We investigate the properties of the 525 spectroscopically confirmed members of the Cl1604 supercluster at z~0.9 as part of the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey. Using extensive Keck LRIS/DEIMOS spectrosc
opy in conjunction with ten-band ground-based, Spitzer, and HST imaging, we investigate the buildup of the red sequence in groups and clusters at high redshift. Nearly all of the brightest and most massive red-sequence galaxies are found within the bounds of the clusters and groups. Despite the prevalence of these red-sequence galaxies, we find that the average cluster galaxy has a spectrum indicative of a star-forming galaxy, with a star formation rate between those of z~1 field galaxies and moderate redshift cluster galaxies. The average group galaxy is even more active, exhibiting properties indicative of a starburst. The presence of massive, red galaxies and the high fraction of starbursting galaxies suggest that significant processing is occurring in the group environment at z~1 and earlier. There is a deficit of low-luminosity red-sequence galaxies in all Cl1604 clusters and groups, suggesting that such galaxies transition to the red sequence at later times. Extremely massive (10^12) red sequence galaxies are also absent from the Cl1604 clusters and groups. We suggest that such galaxies form at later times through merging processes. There are also large populations of transition galaxies at intermediate stellar masses present in the groups and clusters, suggesting that such masses are important in the buildup of the red-sequence mass function at z~1. Through a comparison of the transitional populations present in the Cl1604 clusters and groups, we find evidence that massive blue cloud galaxies are quenched earliest in the most dynamically relaxed systems and at progressively later times in dynamically unrelaxed systems.
We have employed emission-line diagnostics derived from DEIMOS and NIRSPEC spectroscopy to determine the origin of the [OII] emission line observed in six AGN hosts at z~0.9. These galaxies are a subsample of AGN hosts detected in the Cl1604 superclu
ster 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 [OII] emission. Examining the flux ratio of the [NII] to Halpha 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 [OII] line luminosity that could be generated by star formation processes alone given their Halpha line luminosities. This strongly suggests that AGN-excited narrow-line emission is contaminating the [OII] line flux. A comparison of star formation rates calculated from extinction-corrected [OII] and Halpha line luminosities indicates that the former yields a five-fold overestimate of current activity in these galaxies. Our findings reveal the [OII] 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 are 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 suggest AGN feedback may play an increasingly important role in suppressing ongoing activity in large-scale structures at high redshift.
Using Spitzer-MIPS 24um imaging and Keck spectroscopy we examine the nature of the obscured star forming population in three clusters and three groups at z~0.9. These six systems are components of the Cl1604 supercluster, the largest structure imaged
by Spitzer at redshifts near unity. We find that the average density of 24um-detected galaxies within the Cl1604 clusters is nearly twice that of the surrounding field and that this overdensity scales with the clusters dynamical state. The 24um-bright members often appear optically unremarkable and exhibit only moderate [OII] line emission due to severe obscuration. Their spatial distribution suggests they are an infalling population, but an examination of their spectral properties, morphologies and optical colors indicate they are not simply analogs of the field population that have yet to be quenched. Using stacked composite spectra, we find the 24um-detected cluster and group galaxies exhibit elevated levels of Balmer absorption compared to galaxies undergoing normal, continuous star formation. A similar excess is not observed in field galaxies with equivalent infrared luminosities, indicating a greater fraction of the detected cluster and group members have experienced a burst of star formation in the recent past compared to their counterparts in the field. Our results suggest that gas-rich galaxies at high redshift experience a temporary increase in their star formation activity as they assemble into denser environments. Using HST-ACS imaging we find that disturbed morphologies are common among the 24um-detected cluster and group members and become more prevalent in regions of higher galaxy density. We conclude that mergers are the dominant triggering mechanism responsible for the enhanced star formation found in the Cl1604 groups, while a mix of harassment and mergers are likely driving the activity of the cluster galaxies.
We present the first results from a near-IR spectroscopic campaign of the Cl1604 supercluster at z~0.9 and the cluster RX J1821.6+6827 at z~0.82 to investigate the nature of [OII] 3727A emission in cluster galaxies at high redshift. Of the 401 member
s in the two systems, 131 galaxies have detectable [OII] emission with no other signs of current star-formation, as well as strong absorption features indicative of a well-established older stellar population. The combination of these features suggests that the primary source of [OII] emission in these galaxies is not the result of star-formation, but rather due to the presence of a LINER or Seyfert component. Using the NIRSPEC spectrograph on the Keck II 10-m telescope, 19 such galaxies were targeted, as well as six additional [OII]-emitting cluster members that exhibited other signs of ongoing star-formation. Nearly half (~47%) of the 19 [OII]-emitting, absorption-line dominated galaxies exhibit [OII] to Ha equivalent width ratios higher than unity, the typical value for star-forming galaxies. A majority (~68%) of these 19 galaxies are classified as LINER/Seyfert based on the emission-line ratio of [NII] and Ha, increasing to ~85% for red [OII]-emitting, absorption-line dominated galaxies. The LINER/Seyfert galaxies exhibit L([OII])/L(Ha) ratios significantly higher than that observed in populations of star-forming galaxies, suggesting that [OII] is a poor indicator of star-formation in a large fraction of high-redshift cluster members. We estimate that at least ~20% of galaxies in high-redshift clusters contain a LINER/Seyfert component that can be revealed with line ratios. We also investigate the effect this population has on the star formation rate of cluster galaxies and the post-starburst fraction, concluding that LINER/Seyferts must be accounted for if these quantities are to be meaningful.
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 format
ion 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.
To investigate the role of feedback from Active Galactic Nuclei (AGN) in driving the evolution of their host galaxies, we have carried out a study of the environments and optical properties of galaxies harboring X-ray luminous AGN in the Cl1604 super
cluster at z~0.9. Making use of Chandra, HST/ACS and Keck/DEIMOS observations, we examine the integrated colors, morphologies and spectral properties of nine moderate-luminosity (L_x ~ 10^43 erg s^-1) type 2 Seyferts detected in the Cl1604 complex. We find that the AGN are predominantly hosted by luminous spheroids and/or bulge dominated galaxies which have colors that place them in the valley between the blue cloud and red sequence in color-magnitude space, consistent with predictions that AGN hosts should constitute a transition population. Half of the hosts have bluer overall colors as a result of blue resolved cores in otherwise red spheroids and a majority show signs of recent or pending interactions. We also find a substantial number exhibit strong Balmer absorption features indicative of post-starburst galaxies, despite the fact that we detect narrow [OII] emission lines in all of the host spectra. If the [OII] lines are due in part to AGN emission, as we suspect, then this result implies that a significant fraction of these galaxies (44%) have experienced an enhanced level of star formation within the last ~1 Gyr which was rapidly suppressed. Overall we find that the properties of the nine host galaxies are generally consistent with a scenario in which recent interactions have triggered both increased levels of nuclear activity and an enhancement of centrally concentrated star formation, followed by a rapid truncation of the latter, possibly as a result of feedback from the AGN itself. [Abridged]
We present the results of Chandra observations of the Cl1604 supercluster at z~0.9. The system is the largest structure mapped at redshifts approaching unity, containing at least eight spectroscopically confirmed galaxy clusters and groups. Using two
50-ksec ACIS-I pointings we examine both the X-ray point source population and the diffuse emission from individual clusters in the system. We find a 2.5sigma excess of point sources detected in the hard band (2-10 keV) relative to the number of sources found in blank fields observed by Chandra. No such excess is observed in the soft band (0.5-2 keV). The hard-band source density is 1.47 times greater than that of a blank field, in agreement with the previously reported correlation between overdensity amplitude and cluster redshift. Using a maximum likelihood technique we have matched 112 of the 161 detected X-ray point sources to optical counterparts and found 15 sources that are associated with the supercluster. All 15 sources have rest-frame luminosities consistent with emission from active galactic nuclei (AGN). We find that the supercluster AGN largely avoid the densest regions of the system and are instead distributed on the outskirts of massive clusters or within poorer clusters and groups. We have also detected diffuse emission from two of the eight clusters and groups in the system, clusters Cl1604+4304 and Cl1604+4314. The systems have bolometric luminosities of 1.43x10^44 and 8.20x10^43 h70^-2 erg s^-1 and gas temperatures of 3.50 (+1.82-1.08) and 1.64 (+0.65-0.45) keV, respectively. Using updated velocity dispersions, we compare the properties of these systems to the cluster scaling relations followed by other X-ray and optically selected galaxy clusters at high redshift.
