1,858 research outputs found
The Galaxy Cluster Luminosity-Temperature Relationship and Iron Abundances - A Measure of Formation History ?
Both the X-ray luminosity-temperature (L-T) relationship and the iron
abundance distribution of galaxy clusters show intrinsic dispersion. Using a
large set of galaxy clusters with measured iron abundances we find a
correlation between abundance and the relative deviation of a cluster from the
mean L-T relationship. We argue that these observations can be explained by
taking into account the range of cluster formation epochs expected within a
hierarchical universe. The known relationship of cooling flow mass deposition
rate to luminosity and temperature is also consistent with this explanation.
From the observed cluster population we estimate that the oldest clusters
formed at z>~2. We propose that the iron abundance of a galaxy cluster can
provide a parameterization of its age and dynamical history.Comment: 13 pages Latex, 2 figures, postscript. Accepted for publication in
ApJ Letter
X-ray Measurements of the Gravitational Potential Profile in the Central Region of the Abell 1060 Cluster of Galaxies
X-ray spectral and imaging data from ASCA and ROSAT were used to measure the
total mass profile in the central region of Abell 1060, a nearby and relatively
poor cluster of galaxies. The ASCA X-ray spectra, after correcting for the
spatial response of the X-ray telescope, show an isothermal distribution of the
intra-cluster medium (ICM) within at least 12' (or kpc;
km sMpc) in radius of the cluster center. The
azimuthally averaged surface brightness profile from the ROSAT PSPC exhibits a
central excess above an isothermal model. The ring-sorted ASCA GIS
spectra and the radial surface brightness distribution from the ROSAT PSPC were
simultaneously utilized to constrain the gravitational potential profile. Some
analytic models of the total mass density profile were examined. The ICM
density profile was also specified by analytic forms. The ICM temperature
distribution was constrained to satisfy the hydrostatic equilibrium, and to be
consistent with the data. Then, the total mass distribution was found to be
described better by the universal dark halo profile proposed by Navarro, Frenk,
and White (1996;1997) than by a King-type model with a flat density core. A
profile with a central cusp together with a logarithmic radial slope of was also consistent with the data. Discussions are made concerning the
estimated dark matter distribution around the cluster center.Comment: 32 pages. Accepted: ApJ 2000, 35 pages, Title was correcte
Magnetic anisotropy of the spin ice compound Dy2Ti2O7
We report magnetization and ac susceptibility of single crystals of the spin
ice compound Dy2Ti2O7. Saturated moments at 1.8 K along the charasteristic axes
[100] and [110] agree with the expected values for an effective ferromagnetic
nearest-neighbor Ising pyrochlore with local anisotropy, where each
magnetic moment is constrained to obey the `ice-rule'. At high enough magnetic
fields along the [111] axis, the saturated moment exhibits a beaking of the
ice-rule; it agrees with the value expected for a three-in one-out spin
configuration. Assuming the realistic magnetic interaction between Dy ions
given by the dipolar spin ice model, we completely reproduce the results at 2 K
by Monte Carlo calculations. However, down to at least 60 mK, we have not found
any experimental evidence of the long-range magnetic ordering predicted by this
model to occur at around 180 mK. Instead, we confirm the spin freezing of the
system below 0.5 K.Comment: 7 pages, 6 figures, submitted to Phys. Rev.
CAIRNS: The Cluster And Infall Region Nearby Survey III. Environmental Dependence of H-alpha Properties of Galaxies
We investigate the environmental dependence of star formation in cluster
virial regions and infall regions as part of CAIRNS (Cluster And Infall Region
Nearby Survey), a large spectroscopic survey of the infall regions surrounding
nine nearby rich clusters of galaxies. Our long-slit spectroscopy yields
estimates of star formation rates in environments from cluster cores to the
general large-scale structure. The fraction of galaxies with current star
formation in their inner disks as traced by H-alpha emission increases with
distance from the cluster and converges to the ``field'' value only at 2-3
virial radii, in agreement with other investigations. However, among galaxies
with significant current star formation (EW[Ha]geq2\AA), there is no difference
in the distribution of EW[Ha] inside and outside the virial radius. This
surprising result, first seen by Carter et al., suggests that (1) star
formation is truncated on either very short timescales or only at moderate and
high redshifts or (2) that projection effects contaminate the measurement. The
number density profiles of star-forming and non-star-forming galaxies indicate
that, among galaxies projected inside the virial radius, at least half of the
former and 20% of the latter are ``infall interlopers,'' galaxies in the infall
region but outside the virial region. The kinematics of star-forming galaxies
in the infall region closely match those of absorption-dominated galaxies. This
result shows that the star forming galaxies in the infall regions are not
interlopers from the field and excludes one model of the backsplash scenario of
galaxy transformation. Finally, we quantify systematic uncertainties in
estimating the global star formation in galaxies from their inner disks.Comment: 25 pages, 21 figures, accepted for publication in A
X-MAS2: Study Systematics on the ICM Metallicity Measurements
(Abridged)The X-ray measurements of the ICM metallicity are becoming more
frequent due to the availability of powerful X-ray telescope with excellent
spatial and spectral resolutions. The information which can be extracted from
the measurements of the alpha-elements, like Oxygen, Magnesium and Silicon with
respect to the Iron abundance is extremely important to better understand the
stellar formation and its evolutionary history. In this paper we investigate
possible source of bias connected to the plasma physics when recovering metal
abundances from X-ray spectra. To do this we analyze 6 simulated galaxy
clusters processed through the new version of our X-ray MAp Simulator, which
allows to create mock XMM-Newton EPIC MOS1 and MOS2 observations. By comparing
the spectroscopic results to the input values we find that: i) Fe is recovered
with high accuracy for both hot (T>3 keV) and cold (T<2 keV) systems; at
intermediate temperatures, however, we find a systematic overestimate which
depends on the number counts; ii) O is well recovered in cold clusters, while
in hot systems its measure may overestimate by a factor up to 2-3; iii) Being a
weak line, the measurement of Mg is always difficult; despite of this, for cold
systems (T<2 keV) we do not find any systematic behavior, while for very hot
systems (T>5 keV) the spectroscopic measurement may be strongly overestimated
up to a factor of 4; iv) Si is well recovered for all the clusters in our
sample. We investigate in detail the nature of the systematic effects and
biases found. We conclude that they are mainly connected with the
multi-temperature nature of the projected observed spectra and to the intrinsic
limitation of the XMM-Newton EPIC spectral resolution that does not always
allow to disentangle among the emission lines produced by different elements.Comment: (e.g.: 17 pages, 8 figures, accepted for publication in the
Astrophysical Journal, updated discussion to match published version-new
section:6.3
Active Galaxies and Cluster Gas
Two lines of evidence indicate that active galaxies, principally radio
galaxies, have heated the diffuse hot gas in clusters. The first is the general
need for additional heating to explain the steepness of the X-ray
luminosity--temperature relation in clusters, the second is to solve the
cooling flow problem in cluster cores. The inner core of many clusters is
radiating energy as X-rays on a timescale much shorter than its likely age.
Although the temperature in this region drops by a factor of about 3 from that
of the surrounding gas, little evidence is found for gas much cooler than that.
Some form of heating appears to be taking place, probably by energy transported
outward from the central accreting black hole or radio source. How that energy
heats the gas depends on poorly understood transport properties (conductivity
and viscosity) of the intracluster medium. Viscous heating is discussed as a
possibility. Such heating processes have consequences for the truncation of the
luminosity function of massive galaxies.Comment: 14 pages, 16 fig, Feb 2004 talk for Phil Trans Roy So
The Enrichment History of Hot Gas in Poor Galaxy Groups
We have analyzed the ASCA SIS and GIS data for seventeen groups and
determined the average temperature and abundance of the hot x-ray emitting gas.
For groups with gas temperatures less than 1.5 keV we find that the abundance
is correlated with the gas temperature and luminosity. We have also determined
the abundance of the alpha-elements and iron independently for those groups
with sufficient counts. We find that for the cool groups (i.e. kT <1.5 keV) the
ratio of alpha-elements to iron is ~1, about half that seen in clusters.
Spectral fits with the S, Si and Fe abundances allowed to vary separately
suggest the S/Fe ratio is similar to that seen in clusters while the Si/Fe
ratio in groups is half the value determined for richer systems. The mass of
metals per unit blue luminosity drops rapidly in groups as the temperature
drops. There are two possible explanations for this decrease. One is that the
star formation in groups is very different from that in rich clusters. The
other explanation is that groups lose much of their enriched material via winds
during the early evolution of ellipticals. If the latter is true, we find that
poor groups will have contributed significantly (roughly 1/3 of the metals) to
the enrichment of the intergalactic medium.Comment: 19 Pages with 2 figures, Accepted for publication in the
Astrophysical Journa
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