113 research outputs found
The Baryonic Fraction in Groups of Galaxies from X-Ray Measurements
The recent {\sl ROSAT \/} X-ray detections of hot intergalactic gas in three
groups of galaxies are reviewed and the resulting baryonic fraction in these
groups is reevaluated. We show that the baryonic fraction obtained, assuming
hydrostatic equilibrium, should depend, perhaps sensitively, on the radius out
to which the X-rays are detected, and the temperature profile of the gas. We
find that the NGC 2300 group has a baryonic fraction out to of at least
20\%, thus over five times higher than in the original analysis of Mulchaey
\etal\ (1993), and also much higher than one would obtain from big-bang
nucleosynthesis, but similar to the other two groups as well as rich clusters.
With this baryonic fraction, groups would be fair tracers of the distribution
of baryons in the Universe if . A baryonic fraction that
increases with radius is consistent with the X-ray data from all three groups.
However, a detailed analysis of the NGC 2300 group shows that the dependence of
baryonic fraction on radius is not well constrained by the data, in part
because of uncertainties in the estimated background.Comment: ApJ Lett 421 (Feb 1 1994), in press. 4 pages of uuencoded compressed
Postscript (extract on UNIX with 'csh' after removing header: 1st line should
be '#/bin/csh -f') with 3 figures. POP-DAEC-9306
Diffuse Non-thermal X-ray Emission: Evidence for Cosmic-ray Acceleration at the Shock Front in IC1262
We report the first localization of diffuse, non-thermal, X-ray emission from
a nearby galaxy cluster. Using Chandra data, we have isolated a diffuse
non-thermal X-ray component with a photon index, Gamma_ X = 2.21 +0.14 -0.15
and a flux of 9.5 +1.1 -2.5 x 10^-5 photons cm^-2 s^-1 keV^-1 at 1 keV, that
extends from ~1'.5 to ~2'.5 to the south of the X-ray flux peak. Comparison to
simulations implies that the diffuse non-thermal emission is produced by
primary electrons, accelerated at shocks to relativistic velocities. Using
these results and the flux and hardness maps produced with data from the
Chandra Advanced CCD Imaging Spectrometer, we conclude that a smaller subclump
coming from the north merged with IC1262. The offset of the cD galaxy from the
X-ray peak and large peculiar velocity indicate that the subclump's impact
parameter was to the west and on the near side of IC1262.Comment: 5 pages, 4 figures, 2 tables. Accepted by ApJ
Non-isothermal X-ray Emitting Gas in Clusters of Galaxies
We have analyzed X-ray spectra from six galaxy clusters which contain cooling
flows: A85, A478, A1795, A2142, A2147, & A2199. The X-ray spectra were taken
with the HEAO1-A2 Medium and High Energy Detectors and the Einstein Solid State
Spectrometer. For each cluster, we simultaneously fit the spectra from these
three detectors with models incorporating one or more emission components
comprised of either thermal or cooling flow models. Five of the clusters (all
but A2142) are better fit by a multi-component model (a cooling flow plus one
or two thermal components or a two thermal component model) than by isothermal
models. In four of the clusters (A85, A1795, A2147, & A2199), we find evidence
for cool gas outside of the canonical cooling flow region. These latter four
clusters can be characterized by three temperature components: a temperature
inversion in the central region, a hotter region with an emission-weighted
temperature which is higher than that of an isothermal model fit to the entire
cluster, and a cooler region with an emission-weighted temperature of ~1 keV.
The cool component outside the cooling flow region has a large minimum emission
measure which we attribute, in part, to diffuse cool gas in the outer cluster
atmosphere. If at least some of the cool exterior gas is virialized, this would
imply a radially decreasing temperature profile. Together with the density
profiles we have found, this leads to a baryon fraction in gas which increases
with radius and is larger than that for an isothermal cluster atmosphere.
Consequently, if clusters of galaxies trace the mass distribution in the
Universe, the gas mass fraction we have calculated for an isothermal gas (which
is ~15%) together with the nominal galaxy contribution (~5%) gives a baryon
fraction of ~20%. Using the upper limit to the baryon density derived from BigComment: gzipped tar file of 26 PostScript pages, including 2 figures, 7
tables. Also available at
http://www.astr.ua.edu/preprints/white/INDEX_READ_ME_1st.htm
RXTE and ASCA Constraints on Non-thermal Emission from the A2256 Galaxy Cluster
An 8.3 hour observation of the Abell 2256 galaxy cluster using the Rossi
X-ray Timing Explorer proportional counter array produced a high quality
spectrum in the 2 - 30 keV range. Joint fitting with the 0.7 - 11 keV spectrum
obtained with the Advanced Satellite for Astrophysics and Cosmology gas imaging
spectrometer gives an upperlimit of 2.3x10^-7 photons/cm^2/sec/keV for
non-thermal emission at 30 keV. This yields a lower limit to the mean magnetic
field of 0.36 micro Gauss (uG) and an upperlimit of 1.8x10^-13 ergs/cm^3 for
the cosmic-ray electron energy density. The resulting lower limit to the
central magnetic field is ~1 - 3 uG While a magnetic field of ~0.1 - 0.2 uG can
be created by galaxy wakes, a magnetic field of several uG is usually
associated with a cooling flow or, as in the case of the Coma cluster, a
subcluster merger. However, for A2256, the evidence for a merger is weak and
the main cluster shows no evidence of a cooling flow. Thus, there is presently
no satisfactory hypothesis for the origin of an average cluster magnetic field
as high as >0.36 uG in the A2256 cluster.Comment: 8 pages, Astrophysical Journal (in press
Abell 754: A Non-Head On Collision of Subclusters
We have analyzed spatially resolved spectra of A754 obtained with ASCA. The
combination of spectral and imaging capabilities of ASCA has set unprecedented
constraints on the hydrodynamical effects of a cluster merger. We find
significant gas temperature variations over the cluster face, indicating shock
heating of the atmosphere during the merger. The hottest region, >12 keV, is
located in the region of the Northwest galaxy clump though the entire region
along the cluster axis appears to be hotter than the mean cluster temperature
(~9 keV). The cool, ~5 keV gas originally found with the HEAO1-A2 experiment,
resides in the exterior of the cluster atmosphere and in plume of gas we
identify with a stripped cool atmosphere of the infalling subcluster. We have
also attempted to reconstruct an iron abundance map of this merging system.
Though poorly constrained, no significant deviations of abundance from the mean
value are apparent in the individual regions.
A754 is the only cluster so far which shows the significant temperature
pattern expected in a subcluster merger, in both the ROSAT (Henry & Briel 1995)
and ASCA data, providing the first possibility to compare it with theoretical
predictions. The observed temperature and surface brightness maps suggest that
the two colliding subunits have missed each other by about 1 Mpc, and are now
moving perpendicular to the cluster axis in the image plane (as, e.g., in the
simulations by Evrard etal 1996).Comment: Latex, 10 pages, 3 figures incl. color plate, uses aaspp4.sty,
flushrt.sty and pstricks.sty. Submitted to ApJ Letter
Hard X-ray Emission from the NGC 5044 Group
Observations made with the Rossi X-ray Timing Explorer (RXTE) Proportional
Counter Array (PCA) to constrain the hard X-ray emission in the NGC 5044 group
are reported here. Modeling a combined PCA and ROSAT position sensitive
proportional counter (PSPC) spectrum with a 0.5 - 15 keV energy range shows
excess hard emission above 4 keV. Addition of a powerlaw component with
spectral index of 2.6 - 2.8 and luminosity of 2.6 x10^42 ergs/s within 700 kpc
in the observed energy band removes these residuals. Thus, there is a detection
of a significant non-thermal component that is 32% of the total X-ray emission.
Point source emission makes up at most 14% of the non-thermal emission from the
NGC 5044 group. Therefore, the diffuse, point source subtracted, non-thermal
component is 2.2 - 3.0x10^42 ergs/s . The cosmic-ray electron energy density is
3.6 x10^[-12] ergs cm-3 and the average magnetic field is 0.034 \muGauss in the
largest radio emitting region. The ratio of cosmic-ray electron energy density
to magnetic field energy density, ~2.5x10^4, is significantly out of
equipartition and is therefore atypical of radio lobes. In addition, the
group's small size and low non-thermal energy density strongly contradicts the
size-energy relationship found for radio lobes. Thus, it is unlikely to the
related to the active galaxy and is most likely a relic of the merger. The
energy in cosmic-rays and magnetic field is consistent with simulations of
cosmic-ray acceleration by merger shocks.Comment: 17 pages, including 4 figures and 2 table
A Soft X-ray Component in the Abell 754 Cluster
We have analyzed the Chandra, BeppoSax, and ROSAT observations of Abell 754
and report evidence of a soft, diffuse X-ray component. The emission is peaked
in the cluster center and is detected out to 8' from the X-ray center. Fitting
a thermal model to the combined BeppoSax and PSPC spectra show excess emission
below 1 keV in the PSPC and above 100 keV in the BeppoSax PDS. The source 26W20
is in the field of view of the PDS. The addition of a powerlaw with the
spectral parameters measured by Silverman et al. (1998) for 26W20 successfully
models the hard component in the PDS. The remaining excess soft emission can be
modeled by either a low temperature, 0.75 - 1.03 keV component, or by a
powerlaw with a steep spectral index, 2.3. Addition of a second thermal
component model provides a much better fit to the data than does the addition
of a non-thermal component. The Chandra temperature map does not show any
region cooler than 6.9 keV within the region where the cool component was
detected. Simulations of the emission from embedded groups were performed and
compared with the Chandra temperature map which show groups are a plausible
source of ~1 keV emission. The cool component is centrally peaked in the
cluster and the gas density and temperature are relatively high arguing against
the WHIM as the source of the X-ray emission. X-ray emission from elliptical
galaxies is not high enough to provide the total cool component luminosity,
7.0x10^43 ergs s^-1. The peak of the cool component is located between the low
frequency radio halos arguing against a non-thermal interpretation for the
emission. We conclude that emission from embedded groups is the most likely
origin of the cool component in Abell 754.Comment: Submitted to Ap
A BeppoSAX Observation of the IC1262 Galaxy Cluster
We present an analysis of BeppoSAX observations of the IC1262 galaxy cluster
and report the first temperature and abundance measurements, along with
preliminary indications of diffuse, nonthermal emission. By fitting a 6' (~360
h_50^-1 kpc) region with a single Mewe-Kaastra-Liedahl model with photoelectric
absorption, we find a temperature of 2.1 - 2.3 keV, and abundance of 0.45 -
0.77 (both 90% confidence). We find the addition of a power-law component
provides a statistically significant improvement (F-test = 90%) to the fit. The
addition of a second thermal component also improves the fit but we argue that
it is physically implausible. The power-law component has a photon index
(Gamma_X) of 0.4 - 2.8 and a nonthermal flux of (4.1 - 56.7) x 10-5 photons
cm^-2 s^-1 over the 1.5 - 10.5 keV range in the Medium Energy Concentrator
spectrometer detector. An unidentified X-ray source found in the ROSAT High
Resolution Imager observation (~0'.9 from the center of the cluster) is a
possible explanation for the nonthermal flux; however, additional evidence of
diffuse, nonthermal emission comes from the NRAO VLA Sky Survey and the
Westerbork Northern Sky Survey radio measurements, in which excess diffuse,
radio flux is observed after point-source subtraction. The radio excess can be
fitted to a simple power law with a spectral index of ~1.8, which is consistent
with the nonthermal X-ray emission spectral index. The steep spectrum is
typical of diffuse emission and the size of the radio source implies that it is
larger than the cD galaxy and not due to a discreet source
Chandra Observations of the A3266 Galaxy Cluster Merger
Analysis of a 30,000 s X-ray observation of the Abell 3266 galaxy cluster
with the ACIS on board the Chandra Observatory has produced several new
insights into the cluster merger. The intracluster medium has a
non-monotonically decreasing radial abundance profile. We argue that the most
plausible origin for the abundance enhancement is unmixed, high abundance
subcluster gas from the merger. The enrichment consists of two stages:
off-center deposition of a higher abundance material during a subcluster merger
followed by a strong, localized intracluster wind that acts to drive out the
light elements, producing the observed abundance enhancement. The wind is
needed to account for both an increase in the heavy element abundance and the
lack of an enhancement in the gas density. Dynamical evidence for the wind
includes: (1) a large scale, low surface brightness feature perpendicular to
the merger axis that appears to be an asymmetric pattern of gas flow to the
northwest, away from the center of the main cluster, (2) compressed gas in the
opposite direction (toward the cluster center), and (3), the hottest regions
visible in the temperature map coincide with the proposed merger geometry and
the resultant gas flow. The Chandra data for the central region of the main
cluster shows a slightly cooler, filamentary region that is centered on the
central cD galaxy and is aligned with the merger axis directly linking the
dynamical state of the cD to the merger. Overall, the high spectral/spatial
resolution Chandra observations support our earlier hypothesis (Henriksen,
Donnelly, & Davis 1999) that we are viewing a minor merger in the plane of the
sky.Comment: Accepted to ApJ. 13 pages, 4 tables, 11 figure
In Vitro–expanded Antigen-specific Regulatory T Cells Suppress Autoimmune Diabetes
The low number of CD4+ CD25+ regulatory T cells (Tregs), their anergic phenotype, and diverse antigen specificity present major challenges to harnessing this potent tolerogenic population to treat autoimmunity and transplant rejection. In this study, we describe a robust method to expand antigen-specific Tregs from autoimmune-prone nonobese diabetic mice. Purified CD4+ CD25+ Tregs were expanded up to 200-fold in less than 2 wk in vitro using a combination of anti-CD3, anti-CD28, and interleukin 2. The expanded Tregs express a classical cell surface phenotype and function both in vitro and in vivo to suppress effector T cell functions. Most significantly, small numbers of antigen-specific Tregs can reverse diabetes after disease onset, suggesting a novel approach to cellular immunotherapy for autoimmunity
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