224 research outputs found
Stopping Cooling Flows with Jets
We describe two-dimensional gasdynamical models of jets that carry mass as well as energy to the hot gas in galaxy clusters. These flows have many attractive attributes for solving the galaxy cluster cooling flow problem: why the hot gas temperature and density profiles resemble cooling flows but show no spectral evidence of cooling to low temperatures. Using an approximate model for the cluster A1795, we show that mass-carrying jets can reduce the overall cooling rate to or below the low values implied by X-ray spectra. Biconical subrelativistic jets, described by several ad hoc parameters, are assumed to be activated when gas flows toward or cools near a central supermassive black hole. As the jets proceed out from the center, they entrain more and more ambient gas. The jets lose internal pressure by expansion and are compressed by the ambient cluster gas, becoming rather difficult to observe. For a wide variety of initial jet parameters and several feedback scenarios, the global cooling can be suppressed for many gigayears while maintaining cluster temperature profiles similar to those observed. The intermittency of the feedback generates multiple generations of X-ray cavities similar to those observed in the Perseus Cluster and elsewhere
Hot gaseous atmospheres in galaxy groups and clusters are both heated and cooled by X-ray cavities
Expanding X-ray cavities observed in hot gas atmospheres of many galaxy
groups and clusters generate shock waves and turbulence that are primary
heating mechanisms required to avoid uninhibited radiatively cooling flows
which are not observed. However, we show here that the evolution of buoyant
cavities also stimulates radiative cooling of observable masses of
low-temperature gas. During their early evolution, radiative cooling occurs in
the wakes of buoyant cavities in two locations: in thin radial filaments
parallel to the buoyant velocity and more broadly in gas compressed beneath
rising cavities. Radiation from these sustained compressions removes entropy
from the hot gas. Gas experiencing the largest entropy loss cools first,
followed by gas with progressively less entropy loss. Most cooling occurs at
late times, yrs, long after the X-ray cavities have disrupted
and are impossible to detect. During these late times, slightly denser low
entropy gas sinks slowly toward the centers of the hot atmospheres where it
cools intermittently, forming clouds near the cluster center. Single cavities
of energy ergs in the atmosphere of the NGC 5044 group create
of cooled gas, exceeding the mass of extended
molecular gas currently observed in that group. The cooled gas clouds we
compute share many attributes with molecular clouds recently observed in NGC
5044 with ALMA: self-gravitationally unbound, dust-free, quasi-randomly
distributed within a few kpc around the group center.Comment: 12 pages, 11 figure; accepted for publication by Ap
The Mid-Infrared Spectral Energy Distribution, Surface Brightness and Color Profiles in Elliptical Galaxies
We describe photometry at mid-infrared passbands (1.2 - 24 microns) for a
sample of 18 elliptical galaxies. All surface brightness distributions resemble
de Vaucouleurs profiles, indicating that most of the emission arises from the
photospheres or circumstellar regions of red giant stars. The spectral energy
distribution peaks near 1.6 microns, but the half-light or effective radius has
a pronounced minimum near the K band (2.15 microns). Apart from the 24 micron
passband, all sample-averaged radial color profiles have measurable slopes
within about twice the (K band) effective radius. Evidently this variation
arises because of an increase in stellar metallicity toward the galactic cores.
For example, the sampled-averaged color profile (K - 5.8 microns) has a
positive slope although no obvious absorption feature is observed in spectra of
elliptical galaxies near 5.8 microns. This, and the minimum in the effective
radius, suggests that the K band may be anomalously luminous in metal-rich
stars in galaxy cores. Unusual radial color profiles involving the 24 micron
passband may suggest that some 24 micron emission comes from interstellar not
circumstellar dust grains.Comment: 18 pages. Accepted by Ap
The Entire Virial Radius of the Fossil Cluster RXJ1159+5531: I. Gas Properties
Previous analysis of the fossil-group/cluster RXJ1159+5531 with X-ray
observations from a central Chandra pointing and an offset-North Suzaku
pointing indicate a radial intracluster medium (ICM) entropy profile at the
virial radius () consistent with predictions from gravity-only
cosmological simulations, in contrast to other cool-core clusters. To examine
the generality of these results, we present three new Suzaku observations that,
in conjunction with the North pointing, provide complete azimuthal coverage out
to . With two new Chandra ACIS-I observations overlapping the
North Suzaku pointing, we have resolved 50\% of the cosmic X-ray
background there. We present radial profiles of the ICM density, temperature,
entropy, and pressure obtained for each of the four directions. We measure only
modest azimuthal scatter in the ICM properties at between the
Suzaku pointings: 7.6\% in temperature and 8.6\% in density, while the
systematic errors can be significant. The temperature scatter, in particular,
is lower than that studied at for a small number of other
clusters observed with Suzaku. These azimuthal measurements verify that
RXJ1159+5531 is a regular, highly relaxed system. The well-behaved entropy
profiles we have measured for RXJ1159+5531 disfavor the weakening of the
accretion shock as an explanation of the entropy flattening found in other
cool-core clusters but is consistent with other explanations such as gas
clumping, electron-ion non-equilibrium, non-thermal pressure support, and
cosmic ray acceleration. Finally, we mention that the large-scale galaxy
density distribution of RXJ1159+5531 seems to have little impact on its gas
properties near .Comment: Accepted for publication in Ap
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