188 research outputs found
An X-ray and Optical Study of Matter Distribution in the Galaxy Cluster A 2319
A new analysis of velocity distribution, optical photometry and X-ray surface
brightness from ROSAT PSPC data of the galaxy cluster A 2319 is presented. The
temperature profile derived from ASCA data (Markevitch et al.,1996) is taken
into account. A method to check the hydrostatic model in the presence of a
temperature gradient is proposed. Consistency of the hydrostatic isothermal
model and the explanation of the "beta-discrepancy" are discussed. Galaxy and
gas density profiles of the main component A 2319A are derived, allowing for
the effect of the secondary component A 2319B. The inadequacy of a polytropic
model, which would produce a binding mass decrease with respect to the
isothermal beta-model, is discussed. A simple interpolation of the temperature
profile provides instead an increase of the binding mass and a lower baryon
fraction thus mitigating the "baryon catastrophe". Assuming as typical the
value f_b ~ 0.2, a comparison with the most recent estimate of Omega_b(nucl)
implies for the cosmological parameter Omega_o less than 0.4.Comment: 7 pages, 2 tables, 8 figure
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
Is the Butcher-Oemler effect a function of the cluster redshift ?
Using PSPC {\it Rosat} data, we measure x-ray surface brightness profiles,
size and luminosity of the Butcher-Oemler (BO) sample of clusters of galaxies.
The cluster x-ray size, as measured by the Petrosian r_{\eta=2} radius, does
not change with redshift and is independent from x-ray luminosity. On the other
hand, the x-ray luminosity increases with redshift. Considering that fair
samples show no-evolution, or negative luminosity evolution, we conclude that
the BO sample is not formed from the same class of objects observed at
different look-back times. This is in conflict with the usual interpretation of
the Butcher-Oemler as an evolutionary (or redshift-dependent) effect, based on
the assumption that we are comparing the same class of objects at different
redshifts. Other trends present in the BO sample reflect selection criteria
rather than differences in look-back time, as independently confirmed by the
fact that trends loose strength when we enlarge the sample with x-ray selected
sample of clusters. The variety of optical sizes and shapes of the clusters in
the Butcher-Oemler sample, and the Malmquist-like bias, are the reasons for
these selection effects that mimic the trends usually interpreted as changes
due to evolution.Comment: ApJ, in press, scheduled on May, 10 issue. 17 pages & 11 figure
Off-Center Collisions between Clusters of Galaxies
We present numerical simulations of off-center collisions between galaxy
clusters made using a new hydrodynamical code based on the piecewise-parabolic
method (PPM) and an isolated multigrid Poisson solver. We have performed three
gas-only high-resolution simulations of collisions between equal-mass clusters
with different values of the impact parameter (0, 5, and 10 times the core
radius). With these simulations we have studied the observational appearance of
the merging clusters and the variation in equilibration time, luminosity
enhancement during the collision, and structure of the merger remnant with
varying impact parameter. Observational evidence of an ongoing collision is
present for 1-2 sound crossing times after the collision, but only for special
viewing angles. The remnant actually requires at least five crossing times to
reach virial equilibrium. Since the sound crossing time can be as large as 1-2
Gyr, the equilibration time can thus be a large fraction of the age of the
universe. The final merger remnant is very similar for impact parameters of
zero and five core radii. It possesses a roughly isothermal core, with central
density and temperature twice the initial values. Outside the core the
temperature drops as r^-1, and the density roughly as r^-3.8. The core radius
shows a small increase due to shock heating during the merger. For an impact
parameter of ten core radii the core of the remnant possesses a more flattened
density profile, with a steeper dropoff outside the core. In both off-center
cases the merger remnant rotates, but only for the ten-core-radius case does
this appear to have an effect on the structure of the remnant.Comment: 26 pages, 15 figures included, submitted to ApJ; for color figures
and movies see http://www.astro.virginia.edu/~pmr7u/paper_offctr.htm
Extragalactic Foregrounds of the Cosmic Microwave Background: Prospects for the MAP Mission
(Abridged) While the major contribution to the Cosmic Microwave Background
(CMB) anisotropies are the sought-after primordial fluctuations produced at the
surface of last scattering, other effects produce secondary fluctuations at
lower redshifts. Here, we study the extragalactic foregrounds of the CMB in the
context of the upcoming MAP mission. We first survey the major extragalactic
foregrounds and show that discrete sources, the Sunyaev-Zel'dovich (SZ) effect,
and gravitational lensing are the most dominant ones for MAP. We then show that
MAP will detect (>5 sigma) about 46 discrete sources and 10 SZ clusters
directly with 94 GHz fluxes above 2 Jy. The mean SZ fluxes of fainter clusters
can be probed by cross-correlating MAP with cluster positions extracted from
existing catalogs. For instance, a MAP-XBACs cross-correlation will be
sensitive to clusters with S(94GHz)>200mJy, and will thus provide a test of
their virialization state and a measurement of their gas fraction. Finally, we
consider probing the hot gas on supercluster scales by cross-correlating the
CMB with galaxy catalogs. Assuming that galaxies trace the gas, we show that a
cross-correlation between MAP and the APM catalog should yield a marginal
detection, or at least a four-fold improvement on the COBE upper limits for the
rms Compton y-parameter.Comment: 27 LaTeX pages, including 5 ps figures and 2 tables. To appear in
ApJ. Minor revisions to match accepted version. Color figures and further
links available at http://www.astro.princeton.edu/~refreg
A Flux-limited Sample of Bright Clusters of Galaxies from the Southern Part of the ROSAT All-Sky Survey: the Catalog and the LogN-LogS
We describe the selection of an X-ray flux-limited sample of bright clusters
of galaxies in the southern hemisphere, based on the first analysis of the
ROSAT All-Sky Survey data (RASS1). The sample is constructed starting from an
identification of candidate clusters in RASS1, and their X-ray fluxes are
remeasured using the Steepness Ratio Technique. This method is better suited
than the RASS1 standard algorithm for measuring flux from extended sources. The
final sample is count-rate-limited in the ROSAT hard band (0.5-2.0 keV), so
that due to the distribution of NH, its effective flux limit varies between
about 3-4 x 10**-12 ergs cm**-2 s**-1 over the selected area. This covers the
Decl<2.5 deg part of the south Galactic cap region (b<-20 deg) - with the
exclusion of patches of low RASS1 exposure time and of the Magellanic Clouds
area - for a total of 8235 deg**2. 130 candidate sources fulfill our selection
criteria for bonafide clusters of galaxies in this area. Of these, 101 are
Abell/ACO clusters, while 29 do not have a counterpart in these catalogs. Of
these clusters, 126 (97%) clusters have a redshift and for these we compute an
X-ray luminosity. 20% of the cluster redshifts come from new observations, as
part of the ESO Key Program REFLEX Cluster Survey that is under completion.
Considering the intrinsic biases and incompletenesses introduced by the flux
selection and source identification processes, we estimate the overall
completeness to be better than 90%. The observed number count distribution,
LogN-LogS, is well fitted by a power law with slope alpha = 1.34 +/- 0.15 and
normalization A = 11.87 +/- 1.04 sr**-1 (10**-11 ergs cm**-2 s**-1)**alpha, in
good agreement with other measurements.Comment: 27 pages, 8 figures and 3 tables included, LaTex, emulateapj.sty and
epsf.sty, accepted for publication in ApJ: scheduled for the March 20, 1999,
Vol.514. The cluster catalog is available at
http://www.merate.mi.astro.it/~degrand
Characteristic Energy of the Coulomb Interactions and the Pileup of States
Tunneling data on crystals confirm
Coulomb interaction effects through the dependence of the
density of states. Importantly, the data and analysis at high energy, E, show a
pileup of states: most of the states removed from near the Fermi level are
found between ~40 and 130 meV, from which we infer the possibility of universal
behavior. The agreement of our tunneling data with recent photoemission results
further confirms our analysis.Comment: 4 pages, 4 figures, submitted to PR
DT/T beyond linear theory
The major contribution to the anisotropy of the temperature of the Cosmic
Microwave Background (CMB) radiation is believed to come from the interaction
of linear density perturbations with the radiation previous to the decoupling
time. Assuming a standard thermal history for the gas after recombination, only
the gravitational field produced by the linear density perturbations present on
a universe can generate anisotropies at low z (these
anisotropies would manifest on large angular scales). However, secondary
anisotropies are inevitably produced during the nonlinear evolution of matter
at late times even in a universe with a standard thermal history. Two effects
associated to this nonlinear phase can give rise to new anisotropies: the
time-varying gravitational potential of nonlinear structures (Rees-Sciama RS
effect) and the inverse Compton scattering of the microwave photons with hot
electrons in clusters of galaxies (Sunyaev-Zeldovich SZ effect). These two
effects can produce distinct imprints on the CMB temperature anisotropy. We
discuss the amplitude of the anisotropies expected and the relevant angular
scales in different cosmological scenarios. Future sensitive experiments will
be able to probe the CMB anisotropies beyong the first order primary
contribution.Comment: plain tex, 16 pages, 3 figures. Proceedings of the Laredo Advance
School on Astrophysics "The universe at high-z, large-scale structure and the
cosmic microwave background". To be publised by Springer-Verla
A pan-chromatic view of the galaxy cluster XMMU J1230.3+1339 at z=0.975 - Observing the assembly of a massive system
We present a comprehensive galaxy cluster study of XMMU J1230.3+1339 based on
a joint analysis of X-ray data, optical imaging and spectroscopy observations,
weak lensing results, and radio properties for achieving a detailed
multi-component view of this newly discovered system at z=0.975. We find an
optically very rich and massive system with
M200(4.20.8)10^14 M\sun, Tx5.3(+0.7--0.6)keV,
and Lx(6.50.7)10^44 erg/s, for which various widely used
mass proxies are measured and compared. We have identified multiple
cluster-related components including a central fly-through group close to core
passage with associated marginally extended 1.4GHz radio emission possibly
originating from the turbulent wake region of the merging event. On the cluster
outskirts we see evidence for an on-axis infalling group with a second
Brightest Cluster Galaxy (BCG) and indications for an additional off-axis group
accretion event. We trace two galaxy filaments beyond the nominal cluster
radius and provide a tentative reconstruction of the 3D-accretion geometry of
the system. In terms of total mass, ICM structure, optical richness, and the
presence of two dominant BCG-type galaxies, the newly confirmed cluster XMMU
J1230.3+1339 is likely the progenitor of a system very similar to the local
Coma cluster, differing by 7.6 Gyr of structure evolution.Comment: 26 pages, 14 color figures, accepted for publication in A&
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