts, finding that the measured overdensities are consistent with previous results, but we recommend caution in overestimating the precision of the technique. Optical spectroscopy of objects matched to X-ray point sources confirms a total of 27 AGN within the five structures, and we find that their host galaxies tend to be located away from dense cluster cores. More than 36% of host galaxies are located in the `green valley', which suggests they are a transitional population. Based on analysis of OII and Hd line strengths, the average spectral properties of the AGN host galaxies in all structures indicate either on-going star formation or a starburst within ~ 1 Gyr, and the host galaxies are younger than the average galaxy in the parent population. These results indicate a clear connection between starburst and nuclear activity. We use composite spectra of the spectroscopically confirmed members in each structure to separate them based on a measure of the overall evolutionary state of their constituent galaxies. We define structures as having more evolved populations if their average galaxy has lower EW(OII) and EW(Hd). The AGN in the more evolved structures have lower rest-frame 0.5-8 keV X-ray luminosities (all below 10^43.3 erg s^-1) and longer times since a starburst than those in the less evolved structures, suggesting that the peak of both star formation and AGN activity has occurred at earlier times. With the wide range of evolutionary states and timeframes in the structures, we use our results to analyze the evolution of X-ray AGN and evaluate potential triggering mechanisms.Comment: 29 pages, 13 figure

Star Formation Quenching in High-redshift Large-scale Structure: Post-starburst Galaxies in the Cl 1604 Supercluster at z∼0.9

The Cl 1604 supercluster at z ~ 0.9 is one of the most extensively studied high-redshift large-scale structures, with more than 500 spectroscopically confirmed members. It consists of eight clusters and groups, with members numbering from a dozen to nearly a hundred, providing a broad range of environments for investigating the large-scale environmental effects on galaxy evolution. Here we examine the properties of 48 post-starburst galaxies in Cl 1604, comparing them to other galaxy populations in the same supercluster. Incorporating photometry from ground-based optical and near-infrared imaging, along with Spitzer mid-infrared observations, we derive stellar masses for all Cl 1604 members. The colors and stellar masses of the K+A galaxies support the idea that they are progenitors of red sequence galaxies. Their morphologies, residual star formation rates, and spatial distributions suggest that galaxy mergers may be the principal mechanism producing post-starburst galaxies. Interaction between galaxies and the dense intracluster medium (ICM) is also effective, but only in the cores of dynamically evolved clusters. The prevalence of post-starburst galaxies in clusters correlates with the dynamical state of the host cluster, as both galaxy mergers and the dense ICM produce post-starburst galaxies. We also investigate the incompleteness and contamination of K+A samples selected by means of Hδ and [O II] equivalent widths. K+A samples may be up to ~50% incomplete due to the presence of LINERs/Seyferts, and up to ~30% of K+A galaxies could have substantial star formation activity