30 research outputs found
Cold Feedback in Cooling-Flow Galaxy Clusters
We put forward an alternative view to the Bondi-driven feedback between
heating and cooling of the intra-cluster medium (ICM) in cooling flow galaxies
and clusters. We adopt the popular view that the heating is due to an active
galactic nucleus (AGN), i.e. a central black hole accreting mass and launching
jets and/or winds. We propose that the feedback occurs with the entire cool
inner region (5-30 kpc). A moderate cooling flow does exist here, and
non-linear over-dense blobs of gas cool fast and are removed from the ICM
before experiencing the next major AGN heating event. Some of these blobs may
not accrete on the central black hole, but may form stars and cold molecular
clouds. We discuss the conditions under which the dense blobs may cool to low
temperatures and feed the black hole.Comment: 6 pages, no figures, to appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
Observations of metals in the intra-cluster medium
Because of their deep gravitational potential wells, clusters of galaxies
retain all the metals produced by the stellar populations of the member
galaxies. Most of these metals reside in the hot plasma which dominates the
baryon content of clusters. This makes them excellent laboratories for the
study of the nucleosynthesis and chemical enrichment history of the Universe.
Here we review the history, current possibilities and limitations of the
abundance studies, and the present observational status of X-ray measurements
of the chemical composition of the intra-cluster medium. We summarise the
latest progress in using the abundance patterns in clusters to put constraints
on theoretical models of supernovae and we show how cluster abundances provide
new insights into the star-formation history of the Universe.Comment: 28 pages, 12 figures, accepted for publication in Space Science
Reviews, special issue "Clusters of galaxies: beyond the thermal view",
Editor J.S. Kaastra, Chapter 16; work done by an international team at the
International Space Science Institute (ISSI), Bern, organised by J.S.
Kaastra, A.M. Bykov, S. Schindler & J.A.M. Bleeke
Constraints on Decaying Dark Matter from Fermi Observations of Nearby Galaxies and Clusters
We analyze the impact of Fermi gamma-ray observations (primarily
non-detections) of selected nearby galaxies, including dwarf spheroidals, and
of clusters of galaxies on decaying dark matter models. We show that the fact
that galaxy clusters do not shine in gamma rays puts the most stringent limits
available to-date on the lifetime of dark matter particles for a wide range of
particle masses and decay final states. In particular, our results put strong
constraints on the possibility of ascribing to decaying dark matter both the
increasing positron fraction reported by PAMELA and the high-energy feature in
the electron-positron spectrum measured by Fermi. Observations of nearby dwarf
galaxies and of the Andromeda Galaxy (M31) do not provide as strong limits as
those from galaxy clusters, while still improving on previous constraints in
some cases.Comment: 27 pages, 5 figures, submitted to JCAP, revised version with some
additions and correction
The Physics of Cluster Mergers
Clusters of galaxies generally form by the gravitational merger of smaller
clusters and groups. Major cluster mergers are the most energetic events in the
Universe since the Big Bang. Some of the basic physical properties of mergers
will be discussed, with an emphasis on simple analytic arguments rather than
numerical simulations. Semi-analytic estimates of merger rates are reviewed,
and a simple treatment of the kinematics of binary mergers is given. Mergers
drive shocks into the intracluster medium, and these shocks heat the gas and
should also accelerate nonthermal relativistic particles. X-ray observations of
shocks can be used to determine the geometry and kinematics of the merger. Many
clusters contain cooling flow cores; the hydrodynamical interactions of these
cores with the hotter, less dense gas during mergers are discussed. As a result
of particle acceleration in shocks, clusters of galaxies should contain very
large populations of relativistic electrons and ions. Electrons with Lorentz
factors gamma~300 (energies E = gamma m_e c^2 ~ 150 MeV) are expected to be
particularly common. Observations and models for the radio, extreme
ultraviolet, hard X-ray, and gamma-ray emission from nonthermal particles
accelerated in these mergers are described.Comment: 38 pages with 9 embedded Postscript figures. To appear in Merging
Processes in Clusters of Galaxies, edited by L. Feretti, I. M. Gioia, and G.
Giovannini (Dordrecht: Kluwer), in press (2001
Constraints on Dark Matter Annihilation in Clusters of Galaxies with the Fermi Large Area Telescope
Nearby clusters and groups of galaxies are potentially bright sources of
high-energy gamma-ray emission resulting from the pair-annihilation of dark
matter particles. However, no significant gamma-ray emission has been detected
so far from clusters in the first 11 months of observations with the Fermi
Large Area Telescope. We interpret this non-detection in terms of constraints
on dark matter particle properties. In particular for leptonic annihilation
final states and particle masses greater than ~200 GeV, gamma-ray emission from
inverse Compton scattering of CMB photons is expected to dominate the dark
matter annihilation signal from clusters, and our gamma-ray limits exclude
large regions of the parameter space that would give a good fit to the recent
anomalous Pamela and Fermi-LAT electron-positron measurements. We also present
constraints on the annihilation of more standard dark matter candidates, such
as the lightest neutralino of supersymmetric models. The constraints are
particularly strong when including the fact that clusters are known to contain
substructure at least on galaxy scales, increasing the expected gamma-ray flux
by a factor of ~5 over a smooth-halo assumption. We also explore the effect of
uncertainties in cluster dark matter density profiles, finding a systematic
uncertainty in the constraints of roughly a factor of two, but similar overall
conclusions. In this work, we focus on deriving limits on dark matter models; a
more general consideration of the Fermi-LAT data on clusters and clusters as
gamma-ray sources is forthcoming.Comment: accepted to JCAP, Corresponding authors: T.E. Jeltema and S. Profumo,
minor revisions to be consistent with accepted versio
Clumpiness enhancement of charged cosmic rays from dark matter annihilation with Sommerfeld effect
Boost factors of dark matter annihilation into antiprotons and
electrons/positrons due to the clumpiness of dark matter distribution are
studied in detail in this work, taking the Sommerfeld effect into account. It
has been thought that the Sommerfeld effect, if exists, will be more remarkable
in substructures because they are colder than the host halo, and may result in
a larger boost factor. We give a full calculation of the boost factors based on
the recent N-body simulations. Three typical cases of Sommerfeld effects, the
non-resonant, moderately resonant and strongly resonant cases are considered.
We find that for the non-resonant and moderately resonant cases the enhancement
effects of substructures due to the Sommerfeld effect are very small () because of the saturation behavior of the Sommerfeld effect.
For the strongly resonant case the boost factor is typically smaller than . However, it is possible in some very extreme cases that DM
distribution is adopted to give the maximal annihilation the boost factor can
reach up to . The variances of the boost factors due to different
realizations of substructures distribution are also discussed in the work.Comment: 28 pages, 8 figures, 2 table. The detailed fomula of the propagation
and boost factor are moved to the Appendix. Accepted by JCA
Trouble for AGN Feedback? The Puzzle of the Core of the Galaxy Cluster AWM 4
The core of the relaxed cluster AWM 4 is characterized by a unique combination of properties which defy a
popular scenario for AGN heating of cluster cores. A flat inner temperature profile is indicative of a past, major
heating episode which completely erased the cool core, as testified by the high central cooling time (3 Gyr) and
by the high central entropy level ( 3c60 keV cm2
). Yet the presence of a 1.4 GHz active central radio galaxy with
extended radio lobes out to 100 kpc reveals recent feeding of the central massive black hole. A system like AWM
4 should have no radio emission at all if only feedback from the cooling hot gas regulates the AGN activity
Hydrostatic Gas Constraints on Supermassive Black Hole Masses: Implications for Hydrostatic Equilibrium and Dynamical Modeling in a Sample of Early-type Galaxies
We present new mass measurements for the supermassive black holes (SMBHs) in the centers of three early-type
galaxies. The gas pressure in the surrounding, hot interstellar medium (ISM) is measured through spatially resolved
spectroscopy with the Chandra X-ray Observatory, allowing the SMBH mass (MBH) to be inferred directly under the
hydrostatic approximation. This technique does not require calibration against other SMBH measurement methods
and its accuracy depends only on the ISM being close to hydrostatic, which is supported by the smooth X-ray
isophotes of the galaxies. Combined with results from our recent study of the elliptical galaxy NGC 4649, this brings
the number of galaxies with SMBHs measured in this way to four. Of these, three already have mass determinations
from the kinematics of either the stars or a central gas disk, and hence join only a handful of galaxies with MBH
measured by more than one technique. We find good agreement between the different methods, providing support
for the assumptions implicit in both the hydrostatic and the dynamical models. The stellar mass-to-light ratios for
each galaxy inferred by our technique are in agreement with the predictions of stellar population synthesis models
assuming a Kroupa initial mass function (IMF). This concurrence implies that no more than âŒ10%â20% of the ISM
pressure is nonthermal, unless there is a conspiracy between the shape of the IMF and nonthermal pressure. Finally,
we compute Bondi accretion rates (MË bondi), finding that the two galaxies with the highest MË bondi exhibit little evidence
of X-ray cavities, suggesting that the correlation with the active galactic nuclei jet power takes time to be established
Large Scatter in X-Ray Emission among Elliptical Galaxies: Correlations with Mass of Host Halo
Optically similar elliptical galaxies have an enormous range of X-ray luminosities. We show that this range
can be attributed to large variations in the dark halo mass determined from X-ray observations. The Mvir K-band
luminosity of ellipticals varies with virial mass, , but with considerable scatter, probably due to 0.750.22 LK 1d Mvir
the stochastic incidence of massive satellite galaxies that merge by dynamical friction to form group-centered
ellipticals. Both the observed X-ray luminosity and are sufficiently sensitive to the virial 2.4 1.6 L X vir X 1d M L /LK 1d Mvir
mass to explain the wide variation observed in among galaxies of similar . The central galaxy supernova L L X K
energy per particle of diffuse gas increases dramatically with decreasing virial mass, and elliptical galaxies with
the lowest X-ray luminosities (and ) are easily explained by supernova-driven outflows
X-Ray Observations of Cluster Mergers
X-ray observations have played a key role in the study of substructure and
merging in galaxy clusters. I review the evidence for cluster substructure and
mergers obtained from X-ray observations with satellites that operated before
Chandra and XMM. Different techniques to study cluster mergers via X-ray
imaging and spectral data are discussed with an emphasis on the quantitative
analysis of cluster morphologies. I discuss the implications of measurements of
cluster morphologies for cosmology and the origin of radio halos