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ABSTRACT: We use Chandra observations of nine optically and X-ray selected clusters in
five different structures at z ~ 0.7-1.1 from the Observations of Redshift
Evolution in Large-Scale Environments (ORELSE) survey to study diffuse X-ray
emission from galaxy clusters. X-ray gas temperatures and bolometric rest-frame
luminosities are measured for each cluster in the sample. We present new
redshift measurements, derived from dataobtained using the Deep Imaging
Multi-Object Spectrograph on the Keck 10-m telescope, for two clusters in the
RX J0910 supercluster at z ~ 1.1, from which velocity dispersions are measured.
Dispersions for all clusters are combined with X-ray luminosities and gas
temperatures to evaluate how the cluster properties compare to low-redshift
scaling relations. We also measure the degree of substructure in each cluster
by examining the velocity histograms, performing Dressler-Shectman tests, and
computing the offsets between the X-ray emission center and optically-derived
centroids. We find that only two clusters show clear indications of being
unrelaxed, based on their scaling relations and other dynamical state
diagnostics. Using our sample, we evaluate the redshift evolution of the L_x-T
relation and investigate the implications of our results for precision
cosmology surveys.
12/2012;
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ABSTRACT: 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.
The Astrophysical Journal 08/2011; 746(2). · 6.02 Impact Factor
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ABSTRACT: 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 spectroscopy 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.
The Astrophysical Journal 08/2011; 745(2). · 6.02 Impact Factor
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ABSTRACT: 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.
The Astrophysical Journal Letters 08/2011; 737(2):L38. · 5.53 Impact Factor
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ABSTRACT: Using Spitzer-Multiband Imaging Photometer 24 μm imaging and extensive 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 the primary components of the Cl1604 supercluster, the largest structure imaged by Spitzer at redshifts approaching unity. We find that the average density of 24 μm detected galaxies within the Cl1604 clusters is nearly twice that of the surrounding field and that this overdensity scales with the cluster's dynamical state. The 24 μm bright members often appear optically unremarkable and exhibit only moderate [O II] line emission due to severe obscuration. Their spatial distribution suggests that they are an infalling population, but an examination of their spectral properties, morphologies, and optical colors indicates that they are not simply analogs of the field population that have yet to be quenched. Using stacked composite spectra, we find that the 24 μm detected cluster and group galaxies exhibit elevated levels of Balmer absorption compared with 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 Hubble Space Telescope Advanced Camera for Surveys imaging, we find that disturbed morphologies are common among the 24 μm 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.
The Astrophysical Journal 07/2011; 736(1):38. · 6.02 Impact Factor
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ABSTRACT: 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 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 [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. Comment: 5 Pages, 4 Figures, submitted to ApJL
11/2010;
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ABSTRACT: 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 cluster's 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. Comment: 18 pages, 19 figures, submitted to ApJ
09/2010;
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ABSTRACT: 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 members 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. Comment: 33 pages, 17 figures, to appear in ApJ
03/2010;
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ABSTRACT: 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.
10/2009;
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ABSTRACT: We present results of a spectroscopic search for Lyman-alpha emitters (LAEs) in the Cl1604 supercluster field using the extensive spectroscopic Keck/DEIMOS database taken as part of the Observations of Redshift Evolution in Large Scale Environments (ORELSE) survey. A total of 12 slitmasks were observed and inspected in the Cl1604 field, spanning a survey volume of 1.365x10^4 co-moving Mpc^3. We find a total of 17 high redshift (4.39 < z < 5.67) LAE candidates down to a limiting flux of 1.9x10^(-18) ergs/s/cm (~0.1L* at z~5), 13 of which we classify as high quality. The resulting LAE number density is nearly double that of LAEs found in the Subaru deep field at z~4.9 and nearly an order of magnitude higher than in other surveys of LAEs at similar redshifts, an excess that is essentially independent of LAE luminosity. We also report on the discovery of two possible LAE group structures at z~4.4 and z~4.8 and investigate the effects of cosmic variance of LAEs on our results. Fitting a simple truncated single Gaussian model to a composite spectrum of the 13 high quality LAE candidates, we find a best-fit stellar velocity dispersion of 136 km/s. Additionally, we see modest evidence of a second peak in the composite spectrum, possibly caused by galactic outflows, as well as evidence for a non-trivial Lyman-alpha escape fraction. We find an average LAE star formation rate density (SFRD) of ~5x10^(-3) M_solar/yr/Mpc^3 with moderate evidence for negative evolution in the LAE SFRD from z~4.6 to z~5.7. We measure a best-fit luminosity function generally consistent with measurements from other surveys at similar epochs. Finally, we investigate any possible effects from weak or strong gravitational lensing induced by the foreground supercluster, finding that our LAE candidates are minimally affected by lensing processes. Comment: 31 pages, 21 figures, to be published in ApJ
05/2009;
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ABSTRACT: We have undertaken a multiwavelength project to study the relatively unknown properties of groups and poor clusters of galaxies at intermediate redshifts. In this paper, we describe the XMM-Newton observations of six X-ray-selected groups with 0.2 < z < 0.6. The X-ray properties of these systems are generally in good agreement with the properties of low-redshift groups. They appear to follow the scaling relations between luminosity, temperature, and velocity dispersion defined by low-redshift groups and clusters. The X-ray emission in four of the six groups is also centered on a dominant early-type galaxy. The lack of a bright elliptical galaxy at the peak of the group X-ray emission is rare at low redshifts, and the other two groups may be less dynamically evolved. We find indications of excess entropy in these systems over self-similar predictions out to large radii. We also confirm the presence of at least one X-ray-luminous AGN associated with a group member galaxy and find several other potential group AGNs.
The Astrophysical Journal 12/2008; 649(2):649. · 6.02 Impact Factor
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ABSTRACT: We present spectroscopic confirmation of nine moderate-redshift galaxy groups and poor clusters selected from the ROSAT Deep Cluster Survey. The groups span the redshift range z ~ 0.23-0.59 and have between 4 and 20 confirmed members. The velocity dispersions of these groups range from ~125 to 650 km s-1. Similar to X-ray groups at low redshift, these systems contain a significant number of early-type galaxies. Therefore, the trend for X-ray-luminous groups to have high early-type fractions is already in place by at least z ~ 0.5. In four of the nine groups, the X-ray emission is clearly peaked on the most luminous early-type galaxy in the group. However, in several cases the central galaxy is composed of multiple luminous nuclei, suggesting that the brightest group galaxy may still be undergoing major mergers. In at least three (and possibly five) of the groups in our sample, a dominant early-type galaxy is not found at the center of the group potential. This suggests that many of our groups are not dynamically evolved despite their high X-ray luminosities. While similar systems have been identified at low redshift, the X-ray luminosities of the intermediate-redshift examples are 1-3 orders of magnitude higher than those of their low-redshift counterparts. We suggest that this may be evidence for group downsizing: while massive groups are still in the process of collapsing and virializing at intermediate redshifts, only low-mass groups are in the process of forming at the present day.
The Astrophysical Journal 12/2008; 646(1):133. · 6.02 Impact Factor
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ABSTRACT: We present the results from a multiband optical imaging program that has definitively confirmed the existence of a supercluster at z ≈ 0.91. Two massive clusters of galaxies, Cl 1604+4304 at z = 0.897 and Cl 1604+4321 at z = 0.924, were originally observed in the high-redshift cluster survey of Oke, Postman, & Lubin. They are separated by 4300 km s-1 in radial velocity and 17' on the plane of the sky. Their physical and redshift proximity suggested a promising supercluster candidate. Deep BRi imaging of the region between the two clusters indicates a large population of red galaxies. This population forms a tight, red sequence in the color-magnitude diagram at (R-i) ≈ 1.4. The characteristic color is identical to that of the spectroscopically confirmed early-type galaxies in the two member clusters. The red galaxies are spread throughout the 5 h-1 Mpc region between Cl 1604+4304 and Cl 1604+4321. Their spatial distribution delineates the entire large-scale structure with high concentrations at the cluster centers. In addition, we detect a significant overdensity of red galaxies directly between Cl 1604+4304 and Cl 1604+4321 which is the signature of a third, rich cluster associated with this system. The strong sequence of red galaxies and their spatial distribution clearly indicate that we have discovered a supercluster at z ≈ 0.91.
The Astrophysical Journal 12/2008; 531(1):L5. · 6.02 Impact Factor
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ABSTRACT: 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 supercluster 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] Comment: 15 pages, 9 Figures, submitted to ApJ
09/2008;
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ABSTRACT: 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 2.5\sigma 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.
05/2008;
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ABSTRACT: We investigate the galaxy populations in seven X-ray selected, intermediate-redshift groups (0.2<z<0.6). Overall, the galaxy populations in these systems are similar to those in clusters at the same redshift; they have large fractions of early-type galaxies (f_e~70%) and small fractions of galaxies with significant star formation (f_[OII]~30%). We do not observe a strong evolution in the galaxy populations from those seen in X-ray luminous groups at low-redshift. Both f_e and f_[OII] are correlated with radius but do not reach the field value out to ~r_500. However, we find significant variation in the galaxy populations between groups with some groups having field-like populations. Comparisons between the morphological and spectral properties of group galaxies reveal both gas-poor mergers and a population of passive spirals. Unlike low-redshift, X-ray emitting groups, in some of these groups the brightest galaxy does not lie at the center of the X-ray emission, and in several of the groups that do have a central BGG, the BGG has multiple components. These groups appear to represent a range of evolutionary stages in the formation of the BGG. Some groups have relatively large central galaxy densities, and one group contains a string of seven bright galaxies within a radius of 200 kpc that have a lower velocity dispersion than the rest of the system. None of the central galaxies, including those with multiple components, have significant [OII] emission. These observations support a scenario in which BGGs are formed relatively late through gas-poor mergers.
12/2006;
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ABSTRACT: We present the first detailed X-ray observations of optically-selected clusters at high redshift. Two clusters, Cl 1324+3011 at z = 0.76 and Cl 1604+4304 at z = 0.90, were observed with XMM-Newton. The optical center of each cluster is coincident with an extended X-ray source whose emission is detected out to a radius of 0.5 Mpc. The emission from each cluster appears reasonably circular, with some indication of asymmetries and more complex morphologies. Similarly to other optically-selected clusters at redshifts of z > 0.4, both clusters are modest X-ray emitters with bolometric luminosities of only Lx = 1.4 - 2.0 x 10^(44) erg/s. We measure gas temperatures of T = 2.88 (+0.71/-0.49) keV for Cl 1324+3011 and 2.51 (+1.05/-0.69) keV for Cl 1604+4304. The X-ray properties of both clusters are consistent with the high-redshift Lx-T relation measured from X-ray-selected samples at z > 0.5. However, based on the local relations, their X-ray luminosities and temperatures are low for their measured velocity dispersions (sigma). The clusters are cooler by a factor of 2 - 9 compared to the local sigma-T relation. We briefly discuss the possible explanations for these results. Comment: 14 pages, 4 figures; accepted for publication in Astrophysical Journal Letters; version with full resolution figures available at http://bubba.ucdavis.edu/~lubin/xmm.pdf
12/2003;
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ABSTRACT: We have carried out additional spectroscopic observations in the field of cluster Cl 1324+3011 at z = 0.76. Combined with the spectroscopy presented in Postman, Lubin & Oke (2001, AJ, 122, 1125), we now have spectroscopically confirmed 47 cluster members. With this significant number of redshifts, we measure accurately the cluster velocity dispersion to be 1016 (+126/-93) km/s. The distribution of velocity offsets is consistent with a Gaussian, indicating no substantial velocity substructure. As previously noted for other optically-selected clusters at redshifts of z > 0.5, a comparison between the X-ray luminosity (L_x) and the velocity dispersion (sigma) of Cl 1324+3011 implies that this cluster is underluminous in X-rays by a factor of ~3 - 40 when compared to the L_x - sigma relation for local and moderate-redshift clusters. We also examine the morphologies of those cluster members which have available high-angular-resolution imaging with the Hubble Space Telescope (HST). There are 22 spectroscopically-confirmed cluster members within the HST field-of-view. Twelve of these are visually classified as early-type (elliptical or S0) galaxies, implying an early-type fraction of 0.55 (+0.17/-0.14) in this cluster. This fraction is a factor of ~1.5 lower than that observed in nearby rich clusters. Confirming previous cluster studies, the results for cluster Cl 1324+3011, combined with morphological studies of other massive clusters at redshifts less than z = 1, suggest that the galaxy population in massive clusters is strongly evolving with redshift. This evolution implies that early-type galaxies are forming out of the excess of late-type (spiral, irregular, and peculiar) galaxies over the ~7 Gyr timescale. Comment: 22 pages, 7 figures; accepted for publication in Astronomical Journal
06/2002;
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ABSTRACT: We present an extensive photometric and spectroscopic study of two high-redshift clusters of galaxies based on data obtained from the Keck 10m telescopes and the Hubble Space Telescope. The clusters CL0023+0423 (z=0.84) and CL1604+4304 (z=0.90) are part of a multi-wavelength program to study nine candidate clusters at z > 0.6 (Oke, Postman & Lubin 1998). Based on these observations, we study in detail both the field and cluster populations. From the confirmed cluster members, we find that CL0023+0423 actually consists of two components separated by ~2900 km/s. A kinematic analysis indicates that the two components are a poor cluster with ~3 x 10^{14} solar masses and a less massive group with 10^{13} solar masses. CL1604+4304 is a centrally concentrated, rich cluster at z = 0.8967 with a velocity dispersion of 1226 km/s and a mass of ~3 x 10^{15} solar masses. Approximately 57% and 50% of the galaxies show high levels of star formation in CL0023+0423 and CL1604+4304, respectively. These numbers are significantly larger than those found in intermediate redshift clusters. We also observe many old, red galaxies. Found mainly in CL1604+4304, they have spectra consistent with passive stellar evolution, typical of the early-type galaxies in low and intermediate-redshift clusters. We have calculated their ages by comparing their spectral energy distributions to standard Bruzual & Charlot evolutionary models. We find that their colors are consistent with models having an exponentially decreasing star formation rate with a time constant of 0.6 Gyr. We observe a significant luminosity brightening in our brightest cluster galaxies. Compared to BCGs at z < 0.1, we find a luminosity increase of ~1 mag in the rest B-band and ~0.8 mag in the rest V-band. Comment: 65 pages, 21 figures. (Two are large plate images available from the lead author). Accepted for publication in the Astronomical Journal
05/1998;
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Lori M. Lubin
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ABSTRACT: We present a color analysis of the galaxy populations of candidate clusters of galaxies from the Palomar Distant Cluster Survey (Postman et al.\ 1996). The survey was conducted in two broad band filters that closely match $V$ and $I$ and contains a total of 79 candidate clusters of galaxies, covering an estimated redshift range $0.2 \simless z \simless 1.2$. We examine the color evolution in the 57 richest clusters from this survey. The intermediate redshift ($0.2 \simless z \simless 0.4$) clusters show a distinct locus of galaxy colors in the color--magnitude diagram. This ridge line corresponds well with the expected no--evolution color of present--day elliptical galaxies at these redshifts. In clusters at redshifts of $z \simgreat 0.5$, this red envelope has shifted bluewards compared to the ``no--evolution'' prediction. By $z \sim 0.8$ there are only a few galaxies which are as red in their rest-frame as present--day ellipticals. The detected evolution is consistent with passive aging of stellar populations formed at redshifts of $z \simgreat 2$. Though the uncertainties are large, the Butcher--Oemler effect is observed in the Palomar clusters. The fraction of blue galaxies increases with the estimated redshift of the cluster at a 96.2\% confidence level. The measured blue fractions of the intermediate redshift clusters ($f_{b} \sim 0.2 - 0.3$) are consistent with those found previously by Butcher \& Oemler (1984). The trend in the Palomar clusters suggests that $f_{b}$ can be greater than 0.4 in clusters of galaxies at redshifts of $z \simgreat 0.6$. Comment: Latex, 28 pages, aaspp.sty, 11 figures and epsf.sty included, appended as a uuencoded, gzipped tar file, accepted for publication in the Astronomical Journal
04/1996;
