91 research outputs found
The Palomar Distant Cluster Survey : III. The Colors of the Cluster Galaxies
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 and
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
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 () are consistent with
those found previously by Butcher \& Oemler (1984). The trend in the Palomar
clusters suggests that 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 Journa
The Tolman Surface Brightness Test for the Reality of the Expansion. III. HST Profile and Surface Brightness Data for Early-Type Galaxies in Three High-Redshift Clusters
Photometric data for 34 early-type galaxies in the three high-redshift
clusters Cl 1324+3011 (z = 0.76), Cl 1604+4304 (z = 0.90), and Cl 1604+4321 (z
= 0.92), observed with the Hubble Space Telescope (HST) and with the Keck
10-meter telescopes by Oke, Postman & Lubin, are analyzed to obtain the
photometric parameters of mean surface brightness, magnitudes for the growth
curves, and angular radii at various Petrosian eta radii. The angular radii at
eta = 1.3 mag for the program galaxies are all larger than 0.24". All of the
galaxies are well resolved at this angular size using HST whose point-spread
function is 0.05", half width at half maximum. The data for each of the program
galaxies are listed at eta = 1.0, 1.3, 1.5, 1.7, and 2.0 mag. They are
corrected by color equations and K terms for the effects of redshift to the
rest-frame Cape/Cousins I for Cl 1324+3011 and Cl 1604+4304 and R for Cl
1604+4321. The K corrections are calculated from synthetic spectral energy
distributions derived from evolving stellar population models of Bruzual &
Charlot which have been fitted to the observed broad-band (BVRI) AB magnitudes
of each program galaxy. The listed photometric data are independent of all
cosmological parameters. They are the source data for the Tolman surface
brightness test made in Paper IV.Comment: 17 pages, 7 figures; accepted for publication in the Astronomical
Journa
The Tolman Surface Brightness Test for the Reality of the Expansion. II. The Effect of the Point-Spread Function and Galaxy Ellipticity on the Derived Photometric Parameters
To complete the Tolman surface brightness test on the reality of the
expansion of the Universe, we need to measure accurately the surface brightness
profiles of the high-redshift galaxy sample. We, therefore, investigate the
effects of various sizes of point-spread-functions composed of telescope
diffraction, CCD pixel resolutions, and ground-based seeing on the measurements
of mean surface brightness. We have done the calculations using two synthetic
galaxies of effective radii of 0.70" and 0.25" with point-spread functions of
0.1, 0.3, and 0.9 arcseconds. We have also compared actual observations of
three high-redshift galaxies in the cluster Cl 1324 + 3011 (z = 0.76) made both
with the Keck telescopes in seeing of about 0.9" and with HST which has a PSF
that is approximately ten times smaller. The conclusion is that HST data can be
used as far into the galaxy image as a Petrosian metric radius of eta = 1.3
magnitudes, whereas the ground-based data will have systematic errors of up to
2.9 magnitudes in the mean surface brightness at eta values of less than 2.2
magnitudes. In the final section, we compare the differences in derived average
surface brightness for nearly circular galaxy images compared with highly
flattened images. The comparison is made by using the two reduction procedures
of (1) integrating the profile curves using circular apertures, and (2)
approximating an ``equivalent circular'' galaxy that is highly elongated by
using an ``effective'' radius of sqrt{ab}, where a and b are the semi-major and
semi-minor axis, respectively, of the best-fitting ellipse. The conclusion is
that the two methods of reduction give nearly identical results and that either
method can be used to analyze the low and high-redshift galaxy samples used in
the Tolman test.Comment: 15 pages, 9 figures; accepted for publication in Astronomical Journa
Keck Spectroscopy of the Gravitational Lens System PG 1115+080: Redshifts of the Lensing Galaxies
The quadruple system PG 1115+080 is the second gravitational lens with a
reported measurement of the Hubble constant. In addition to the primary lens,
three nearby galaxies are believed to contribute significantly to the lensing
potential. In this paper we report accurate redshifts for all four galaxies and
show that they belong to a single group at z_d = 0.311. This group has very
similar properties to Hickson's compact groups of galaxies found at lower
redshifts. We briefly discuss implications for the existing lens models and
derive H_0 = 52 +/- 14 km/s/Mpc.Comment: revised to use the updated model of Keeton & Kochanek
(astro-ph/9611216) and to correct the velocity dispersion of the group; 10
pages including 2 eps figures and 2 tables. Submitted to the Astronomical
Journa
Resolving the Beta-Discrepancy for Clusters of Galaxies
Previous comparisons of optical and X-ray observations of clusters of
galaxies have lead to the so-called `` - discrepancy'' that has
persisted for the last decade. The standard hydrostatic-isothermal model for
clusters predicts that the parameter , which describes the ratio of energy per unit mass in galaxies to that
in the gas, should equal the parameter (where ) determined from the X-ray surface
brightness distribution. The observations suggest an apparent discrepancy :
(i.e., the galaxies are ``hotter'' than the gas) while
(i.e., the gas is ``hotter'' and more extended than the
galaxies). Here we show that the discrepancy is resolved when the actual
observed galaxy distribution in clusters is used, , instead of the previously assumed steeper King
approximation, . Using a large sample of
clusters, we find best-fit mean values of and
. These
results resolve the - discrepancy and provide support for the
hydrostatic cluster model.Comment: (to appear in ApJ May 10, 1994), 10 pages of TeX, 3 figures available
on request to [email protected]
The Violent Youth of Bright and Massive Cluster Galaxies and their Maturation over 7 Billion Years
In this study we investigate the formation and evolution mechanisms of the
brightest cluster galaxies (BCGs) over cosmic time. At high redshift
(), we selected BCGs and most massive cluster galaxies (MMCGs) from
the Cl1604 supercluster and compared them to low-redshift ()
counterparts drawn from the MCXC meta-catalog, supplemented by SDSS imaging and
spectroscopy. We observed striking differences in the morphological, color,
spectral, and stellar mass properties of the BCGs/MMCGs in the two samples.
High-redshift BCGs/MMCGs were, in many cases, star-forming, late-type galaxies,
with blue broadband colors, properties largely absent amongst the low-redshift
BCGs/MMCGs. The stellar mass of BCGs was found to increase by an average factor
of from to . Through this and other
comparisons we conclude that a combination of major merging (mainly wet or
mixed) and \emph{in situ} star formation are the main mechanisms which build
stellar mass in BCGs/MMCGs. The stellar mass growth of the BCGs/MMCGs also
appears to grow in lockstep with both the stellar baryonic and total mass of
the cluster. Additionally, BCGs/MMCGs were found to grow in size, on average, a
factor of , while their average S\'ersic index increased by 0.45
from to , also supporting a scenario involving major
merging, though some adiabatic expansion is required. These observational
results are compared to both models and simulations to further explore the
implications on processes which shape and evolve BCGs/MMCGs over the past
7 Gyr.Comment: Accepted for publication in MNRA
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
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
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