71 research outputs found
Synchrotron Emission from Dark Matter Annihilation: Predictions for Constraints from Non-detections of Galaxy Clusters with New Radio Surveys
The annihilation of dark matter particles is expected to yield a broad
radiation spectrum via the production of Standard Model particles in
astrophysical environments. In particular, electrons and positrons from dark
matter annihilation produce synchrotron radiation in the presence of magnetic
fields. Galaxy clusters are the most massive collapsed structures in the
universe, and are known to host G-scale magnetic fields. They are
therefore ideal targets to search for, or to constrain the synchrotron signal
from dark matter annihilation. In this work we use the expected sensitivities
of several planned surveys from the next generation of radio telescopes to
predict the constraints on dark matter annihilation models which will be
achieved in the case of non-detections of diffuse radio emission from galaxy
clusters. Specifically, we consider the Tier 1 survey planned for the Low
Frequency Array (LOFAR) at 120 MHz, the EMU survey planned for the Australian
Square Kilometre Array Pathfinder (ASKAP) at 1.4 GHz, and planned surveys for
APERTIF at 1.4 GHz. We find that, for massive clusters and dark matter masses
GeV, the predicted limits on the annihilation cross section
would rule out vanilla thermal relic models for even the shallow LOFAR Tier 1,
ASKAP, and APERTIF surveys.Comment: accepted to ApJ; removal of LOFAR Tier 2 limits; other minor text
changes; conclusions largely unchange
A Radio and X-ray Study of the Merging Cluster A2319
A2319 is a massive, merging galaxy cluster with a previously detected radio
halo that roughly follows the X-ray emitting gas. We present the results from
recent observations of A2319 at 20 cm with the Jansky Very Large Array (VLA)
and a re-analysis of the X-ray observations from XMM-Newton, to investigate the
interactions between the thermal and nonthermal components of the ICM . We
confirm previous reports of an X-ray cold front, and report on the discovery of
a distinct core to the radio halo, 800 kpc in extent, that is strikingly
similar in morphology to the X-ray emission, and drops sharply in brightness at
the cold front. We detect additional radio emission trailing off from the core,
which blends smoothly into the 2 Mpc halo detected with the Green Bank
Telescope (GBT; Farnsworth et al., 2013). We speculate on the possible
mechanisms for such a two-component radio halo, with sloshing playing a
dominant role in the core. By directly comparing the X-ray and radio emission,
we find that a hadronic origin for the cosmic ray electrons responsible for the
radio halo would require a magnetic field and/or cosmic ray proton distribution
that increases with radial distance from the cluster center, and is therefore
disfavored.Comment: 9 pages, 5 figures. Submitted to MNRA
The Evolution of Cluster Substructure with Redshift
Using Chandra archival data, we quantify the evolution of cluster morphology
with redshift. To quantify cluster morphology, we use the power ratio method
developed by Buote and Tsai (1995). Power ratios are constructed from moments
of the two-dimensional gravitational potential and are, therefore, related to a
cluster's dynamical state. Our sample will include 40 clusters from the Chandra
archive with redshifts between 0.11 and 0.89. These clusters were selected from
two fairly complete flux-limited X-ray surveys (the ROSAT Bright Cluster Sample
and the Einstein Medium Sensitivity Survey), and additional high-redshift
clusters were selected from recent ROSAT flux-limited surveys. Here we present
preliminary results from the first 28 clusters in this sample. Of these, 16
have redshifts below 0.5, and 12 have redshifts above 0.5.Comment: 5 pages, 1 figure, corrected a reference, to appear in the proceeding
of Multiwavelength Cosmology, ed. M. Plioni
The Hot Gas Halos of Galaxies in Groups
We use Chandra observations of 13 nearby groups of galaxies to investigate
the hot gas content of their member galaxies. We find that a large fraction of
near-IR bright, early-type galaxies in groups have extended X-ray emission,
indicating that they retain significant hot gas halos even in these dense
environments. In particular, we detect hot gas halos in ~80% of L_K > L_star
galaxies. We do not find a significant difference in the L_K-L_X relation for
detected group and cluster early-type galaxies. However, we detect X-ray
emission from a significantly higher fraction of galaxies brighter than L_star
in groups compared to clusters, indicating that a larger fraction of galaxies
in clusters experience significant stripping of their hot gas. In addition,
group and cluster galaxies appear to be X-ray faint compared to field galaxies,
though a Chandra based field sample is needed to confirm this result. The
near-IR bright late-types galaxies in clusters and groups appear to follow the
L_K-L_X relation for early-type galaxies, while near-IR fainter late-type
galaxies are significantly more X-ray luminous than this relation likely due to
star formation. Finally, we find individual examples of ongoing gas stripping
of group galaxies. One galaxy shows a 40-50 kpc X-ray tail, and two merging
galaxy systems show tidal bridges/tails of X-ray emission. Therefore, stripping
of hot galactic gas through both ram pressure and tidal forces does occur in
groups and clusters, but the frequency or efficiency of such events must be
moderate enough to allow hot gas halos in a large fraction of bright galaxies
to survive even in group and cluster cores.Comment: 33 pages, 7 figures, accepted to ApJ, for version with full
resolution figures see http://www.ucolick.org/~tesla/groupgals.ps.g
Cluster Structure in Cosmological Simulations I: Correlation to Observables, Mass Estimates, and Evolution
We use Enzo, a hybrid Eulerian AMR/N-body code including non-gravitational
heating and cooling, to explore the morphology of the X-ray gas in clusters of
galaxies and its evolution in current generation cosmological simulations. We
employ and compare two observationally motivated structure measures: power
ratios and centroid shift. Overall, the structure of our simulated clusters
compares remarkably well to low-redshift observations, although some
differences remain that may point to incomplete gas physics. We find no
dependence on cluster structure in the mass-observable scaling relations, T_X-M
and Y_X-M, when using the true cluster masses. However, estimates of the total
mass based on the assumption of hydrostatic equilibrium, as assumed in
observational studies, are systematically low. We show that the hydrostatic
mass bias strongly correlates with cluster structure and, more weakly, with
cluster mass. When the hydrostatic masses are used, the mass-observable scaling
relations and gas mass fractions depend significantly on cluster morphology,
and the true relations are not recovered even if the most relaxed clusters are
used. We show that cluster structure, via the power ratios, can be used to
effectively correct the hydrostatic mass estimates and mass-scaling relations,
suggesting that we can calibrate for this systematic effect in cosmological
studies. Similar to observational studies, we find that cluster structure,
particularly centroid shift, evolves with redshift. This evolution is mild but
will lead to additional errors at high redshift. Projection along the line of
sight leads to significant uncertainty in the structure of individual clusters:
less than 50% of clusters which appear relaxed in projection based on our
structure measures are truly relaxed.Comment: 57 pages, 18 figures, accepted to ApJ, updated definition of T_X and
M_gas but results unchanged, for version with full resolution figures, see
http://www.ociw.edu/~tesla/sims.ps.g
The Evolution of Structure in X-ray Clusters of Galaxies
Using Chandra archival data, we quantify the evolution of cluster morphology
with redshift. Clusters form and grow through mergers with other clusters and
groups, and the amount of substructure in clusters in the present epoch and how
quickly it evolves with redshift depend on the underlying cosmology. Our sample
includes 40 X-ray selected, luminous clusters from the Chandra archive, and we
quantify cluster morphology using the power ratio method (Buote & Tsai 1995).
The power ratios are constructed from the moments of the X-ray surface
brightness and are related to a cluster's dynamical state. We find that, as
expected qualitatively from hierarchical models of structure formation,
high-redshift clusters have more substructure and are dynamically more active
than low-redshift clusters. Specifically, the clusters with z>0.5 have
significantly higher average third and fourth order power ratios than the lower
redshift clusters. Of the power ratios, is the most unambiguous
indicator of an asymmetric cluster structure, and the difference in
between the two samples remains significant even when the effects of noise and
other systematics are considered. After correcting for noise, we apply a linear
fit to versus redshift and find that the slope is greater than zero
at better than 99% confidence. This observation of structure evolution
indicates that dynamical state may be an important systematic effect in cluster
studies seeking to constrain cosmology, and when calibrated against numerical
simulations, structure evolution will itself provide interesting bounds on
cosmological models.Comment: 42 pages, 6 figures, ApJ accepted. For a version of the paper
containing an appendix with images of all of the clusters, see
http://www.ociw.edu/~tesla/structure.ps.g
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