383 research outputs found
X-ray Properties of the Abell 644 Cluster of Galaxies
We use new ASCA observations and archival ROSAT Position Sensitive
Proportional Counter (PSPC) data to determine the X-ray spectral properties of
the intracluster gas in Abell 644. From the overall spectrum, we determine the
average gas temperature to be 8.64 (+0.67,-0.56) keV, and an abundance of 0.32
(+/-0.04) . The global ASCA and ROSAT spectra imply a cooling rate
of 214 (+100,-91) yr. The PSPC X-ray surface brightness
profile and the ASCA data suggest a somewhat higher cooling rate. We determine
the gravitational mass and gas mass as a function of radius. The total
gravitating mass within 1.2 Mpc is , of which 20%
is in the form of hot gas. There is a region of elevated temperature 1.5-5
arcmin to the west of the cluster center. The south-southwest region of the
cluster also shows excess emission in the ROSAT PSPC X-ray image, aligned with
the major axis of the optical cD galaxy in the center of the cluster. We argue
that the cluster is undergoing or has recently undergone a minor merger. The
combination of a fairly strong cooling flow and evidence for a merger make this
cluster an interesting case to test the disruption of cooling flow in mergers.Comment: 26 pages LaTeX including 9 eps figures + 4 pages LaTeX tables
(landscape); accepted to ApJ, uses aaspp
The North Ecliptic Pole Supercluster
We have used the ROSAT All-Sky Survey to detect a known supercluster at
z=0.087 in the North Ecliptic Pole region. The X-ray data greatly improve our
understanding of this supercluster's characteristics, approximately doubling
our knowledge of the structure's spatial extent and tripling the cluster/group
membership compared to the optical discovery data. The supercluster is a rich
structure consisting of at least 21 galaxy clusters and groups, 12 AGN, 61 IRAS
galaxies, and various other objects. A majority of these components were
discovered with the X-ray data, but the supercluster is also robustly detected
in optical, IR, and UV wavebands. Extending 129 x 102 x 67 (1/h50 Mpc)^3, the
North Ecliptic Pole Supercluster has a flattened shape oriented nearly edge-on
to our line-of-sight. Owing to the softness of the ROSAT X-ray passband and the
deep exposure over a large solid angle, we have detected for the first time a
significant population of X-ray emitting galaxy groups in a supercluster. These
results demonstrate the effectiveness of X-ray observations with contiguous
coverage for studying structure in the Universe.Comment: Accepted for publication in The Astrophysical Journal; 5 pages with 2
embedded figures; uses emulateapj.sty; For associated animations, see
http://www.ifa.hawaii.edu/~mullis/nep3d.html; A high-resolution color
postscript version of the full paper is available at
http://www.ifa.hawaii.edu/~mullis/papers/nepsc.ps.g
The ROSAT North Ecliptic Pole Survey: The Optical Identifications
The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All Sky
Survey have been used to construct a contiguous area survey consisting of a
sample of 445 individual X-ray sources above a flux of ~2x10^-14 erg cm^-2 s^-1
in the 0.5-2.0 keV energy band. The NEP survey is centered at RA (2000) = 18h
00m, DEC(2000) = +66deg 33arcmin and covers a region of 80.7 sq. deg at a
moderate Galactic latitude of b = 29.8deg. Hence, the NEP survey is as deep and
covers a comparable solid angle to the ROSAT serendipitous surveys, but is also
contiguous. We have identified 99.6% of the sources and determined redshifts
for the extragalactic objects. In this paper we present the optical
identifications of the NEP catalog of X-ray sources including basic X-ray data
and properties of the sources. We also describe with some detail the optical
identification procedure. The classification of the optical counterparts to the
NEP sources is very similar to that of previous surveys, in particular the
Einstein Extended Medium Sensitivity Survey (EMSS). The main constituents of
the catalog are active galactic nuclei (~49%), either type 1 or type 2
according to the broadness of their permitted emission lines. Stellar
counterparts are the second most common identification class (~34%). Clusters
and groups of galaxies comprise 14%, and BL Lacertae objects 2%. One non-AGN
galaxy, and one planetary nebula have also been found. The NEP catalog of X-ray
sources is a homogeneous sample of astronomical objects featuring complete
optical identification.Comment: Accepted for publication in the ApJS; 33 pages including 12
postscript figures and 3 tables; uses emulateapj.sty. On-line source catalog
at http://www.eso.org/~cmullis/research/nep-catalog.htm
A Richness Study of 14 Distant X-ray Clusters From the 160 Square Degree Survey
We have measured the surface density of galaxies toward 14 X-ray-selected
cluster candidates at redshifts greater than z=0.46, and we show that they are
associated with rich galaxy concentrations. We find that the clusters range
between Abell richness classes 0-2, and have a most probable richness class of
one. We compare the richness distribution of our distant clusters to those for
three samples of nearby clusters with similar X-ray luminosities. We find that
the nearby and distant samples have similar richness distributions, which shows
that clusters have apparently not evolved substantially in richness since
redshift z =0.5. We compare the distribution of distant X-ray clusters in the
L_x--richness plane to the distribution of optically-selected clusters from the
Palomar Distant Cluster Survey. The optically-selected clusters appear overly
rich for their X-ray luminosities when compared to X-ray-selected clusters.
Apparently, X-ray and optical surveys do not necessarily sample identical mass
concentrations at large redshifts. This may indicate the existence of a
population of optically rich clusters with anomalously low X-ray emission. More
likely, however, it reflects the tendency for optical surveys to select
unvirialized mass concentrations, as might be expected when peering along
large-scale filaments.Comment: The abstract has been abridged. Accepted for publication in the
Astrophysical Journa
The X-ray Luminosity Function of Bright Clusters in the Local Universe
We present the X-ray luminosity function (XLF) for clusters of galaxies
derived from the RASS1 Bright Sample. The sample, selected from the ROSAT
All-Sky Survey in a region of 2.5 sr within the southern Galactic cap, contains
130 clusters with flux limits in the range ~ 3-4 x 10^-12 ergs/cm^2/s in the
0.5-2.0 keV band. A maximum-likelihood fit with a Schechter function of the XLF
over the entire range of luminosities (0.045 - 28. x 10^44 ergs/s), gives alpha
= 1.52 +/- 0.11, L_* = 3.80 +0.70 -0.55 x 10^44 ergs/s, and A = 5.07 +/- 0.45 x
10^-7 Mpc^-3 (10^44 ergs/s)^(\alpha-1). We investigate possible evolutionary
effects within the sample, out to our redshift limit (z ~ 0.3), finding no
evidence for evolution. Our results are in good agreement with other local
estimates of the XLF, implying that this statistic for the local universe is
now well determined. Comparison with XLFs for distant clusters (0.3 < z < 0.6),
shows that no evolution is present for L_X < 10^{44} ergs/s. However, we detect
differences at the 3 sigma level, between our local XLF and the distant one
estimated by Henry et al. for the EMSS sample. This difference is still present
when considering the EMSS sample revised by Nichol et al.Comment: 13 pages with 3 figures included, LaTex, aaspp4.sty and epsf.sty,
accepted for publication in ApJ Letters, only minor changes, added reference
ROSAT PSPC Observations of the Richest () ACO Clusters
We have compiled an X-ray catalog of optically selected rich clusters of
galaxies observed by the PSPC during the pointed GO phase of the ROSAT mission.
This paper contains a systematic X-ray analysis of 150 clusters with an optical
richness classification of from the ACO catalog (Abell, Corwin, and
Olowin 1989). All clusters were observed within 45' of the optical axis of the
telescope during pointed PSPC observations. For each cluster, we calculate: the
net 0.5-2.0 keV PSPC count rate (or upper limit) in a 1 Mpc radius
aperture, 0.5-2.0 keV flux and luminosity, bolometric luminosity, and X-ray
centroid. The cluster sample is then used to examine correlations between the
X-ray and optical properties of clusters, derive the X-ray luminosity function
of clusters with different optical classifications, and obtain a quantitative
estimate of contamination (i.e, the fraction of clusters with an optical
richness significantly overestimated due to interloping galaxies) in the ACO
catalog
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
A Hot Spot in Coma
We study the temperature structure of the central part (r<18' ~0.7 h50**-1
Mpc) of the Coma cluster of galaxies using ASCA data. Two different analysis
methods produce results in good agreement with each other and reveal the
presence of interesting structures in the gas temperature distribution.
Globally, the average temperature in the center of the cluster is 9.0 +/- 0.6
keV in good agreement with previous results. Superimposed on this, we find a
cool area with temperatures of 4-6 keV associated with a filament of X-ray
emission extending southeast from the cluster center detected by Vikhlinin and
coworkers. We also find a hot spot with a temperature of around 13 keV
displaced north from the central peak of emission. The distribution of the gas
temperatures and relative specific entropies suggests that the cool features
are most likely gas stripped from a galaxy group centered on NGC 4874 falling
toward the core from outside, while the hot spot located ``ahead'' of this
in-falling gas is due to shock heating. Thus our results suggest that we are
observing Coma during a minor merger with a small group of galaxies associated
with NGC 4874 shortly before the initial core passage.Comment: 5 pages, 2 figures, accepted for publication in the Astrophysical
Journa
A Multiwavelength Study of the Extreme AGN J2310-437
We present new X-ray, radio, and optical data for the unusual galaxy/cluster
system J2310-437. Our results confirm the presence of an active nucleus, and
suggest an interpretation as an anomalous BL Lac object of bulk relativistic
Doppler factor < 2, with an optically deficient radio-to-X-ray spectrum. The
radio, optical, and soft X-ray flux densities could lie along a single
power-law function, lacking the curvature typical of BL Lac objects. Compared
with other known sources that may have comparable multifrequency spectra,
J2310-437 is the most extreme. Its low isotropic optical/UV radiation is
consistent with the intensity of external photons governing the electron
spectral break through Compton cooling; in this source the external photon
density would be too low to produce a spectral break below the X-ray.Comment: 19 pages, including 10 figures, using emulateapj.sty and
apjfonts.sty. To appear in the Ap
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