1,088 research outputs found
Cosmic Ray Feedback
Cosmic rays produced or deposited at sites in hot cluster gas are thought to
provide the pressure that forms X-ray cavities. While cavities have a net
cooling effect on cluster gas, young, expanding cavities drive shocks that
increase the local entropy. Cavities also produce radial filaments of thermal
gas and are sources of cluster cosmic rays that diffuse through cavity walls,
as in Virgo where a radio lobe surrounds a radial thermal filament. Cosmic rays
also make the hot gas locally buoyant, allowing large masses of low entropy gas
to be transported out beyond the cooling radius. Successive cavities maintain a
buoyant outflow that preserves the cluster gas temperature and gas fraction
profiles and dramatically reduces the cooling rate onto the central black hole.Comment: 4 pages, 1 figure, to appear in proceedings of the conference "The
Monster's Fiery Breath: Feedback in Galaxies, Groups, and Clusters", June
2009, Madison Wisconsi
Cosmic ray diffusion fronts in the Virgo cluster
The pair of large radio lobes in the Virgo cluster, each about 23 kpc in
radius, have curiously sharp outer edges where the radio-synchrotron continuum
flux declines abruptly. However, just adjacent to this sharp transition, the
radio flux increases. This radio limb-brightening is observed over at least
half of the perimeter of both lobes. We describe slowly propagating steady
state diffusion fronts that explain these counterintuitive features. Because of
the natural buoyancy of radio lobes, the magnetic field is largely tangent to
the lobe boundary, an alignment that polarizes the radio emission and
dramatically reduces the diffusion coefficient of relativistic electrons. As
cosmic ray electrons diffuse slowly into the cluster gas, the local magnetic
field and gas density are reduced as gas flows back toward the radio lobe.
Radio emission peaks can occur because the synchrotron emissivity increases
with magnetic field and then decreases with the density of non-thermal
electrons. A detailed comparison of steady diffusion fronts with quantitative
radio observations may reveal information about the spatial variation of
magnetic fields and the diffusion coefficient of relativistic electrons. On
larger scales, some reduction of the gas density inside the Virgo lobes due to
cosmic ray pressure must occur and may be measurable. Such X-ray observations
could reveal important information about the presence of otherwise unobservable
non-thermal components such as relativistic electrons of low energy or proton
cosmic rays.Comment: 11 pages, 5 figures, Accepted 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
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
It Looks Like a Vessel, It Moves Like a Vessel, But It’s Not a Vessel: Revisiting Vessel Status in Louisiana after Caldwell v. St. Charles Gaming Company
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