302 research outputs found
Gamma-rays from dark matter annihilations strongly constrain the substructure in halos
Recently, it has been shown that electrons and positrons from dark matter
(DM) annihilations provide an excellent fit to the Fermi, PAMELA, and HESS
data. Using this DM model, which requires an enhancement of the annihilation
cross section over its standard value to match the observations, we show that
it immediately implies an observable level of gamma-ray emission for the Fermi
telescope from nearby galaxy clusters such as Virgo and Fornax. We show that
this DM model implies a peculiar feature from final state radiation that is a
distinctive signature of DM. Using the EGRET upper limit on the gamma-ray
emission from Virgo, we constrain the minimum mass of substructures within DM
halos to be > 5x10^-3 M_sun -- about four orders of magnitudes larger than the
expectation for cold dark matter. This limits the cutoff scale in the linear
matter power spectrum to k < 35/kpc which can be explained by e.g., warm dark
matter. Very near future Fermi observations will strongly constrain the minimum
mass to be > 10^3 M_sun: if the true substructure cutoff is much smaller than
this, the DM interpretation of the Fermi/PAMELA/HESS data must be wrong. To
address the problem of astrophysical foregrounds, we performed high-resolution,
cosmological simulations of galaxy clusters that include realistic cosmic ray
(CR) physics. We compute the dominating gamma-ray emission signal resulting
from hadronic CR interactions and find that it follows a universal spectrum and
spatial distribution. If we neglect the anomalous enhancement factor and assume
standard values for the cross section and minimum subhalo mass, the same model
of DM predicts comparable levels of the gamma-ray emission from DM
annihilations and CR interactions. This suggests that spectral subtraction
techniques could be applied to detect the annihilation signal.Comment: 5 pages, 2 figures (published version; minor corrections to figures
and result, equation added
Probing Turbulence in the Coma Galaxy Cluster
Spatially-resolved gas pressure maps of the Coma galaxy cluster are obtained
from a mosaic of XMM-Newton observations in the scale range between a
resolution of 20 kpc and an extent of 2.8 Mpc. A Fourier analysis of the data
reveals the presence of a scale-invariant pressure fluctuation spectrum in the
range between 40 and 90 kpc and is found to be well described by a projected
Kolmogorov/Oboukhov-type turbulence spectrum. Deprojection and integration of
the spectrum yields the lower limit of percent of the total
intracluster medium pressure in turbulent form. The results also provide
observational constraints on the viscosity of the gas.Comment: 12 pages, 13 figures (low resolution), version accepted by Astron.
Astrophy
The Chameleonic Contribution to the SZ Radial Profile of the Coma Cluster
We constrain the chameleonic Sunyaev--Zel'dovich (CSZ) effect in the Coma
cluster from measurements of the Coma radial profile presented in the WMAP
7-year results. The CSZ effect arises from the interaction of a scalar (or
pseudoscalar) particle with the cosmic microwave background in the magnetic
field of galaxy clusters. We combine this radial profile data with SZ
measurements towards the centre of the Coma cluster in different frequency
bands, to find Delta T_{SZ,RJ}(0)=-400+/-40 microKelvin and Delta T_{CSZ}^{204
GHz}(0)=-20+/-15 microKelvin (68% CL) for the thermal SZ and CSZ effects in the
cluster respectively. The central value leads to an estimate of the photon to
scalar (or pseudoscalar) coupling strength of g = (5.2 - 23.8) x 10^{-10}
GeV^{-1}, while the 95% confidence bound is estimated to be g < (8.7 - 39.4) x
10^{-10} GeV^{-1}.Comment: 13 pages, 3 figure
Calibration of the Mass-Temperature Relation for Clusters of Galaxies Using Weak Gravitational Lensing
The main uncertainty in current determinations of the power spectrum
normalization, sigma_8, from abundances of X-ray luminous galaxy clusters
arises from the calibration of the mass-temperature relation. We use our weak
lensing mass determinations of 30 clusters from the hitherto largest sample of
clusters with lensing masses, combined with X-ray temperature data from the
literature, to calibrate the normalization of this relation at a temperature of
8 keV, M_{500c,8 keV}=(8.7 +/- 1.6) h^{-1} 10^{14} M_sun. This normalization is
consistent with previous lensing-based results based on smaller cluster
samples, and with some predictions from numerical simulations, but higher than
most normalizations based on X-ray derived cluster masses. Assuming the
theoretically expected slope alpha=3/2 of the mass-temperature relation, we
derive sigma_8 = 0.88 +/-0.09 for a spatially-flat LambdaCDM universe with
Omega_m = 0.3. The main systematic errors on the lensing masses result from
extrapolating the cluster masses beyond the field-of-view used for the
gravitational lensing measurements, and from the separation of
cluster/background galaxies, contributing each with a scatter of 20%. Taking
this into account, there is still significant intrinsic scatter in the
mass-temperature relation indicating that this relation may not be very tight,
at least at the high mass end. Furthermore, we find that dynamically relaxed
clusters are 75 +/-40% hotter than non-relaxed clusters.Comment: 8 pages, 4 figures, revised version submitted to Ap
The Density Profile of Cluster-scale Dark Matter Halos
We measure the average gravitational shear profile of 6 massive clusters
(M_vir ~ 10^15 M_sun) at z=0.3 out to a radius ~2h^-1 Mpc. The measurements are
fitted to a generalized NFW-like halo model \rho(r) with an arbitrary r -> 0
slope \alpha. The data are well fitted by such a model with a central cusp with
\alpha ~ 0.9 - 1.6 (68% confidence interval). For the standard-NFW case \alpha
= 1.0, we find a concentration parameter c_vir that is consistent with recent
predictions from high-resolution CDM N-body simulations. Our data are also well
fitted by an isothermal sphere model with a softened core. For this model, our
1\sigma upper limit for the core radius corresponds to a limit \sigma_star \leq
0.1 cm^2 g^-1 on the elastic collision cross-section in a self-interacting dark
matter model.Comment: 4 pages, 3 figures; version accepted for publication by ApJ Letters.
Three figures omitted to allow space for new fig. 3 and expanded results and
discussion sections, including NSIS model fi
A Puzzling Merger in A3266: the Hydrodynamic Picture from XMM-Newton
Using the mosaic of nine XMM-Newton observations, we study the hydrodynamic
state of the merging cluster of galaxies Abell 3266. The high quality of the
spectroscopic data and large field of view of XMM-Netwon allow us to determine
the thermodynamic conditions of the intracluster medium on scales of order of
50 kpc. A high quality entropy map reveals the presence of an extended region
of low entropy gas, running from the primary cluster core toward the northeast
along the nominal merger axis. The mass of the low entropy gas amounts to
approximately 2e13 solar masses, which is comparable to the baryonic mass of
the core of a rich cluster. We test the possibility that the origin of the
observed low entropy gas is either related to the disruption a preexisting
cooling core in Abell 3266 or to the stripping of gas from an infalling
subcluster companion. We find that both the radial pressure and entropy
profiles as well as the iron abundance of Abell 3266 do not resemble those in
other known cooling core clusters (Abell 478). Thus we conclude that the low
entropy region is subcluster gas in the process of being stripped off from its
dark matter halo. In this scenario the subcluster would be falling onto the
core of A3266 from the foreground. This would also help interpret the observed
high velocity dispersion of the galaxies in the cluster center, provided that
the mass of the subcluster is at most a tenth of the mass of the main cluster.Comment: 6 pages, ApJ sub
The X-ray luminosity function of galaxies in the Coma cluster
The XMM-Newton survey of the Coma cluster of galaxies covers an area of 1.86
square degrees with a mosaic of 16 pointings and has a total useful integration
time of 400 ksec. Detected X-ray sources with extent less than 10" were
correlated with cataloged galaxies in the Coma cluster region. The redshift
information, which is abundant in this region of the sky, allowed us to
separate cluster members from background and foreground galaxies. For the
background sources, we recover a typical LogN-LogS in the flux range 1.e-15 -
1.e-13 ergs/s/cm^2 in the 0.5-2.0 keV band. The X-ray emission from the cluster
galaxies exhibits X-ray colors typical of thermal emission. The luminosities of
Coma galaxies lie in the 1.e39-1.e41 ergs/s interval in the 0.5-2.0 keV band.
The luminosity function of Coma galaxies reveals that their X-ray activity is
suppressed with respect to the field by a factor of 5.6, indicating a lower
level of X-ray emission for a given stellar mass.Comment: 16 pages, 2004 A&A in pres
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
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