1,618 research outputs found
Cluster Evolution in the ROSAT North Ecliptic Pole Survey
The deepest region of the ROSAT All-Sky Survey, at the North Ecliptic Pole,
has been studied to produce a complete and unbiased X-ray selected sample of
clusters of galaxies. This sample is used to investigate the nature of cluster
evolution and explore potential implications for large-scale structure models.
The survey is 99.6% optically identified. Spectroscopic redshifts have been
measured for all the extragalactic identifications. In this Letter, first
results on cluster evolution are presented based on a comparison between the
number of the observed clusters in the North Ecliptic Pole survey and the
number of expected clusters assuming no-evolution models. At z>0.3 there is a
deficit of clusters with respect to the local universe which is significant at
> 4.7sigma. The evolution appears to commence at L_{0.5-2.0} > 1.8x10^{44} erg
s^{-1} in our data. The negative evolution goes in the same direction as the
original EMSS result, the results from the 160 deg^{2} survey by Vikhlinin et
al. (1998) and the recent results from the RDCS (Rosati et al. 2000). At lower
redshifts there is no evidence for evolution, a result in agreement with these
and other cluster surveys.Comment: 17 pages, 3 figures. Accepted for publication in ApJ Letter
Synthesis of ATP by an artificially imposed electrochemical proton gradient in chromaffin granule ghosts
On the Influence of X-Ray Galaxy Clusters in the Fluctuations of the Cosmic Microwave Background
The negative evolution found in X--ray clusters of galaxies limits the amount
of available hot gas for the inverse Compton scattering of the Cosmic Microwave
Background (the Sunyaev--Zel'dovich effect). Using a parametrisation of the
X-ray luminosity function and its evolution in terms of a coalescence model (as
presented in the analysis of a flux limited X-ray cluster sample by Edge et al.
1990), as well as a simple virialised structure for the clusters (which
requires a gas to total mass fraction \approxgt 0.1 in order to reproduce
observed properties of nearby clusters) we show that the Compton distortion
parameter is about two orders of magnitude below the current FIRAS upper
limits. Concerning the anisotropies imprinted on arcmin scales they are
dominated by the hottest undetected objects. We show that they are negligible
({\Delta T\over T}\approxlt 10^{-7}) at wavelengths \lambda\approxgt 1~mm.
At shorter wavelengths they become more important ( at ~mm), but in fact most clusters will produce an
isolated and detectable feature in sky maps. After removal of these signals,
the fluctuations imprinted by the remaining clusters on the residual radiation
are still much smaller. The conclusion is that X-ray clusters can be ignored as
sources of Cosmic Microwave Background fluctuations.Comment: 20 pages Plain Tex, 7 figures available upon request,UCAST-94-0
The WARPS survey: III. The discovery of an X-ray luminous galaxy cluster at z=0.833 and the impact of X-ray substructure on cluster abundance measurements
The WARPS team reviews the properties and history of discovery of
ClJ0152.7-1357, an X-ray luminous, rich cluster of galaxies at z=0.833. At L_X
= 8 x 10^44 h^(-2) erg/s (0.5-2.0 keV) ClJ0152.7-1357 is the most X-ray
luminous cluster known at redshifts z>0.55. The high X-ray luminosity of the
system suggests that massive clusters may begin to form at redshifts
considerably greater than unity. This scenario is supported by the high degree
of optical and X-ray substructure in ClJ0152.7-1357, which is similarly complex
as that of other X-ray selected distant clusters and consistent with the
picture of cluster formation by mass infall along large-scale filaments. X-ray
emission from ClJ0152.7-1357 was detected already in 1980 with the EINSTEIN
IPC. However, because the complex morphology of the emission caused its
significance to be underestimated, the corresponding source was not included in
the EMSS cluster sample and hence not previously identified. Simulations of the
EMSS source detection and selection procedure suggest a general bias of the
EMSS against X-ray luminous clusters with pronounced substructure. If highly
unrelaxed, merging clusters are common at high redshift, they could create a
bias in some samples as the morphological complexity of mergers may cause them
to fall below the flux limit of surveys that assume a unimodal spatial source
geometry. Conversely, the enhanced X-ray luminosity of mergers might cause them
to, temporarily, rise above the flux limit. Either effect could lead to
erroneous conclusions about the evolution of the comoving cluster space
density. A high fraction of morphologically complex clusters at high redshift
would also call into question the validity of cosmological studies that assume
that the systems under investigation are virialized.Comment: 17 pages, 7 figures; revised to focus on possible detection biases
caused by substructure in clusters; accepted for publication in ApJ; uses
emulateapj.sty; eps files of figures 1 and 2 can be obtained from
ftp://hubble.ifa.hawaii.edu/pub/ebeling/warp
Determining Gravitational Masses of Galaxy Clusters With (1)Virial Equilibrium And (2)Arc-like Images
The mass derived from gravitational lensing reflects the total mass contained
in the lensing system, independent of the specific matter contents and states.
A comparison of the dynamical masses from hydrostatic equilibrium with the
gravitational masses from arc-like images of background galaxies is made for
four clusters of galaxies at intermediate redshits. It is found that virial
analysis has underestimated the total cluster masses (from lensing) by a factor
of within a radius of Mpc around the cluster
centers, indicating that clusters of galaxies might not be regarded as the well
relaxed virialized systems. The increase of the total cluster masses obtained
from lensing leads to the decrease of the baryon fractions of clusters of
galaxies, which provides a crue for solving the `` disprepancy
puzzle" in cosmology.Comment: 11 pages plus 1 Table. LATEX style, submitted to ApJ, BAO-BGGC-940
Galaxy Aggregates in the Coma Cluster
We present evidence for a new morphologically defined form of small-scale
substructure in the Coma Cluster, which we call galaxy aggregates. These
aggregates are dominated by a central galaxy, which is on average three
magnitudes brighter than the smaller aggregate members nearly all of which lie
to one side of the central galaxy. We have found three such galaxy aggregates
dominated by the S0 galaxies RB 55, RB 60, and the star-bursting SBb, NGC 4858.
RB 55 and RB 60 are both equi-distant between the two dominate D galaxies NGC
4874 and NGC 4889, while NGC 4858 is located next to the larger E0 galaxy NGC
4860. All three central galaxies have redshifts consistant with Coma Cluster
membership. We describe the spatial structures of these unique objects and
suggest several possible mechanisms to explain their origin. These include:
chance superpositions from background galaxies, interactions between other
galaxies and with the cluster gravitational potential, and ram pressure. We
conclude that the most probable scenario of creation is an interaction with the
cluster through its potential.Comment: Ten Pages with six figures; submitted to MNRAS letter
The Butcher-Oemler Effect at Moderate Redshift
We present the results of Butcher-Oemler-style analysis of three moderate-
redshift (0.1<z<0.2) clusters which have bimodal X-ray surface brightness
profiles. We find that at least two of these clusters exhibit unusually high
fractions of blue galaxies as compared to clusters at comparable redshifts
studied by Butcher and Oemler (1984). This implies that star formation is
occurring in a high fraction of the galaxies in the two clusters. Our results
are consistent with hierarchical clustering models in which subcluster-
subcluster mergers create shocks in the intracluster medium. The shocks, in
turn, induce simultaneous starbursts in a large fraction of cluster galaxies.
Our study therefore lends weight to the hypothesis that the Butcher-Oemler
effect is an environmental, as well as evolutionary, phenomenon.Comment: 22 pages, 8 figures; accepted for publication in A
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