448 research outputs found
Detecting Star Formation in Brightest Cluster Galaxies with GALEX
We present the results of GALEX observations of 17 cool core (CC) clusters of
galaxies. We show that GALEX is easily capable of detecting star formation in
brightest cluster galaxies (BCGs) out to and 50-100 kpc. In most of
the CC clusters studied, we find significant UV luminosity excesses and colors
that strongly suggest recent and/or current star formation. The BCGs are found
to have blue UV colors in the center that become increasingly redder with
radius, indicating that the UV signature of star formation is most easily
detected in the central regions. Our findings show good agreement between UV
star formation rates and estimates based on H observations. IR
observations coupled with our data indicate moderate-to-high dust attenuation.
Comparisons between our UV results and the X-ray properties of our sample
suggest clear correlations between UV excess, cluster entropy, and central
cooling time, confirming that the star formation is directly and
incontrovertibly related to the cooling gas.Comment: 39 pages, 11 figures; accepted for publication in The Astrophysical
Journal. Figure quality reduced to comply with arXiv file size requirement
A Powerful AGN Outburst in RBS 797
Utilizing ks of Chandra X-ray Observatory imaging, we present an
analysis of the intracluster medium (ICM) and cavity system in the galaxy
cluster RBS 797. In addition to the two previously known cavities in the
cluster core, the new and deeper X-ray image has revealed additional structure
associated with the active galactic nucleus (AGN). The surface brightness
decrements of the two cavities are unusually large, and are consistent with
elongated cavities lying close to our line-of-sight. We estimate a total AGN
outburst energy and mean jet power of erg and
erg s, respectively, depending on the
assumed geometrical configuration of the cavities. Thus, RBS 797 is apparently
among the the most powerful AGN outbursts known in a cluster. The average mass
accretion rate needed to power the AGN by accretion alone is
yr. We show that accretion of cold gas onto the AGN at this level is
plausible, but that Bondi accretion of the hot atmosphere is probably not. The
BCG harbors an unresolved, non-thermal nuclear X-ray source with a bolometric
luminosity of erg s. The nuclear emission is
probably associated with a rapidly-accreting, radiatively inefficient accretion
flow. We present tentative evidence that star formation in the BCG is being
triggered by the radio jets and suggest that the cavities may be driving weak
shocks () into the ICM, similar to the process in the galaxy
cluster MS 0735.6+7421.Comment: Accepted to ApJ; 20 pages, 11 low-resolution figure
Removing Cool Cores and Central Metallicity Peaks in Galaxy Clusters with Powerful AGN Outbursts
Recent X-ray observations of galaxy clusters suggest that cluster populations
are bimodally distributed according to central gas entropy and are separated
into two distinct classes: cool core (CC) and non-cool core (NCC) clusters.
While it is widely accepted that AGN feedback plays a key role in offsetting
radiative losses and maintaining many clusters in the CC state, the origin of
NCC clusters is much less clear. At the same time, a handful of extremely
powerful AGN outbursts have recently been detected in clusters, with a total
energy ~10^{61}-10^{62} erg. Using two dimensional hydrodynamic simulations, we
show that if a large fraction of this energy is deposited near the centers of
CC clusters, which is likely common due to dense cores, these AGN outbursts can
completely remove CCs, transforming them to NCC clusters. Our model also has
interesting implications for cluster abundance profiles, which usually show a
central peak in CC systems. Our calculations indicate that during the CC to NCC
transformation, AGN outbursts efficiently mix metals in cluster central
regions, and may even remove central abundance peaks if they are not broad
enough. For CC clusters with broad central abundance peaks, AGN outbursts
decrease peak abundances, but can not effectively destroy the peaks. Our model
may simultaneously explain the contradictory (possibly bimodal) results of
abundance profiles in NCC clusters, some of which are nearly flat, while others
have strong central peaks similar to those in CC clusters. A statistical
analysis of the sizes of central abundance peaks and their redshift evolution
may shed interesting insights on the origin of both types of NCC clusters and
the evolution history of thermodynamics and AGN activity in clusters.Comment: Slightly revised version, accepted for publication in ApJ. 12 pages,
11 figure
Anisotropic Thermal Conduction and the Cooling Flow Problem in Galaxy Clusters
We examine the long-standing cooling flow problem in galaxy clusters with 3D
MHD simulations of isolated clusters including radiative cooling and
anisotropic thermal conduction along magnetic field lines. The central regions
of the intracluster medium (ICM) can have cooling timescales of ~200 Myr or
shorter--in order to prevent a cooling catastrophe the ICM must be heated by
some mechanism such as AGN feedback or thermal conduction from the thermal
reservoir at large radii. The cores of galaxy clusters are linearly unstable to
the heat-flux-driven buoyancy instability (HBI), which significantly changes
the thermodynamics of the cluster core. The HBI is a convective,
buoyancy-driven instability that rearranges the magnetic field to be
preferentially perpendicular to the temperature gradient. For a wide range of
parameters, our simulations demonstrate that in the presence of the HBI, the
effective radial thermal conductivity is reduced to less than 10% of the full
Spitzer conductivity. With this suppression of conductive heating, the cooling
catastrophe occurs on a timescale comparable to the central cooling time of the
cluster. Thermal conduction alone is thus unlikely to stabilize clusters with
low central entropies and short central cooling timescales. High central
entropy clusters have sufficiently long cooling times that conduction can help
stave off the cooling catastrophe for cosmologically interesting timescales.Comment: Submitted to ApJ, 14 pages, 14 figure
Quasisteady Configurations of Conductive Intracluster Media
The radial distributions of temperature, density, and gas entropy among
cool-core clusters tend to be quite similar, suggesting that they have entered
a quasi-steady state. If that state is regulated by a combination of thermal
conduction and feedback from a central AGN, then the characteristics of those
radial profiles ought to contain information about the spatial distribution of
AGN heat input and the relative importance of thermal conduction. This paper
addresses those topics by deriving steady-state solutions for clusters in which
radiative cooling, electron thermal conduction, and thermal feedback fueled by
accretion are all present, with the aim of interpreting the configurations of
cool-core clusters in terms of steady-state models. It finds that the core
configurations of many cool-core clusters have entropy levels just below those
of conductively balanced solutions in which magnetic fields have suppressed
electron thermal conduction to ~1/3 of the full Spitzer value, suggesting that
AGN feedback is triggered when conduction can no longer compensate for
radiative cooling. And even when feedback is necessary to heat the central ~30
kpc, conduction may still be the most important heating mechanism within a
cluster's central ~100 kpc.Comment: ApJ in press, 13 pages, 5 figure
The first bent double lobe radio source in a known cluster filament: Constraints on the intra-filament medium
We announce the first discovery of a bent double lobe radio source (DLRS) in
a known cluster filament. The bent DLRS is found at a distance of 3.4 Mpc from
the center of the rich galaxy cluster, Abell~1763. We derive a bend angle
alpha=25deg, and infer that the source is most likely seen at a viewing angle
of Phi=10deg. From measuring the flux in the jet between the core and further
lobe and assuming a spectral index of 1, we calculate the minimum pressure in
the jet, (8.0+-3.2)x10^-13 dyn/cm^2, and derive constraints on the
intra-filament medium (IFM) assuming the bend of the jet is due to ram
pressure. We constrain the IFM to be between (1-20)x10^-29 gm/cm^3. This is
consistent with recent direct probes of the IFM and theoretical models. These
observations justify future searches for bent double lobe radio sources located
several Mpc from cluster cores, as they may be good markers of super cluster
filaments.Comment: 13 pages, 4 figures, accepted in ApJ Letter
Abundances of s-process elements in planetary nebulae: Br, Kr & Xe
We identify emission lines of post-iron peak elements in very high
signal-to-noise spectra of a sample of planetary nebulae. Analysis of lines
from ions of Kr and Xe reveals enhancements in most of the PNe, in agreement
with the theories of s-process in AGB star. Surprisingly, we did not detect
lines from Br even though s-process calculations indicate that it should be
produced with Kr at detectable levels.Comment: 2 pages, 1 figure, to be published in the Proceedings of the IAU
Symposium 234: Planetary Nebulae in Our Galaxy and Beyond, eds. M.J. Barlow,
R.H. Mende
Entropy Limit and the Cold Feedback Mechanism in Cooling Flow Clusters
I propose an explanation to the finding that star formation and visible
filaments strong in Halpha emission in cooling flow clusters occur only if the
minimum specific entropy and the radiative cooling time of the intracluster
medium (ICM), are below a specific threshold. The explanation is based on the
cold feedback mechanism. In this mechanism the mass accreted by the central
black hole originates in non-linear over-dense blobs of gas residing in an
extended region of the cooling flow region. I use the criterion that the
feedback cycle period must be longer than the radiative cooling time of dense
blobs for large quantities of gas to cool to low temperature. The falling time
of the dense blobs is parameterized by the ratio of the infall velocity to the
sound speed. Another parameter is the ratio of the blobs' density to that of
the surrounding ICM. By taking the values of the parameters as in previous
papers on the cold feedback model, I derive an expression that gives the right
value of the entropy threshold. Future studies will have to examine in more
detail the role of these parameters, and to show that the observed sharp change
in the behavior of clusters across the entropy, or radiative cooling time,
threshold can be reproduced by the model.Comment: Accepted by ApJ Letter
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