37 research outputs found
The Dark Matter Distribution in Galaxy Cluster Cores
Determining the structure of galaxy clusters is essential for an
understanding of large scale structure in the universe, and may hold important
clues to the identity and nature of dark matter particles. Moreover, the core
dark matter distribution may offer insight into the structure formation
process. Unfortunately, cluster cores also tend to be the site of complicated
astrophysics. X-ray imaging spectroscopy of relaxed clusters, a standard
technique for mapping their dark matter distributions, is often complicated by
the presence of their putative ``cooling flow'' gas, and the dark matter
profile one derives for a cluster is sensitive to assumptions made about the
distribution of this gas. Here we present a statistical analysis of these
assumptions and their effect on our understanding of dark matter in galaxy
clusters.Comment: Poster contribution to the 13th Annual Astrophysics Conference in
Maryland, The Emergence of Cosmic Structure; 4 page
Extracting the Dark Matter Profile of a Relaxed Galaxy Cluster
Knowledge of the structure of galaxy clusters is essential for an
understanding of large scale structure in the universe, and may provide
important clues to the nature of dark matter. Moreover, the shape of the dark
matter distribution in the cluster core may offer insight into the structure
formation process. Unfortunately, cluster cores also tend to be the site of
complicated astrophysics. X-ray imaging spectroscopy of relaxed clusters, a
standard technique for mapping their dark matter distributions, is often
complicated by the presence of cool components in cluster cores, and the dark
matter profile one derives for a cluster is sensitive to assumptions made about
the distribution of this component. In addition, fluctuations in the
temperature measurements resulting from normal statistical variance can produce
results which are unphysical. We present here a procedure for extracting the
dark matter profile of a spherically symmetric, relaxed galaxy cluster which
deals with both of these complications. We apply this technique to a sample of
galaxy clusters observed with the Chandra X-ray Observatory, and comment on the
resulting mass profiles. For some of the clusters we compare their masses with
those derived from weak and strong gravitational measurements.Comment: final version to match accepted ApJ version; 29 page
The Spectra and Variability of X-ray Sources in a Deep Chandra Observation of the Galactic Center
We examine the X-ray spectra and variability of the sample of X-ray sources
with L_X = 10^{31}-10^{33} erg s^{-1} identified within the inner 9' of the
Galaxy. Very few of the sources exhibit intra-day or inter-month variations. We
find that the spectra of the point sources near the Galactic center are very
hard between 2--8 keV, even after accounting for absorption. When modeled as
power laws the median photon index is Gamma=0.7, while when modeled as thermal
plasma we can only obtain lower limits to the temperature of kT>8 keV. The
combined spectra of the point sources is similarly hard, with a photon index of
Gamma=0.8. Strong line emission is observed from low-ionization, He-like, and
H-like Fe, both in the average spectra and in the brightest individual sources.
The line ratios of the highly-ionized Fe in the average spectra are consistent
with emission from a plasma in thermal equilibrium. This line emission is
observed whether average spectra are examined as a function of the count rate
from the source, or as a function of the hardness ratios of individual sources.
This suggests that the hardness of the spectra may in fact to due local
absorption that partially-covers the X-ray emitting regions in the Galactic
center systems. We suggest that most of these sources are intermediate polars,
which (1) often exhibit hard spectra with prominent Fe lines, (2) rarely
exhibit either flares on short time scales or changes in their mean X-ray flux
on long time scales, and (3) are the most numerous hard X-ray sources with
comparable luminosities in the Galaxy.Comment: 27 pages, including 13 figures. To appear in ApJ, 1 October 2004,
v613 issue. An electronic version of table 2 is on
http://astro.ucla.edu/~mmuno/sgra/table2_electronic.txt and reduced data
files for each source are available on
http://www.astro.psu.edu/users/niel/galcen-xray-data/galcen-xray-data.htm
Measuring Molecular, Neutral Atomic, and Warm Ionized Galactic Gas Through X-Ray Absorption
We study the column densities of neutral atomic, molecular, and warm ionized
Galactic gas through their continuous absorption of extragalactic X-ray spectra
at |b| > 25 degrees. For N(H,21cm) < 5x10^20 cm^-2 there is an extremely tight
relationship between N(H,21cm) and the X-ray absorption column, N(xray), with a
mean ratio along 26 lines of sight of N(xray)/N(H,21cm) = 0.972 +- 0.022. This
is significantly less than the anticpated ratio of 1.23, which would occur if
He were half He I and half He II in the warm ionized component. We suggest that
the ionized component out of the plane is highly ionized, with He being mainly
He II and He III. In the limiting case that H is entirely HI, we place an upper
limit on the He abundance in the ISM of He/H <= 0.103.
At column densities N(xray) > 5x10^20 cm^-2, which occurs at our lower
latitudes, the X-ray absorption column N(xray) is nearly double N(H,21cm). This
excess column cannot be due to the warm ionized component, even if He were
entirely He I, so it must be due to a molecular component. This result implies
that for lines of sight out of the plane with |b| ~ 30 degrees, molecular gas
is common and with a column density comprable to N(H,21cm).
This work bears upon the far infrared background, since a warm ionized
component, anticorrelated with N(H,21cm), might produce such a background. Not
only is such an anticorrelation absent, but if the dust is destroyed in the
warm ionized gas, the far infrared background may be slightly larger than that
deduced by Puget et al. (1996).Comment: 1 AASTeX file, 14 PostScript figure files which are linked within the
TeX fil
Discrepant Mass Estimates in the Cluster of Galaxies Abell 1689
We present a new mass estimate of a well-studied gravitational lensing
cluster, Abell 1689, from deep Chandra observations with a total exposure of
200 ks. Within r=200 h-1 kpc, the X-ray mass estimate is systematically lower
than that of lensing by 30-50%. At r>200 h-1 kpc, the mass density profiles
from X-ray and weak lensing methods give consistent results. The most recent
weak lensing work suggest a steeper profile than what is found from the X-ray
analysis, while still in agreement with the mass at large radii. Previous
studies have suggested that cooler small-scale structures can bias X-ray
temperature measurements or that the northern part of the cluster is disturbed.
We find these scenarios unlikely to resolve the central mass discrepancy since
the former requires 70-90% of the space to be occupied by these cool structures
and excluding the northern substructure does not significantly affect the total
mass profiles. A more plausible explanation is a projection effect. We also
find that the previously reported high hard-band to broad-band temperature
ratio in A1689, and many other clusters observed with Chandra, may be resulting
from the instrumental absorption that decreases 10-15% of the effective area at
~1.75 keV.Comment: 18 pages, 15 figures. ApJ accepte
The structural and scaling properties of nearby galaxy clusters: I - The universal mass profile
We present the integrated mass profiles for a sample of ten nearby (z<=0.15),
relaxed galaxy clusters, covering a temperature range of [2-9]keV, observed
with XMM-Newton. The mass profiles were derived from the observed gas density
and temperature profiles under the hypothesis of spherical symmetry and
hydrostatic equilibrium. All ten mass profiles are well described by an
NFW-type profile over the radial range from 0.01 to 0.5 R_200, where R_200 is
the radius corresponding to a density contrast of 200 with respect to the
critical density at the cluster redshift. A King model is inconsistent with
these data. The derived concentration parameters and total masses are in the
range c_200=4-6 and M_200=1.2 10^14-1.2 10^15 Msol, respectively. Our
qualitative and quantitative study of the mass profile shape shows, for the
first time, direct and clear observational evidence for the universality of the
total mass distribution in clusters. The mass profiles scaled in units of R_200
and M_200 nearly coincide, with a dispersion of less than 15% at 0.1 R_200. The
c_200--M_200 relation is consistent with the predictions of numerical
simulations for a LCDM cosmology, taking into account the measurement errors
and expected intrinsic scatter. Our results provide further strong evidence in
favour of the Cold Dark Matter cosmological scenario and show that the dark
matter collapse is well understood at least down to the cluster scale.Comment: 8 pages, 5 figures. Accepted for publication in A&
Arc sensitivity to cluster ellipticity, asymmetries and substructures
We investigate how ellipticity, asymmetries and substructures separately
affect the ability of galaxy clusters to produce strong lensing events, i.e.
gravitational arcs, and how they influence the arc morphologies and fluxes.
This is important for those studies aiming, for example, at constraining
cosmological parameters from statistical lensing, or at determining the inner
structure of galaxy clusters through gravitational arcs. We do so by creating
two-dimensional gradually smoothed, differently elliptical and asymmetric
versions of some numerical models. On average, we find that the contributions
of ellipticity, asymmetries and substructures amount to ~40%, ~10% and ~30% of
the total strong lensing cross section, respectively. However, our analysis
shows that substructures play a more important role in less elliptical and
asymmetric clusters, even if located at large distances from the cluster
centers (~1Mpc/h). Conversely, their effect is less important in highly
asymmetric lenses. The morphology, position and flux of individual arcs are
strongly affected by the presence of substructures in the clusters. Removing
substructures on spatial scales <~50kpc/h, roughly corresponding to mass scales
<~5 10^{10}M_\odot/h, alters the image multiplicity of ~35% of the sources used
in the simulations and causes position shifts larger than 5'' for ~40% of the
arcs longer than 5''. We conclude that any model for cluster lens cannot
neglect the effects of ellipticity, asymmetries and substructures. On the other
hand, the high sensitivity of gravitational arcs to deviations from regular,
smooth and symmetric mass distributions suggests that strong gravitational
lensing is potentially a powerfull tool to measure the level of substructures
and asymmetries in clusters.Comment: 16 pages, 18 figures. Accepted version. Version with full resolution
images can be found at
http://www.ita.uni-heidelberg.de/~massimo/sub/publications.htm
Further evidence for a merger in Abell 2218 from an XMM-Newton observation
(Abridged) The galaxy cluster Abell 2218, at z=0.171, is well-known for the
discrepancy between mass estimates derived from X-ray and strong lensing
analyses. With the present XMM observation, we trace the gas density and
temperature profiles out to a radius of ~ 1400 h_70^-1 kpc (approximately the
virial radius of the cluster). The surface brightness profile is well fitted
over three orders of magnitude with a beta model, with a core radius of 0.'95
and \beta=0.63. The projected temperature profile declines steeply with radius
(by ~50%), and is well described by a polytrope with parameters t_0=8.09 keV
and \gamma=1.15. The temperature map shows a pronounced peak in the central
arcminute, with an increase of a factor of two (from ~5 to ~10 keV). The mass
profile, calculated assuming hydrostatic equilibrium and spherical symmetry, is
best fitted with a King approximation to an isothermal sphere, implying a dark
matter distribution with a central core, in contrast with the cusped cores
found in more obviously relaxed clusters. The X-ray mass is two times less than
the strong lensing mass at r ~ 80 h_50^-1 kpc, although the agreement between
X-ray and weak lensing mass measurements at larger radius (r ~ 400 h_50^-1 kpc)
is slightly better. While the X-ray total mass estimates can vary by 30%
depending on the mass model, all measurements are lower than the corresponding
total mass from optical measurements. Given the X-ray results indicating
disturbance of the intracluster gas, leading to a likely violation of the
assumption of hydrostatic equilibrium, and the observed substructure in the
optical, suggesting a line-of-sight merger, it is unlikely that the different
mass estimates of this cluster can be reconciled, at least with standard
modelling assumptions.Comment: 9 pages, 7 figures; to appear in A&