624 research outputs found
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
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
Measuring Cluster Temperature Profiles with XMM/EPIC
Using the PV observation of A1795, we illustrate the capability of XMM-EPIC
to measure cluster temperature profiles, a key ingredient for the determination
of cluster mass profiles through the equation of hydrostatic equilibrium. We
develop a methodology for spatially resolved spectroscopy of extended sources,
adapted to XMM background and vignetting characteristics. The effect of the
particle induced background is discussed. A simple unbiased method is proposed
to correct for vignetting effects, in which every photon is weighted according
to its energy and location on the detector. We were able to derive the
temperature profile of A1795 up to 0.4 times the virial radius. A significant
and spatially resolved drop in temperature towards the center (r<200 kpc) is
observed, which corresponds to the cooling flow region of the cluster. Beyond
that region, the temperature is constant with no indication of a fall-off at
large radii out to 1.2 Mpc.Comment: 7 pages, 8 figures, Accepted for publication in A&A (special Letter
issue on XMM
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
The discovery of diffuse steep spectrum sources in Abell 2256
Context: Hierarchical galaxy formation models indicate that during their
lifetime galaxy clusters undergo several mergers. Here we report on the
discovery of three diffuse radio sources in the periphery of Abell 2256, using
the GMRT.
Aims: The aim of the observations was to search for diffuse ultra-steep
spectrum radio sources within the galaxy cluster Abell 2256.
Methods: We have carried out GMRT 325 MHz radio continuum observations of
Abell 2256. V, R and I band images of the cluster were taken with the 4.2m WHT.
Results: We have discovered three diffuse elongated radio sources located
about 1 Mpc from the cluster center. Two are located to the west of the cluster
center, and one to the southeast. The sources have a measured physical extent
of 170, 140 and 240 kpc, respectively. The two western sources are also visible
in deep low-resolution 115-165 MHz WSRT images, although they are blended into
a single source. For the combined emission of the blended source we find an
extreme spectral index of -2.05\pm 0.14 between 140 and 351 MHz. The extremely
steep spectral index suggests these two sources are most likely the result of
adiabatic compression of fossil radio plasma due to merger shocks.
Conclusions: The discovery of the steep spectrum sources implies the
existence of a population of faint diffuse radio sources in (merging) clusters
with such steep spectra that they have gone unnoticed in higher frequency
(\gtrsim 1 GHz) observations. An exciting possibility therefore is that such
sources will determine the general appearance of clusters in low-frequency high
resolution radio maps as will be produced by for example LOFAR or LWA.Comment: 5 pages, 2 figures, accepted for publication in A&A on October 16,
200
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
TeX fil
Non-thermal Origin of the EUV and Soft X-rays from the Coma Cluster - Cosmic Rays in Equipartition with the Thermal Medium
The role of cosmic rays (CR) in the formation and evolution of clusters of
galaxies has been much debated. It may well be related to other fundamental
questions, such as the mechanism which heats and virializes the intracluster
medium (ICM), and the frequency at which the ICM is shocked. There is now
compelling evidence both from the cluster soft excess (CSE) and the `hard-tail'
emissions at energies above 10 keV, that many clusters are luminous sources of
inverse-Compton (IC) emission. This is the first direct measurement of cluster
CR: the technique is free from our uncertainties in the ICM magnetic field, and
is not limited to the small subset of clusters which exhibit radio halos. The
CSE emitting electrons fall within a crucial decade of energy where they have
the least spectral evolution, and where most of the CR pressure resides.
However their survival times do not date them back to the relic CR population.
By using the CSE data of the Coma cluster, we demonstrate that the CR are
energetically as important as the thermal ICM: the two components are in
pressure equiparition. Thus, contrary to previous expectations, CR are a
dominant component of the ICM, and their origin and effects should be explored.
The best-fit CR spectral index is in agreement with the Galactic value.Comment: ApJ accepted; 10 pages LaTeX; 2 figures and 1 table in PostScrip
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
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
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