298 research outputs found
A New Radio - X-Ray Probe of Galaxy Cluster Magnetic Fields
Results are presented of a new VLA-ROSAT study that probes the magnetic field
strength and distribution over a sample of 16 ``normal'' low redshift (z < 0.1)
galaxy clusters. The clusters span two orders of magnitude in X-ray luminosity,
and were selected to be free of (unusual) strong radio cluster halos, and
widespread cooling flows. Consistent with these criteria, most clusters show a
relaxed X-ray morphology and little or no evidence for recent merger activity.
Analysis of the rotation measure (RM) data shows cluster-generated Faraday RM
excess out to ~0.5 Mpc from cluster centers. The results, combined with RM
imaging of cluster-embedded sources and ROSAT X-ray profiles indicates that the
hot intergalactic gas within these ``normal'' clusters is permeated with a high
filling factor by magnetic fields at levels of = 5-10 (l/10 kpc)^{-1/2}
microGauss, where l is the field correlation length. These results lead to a
global estimate of the total magnetic energy in clusters, and give new insight
into the ultimate energy origin, which is likely gravitational. These results
also shed some light on the cluster evolutionary conditions that existed at the
onset of cooling flows.Comment: 6 pages, 1 figure, uses emulateapj5.sty, accepted by ApJ
Hot Gas Structure in the Elliptical Galaxy NGC 4472
We present X-ray spectroscopic and morphological analyses using Chandra ACIS
and ROSAT observations of the giant elliptical galaxy NGC 4472 in the Virgo
cluster. We discuss previously unobserved X-ray structures within the extended
galactic corona. In the inner 2' of the galaxy, we find X-ray holes or cavities
with radii of ~2 kpc, corresponding to the position of radio lobes. These holes
were produced during a period of nuclear activity that began 1.2 x 10^7 years
ago and may be ongoing. We also find an asymmetrical edge in the galaxy X-ray
emission 3' (14 kpc) northeast of the core and an ~8' tail (36 kpc) extending
southwest of the galaxy. These two features probably result from the
interaction of NGC 4472 gas with the Virgo gas, which produces compression in
the direction of NGC 4472's infall and an extended tail from ram pressure
stripping. Assuming the tail is in pressure equilibrium with the surrounding
gas, we compute its angle to our line of sight and estimate that its true
extent exceeds 100 kpc. Finally, in addition to emission from the nucleus
(first detected by Soldatenkov, Vikhlinin & Pavlinsky), we detect two small
extended sources within 10'' of the nucleus of the galaxy, both of which have
luminosities of ~7 x 10^38 erg/s.Comment: 25 pages, 11 figures, accepted by Ap
Diffuse Galactic Emission from Spinning Dust Grains
Spinning interstellar dust grains produce detectable rotational emission in
the 10-100 GHz frequency range. We calculate the emission spectrum, and show
that this emission can account for the ``anomalous'' Galactic background
component which correlates with 100um thermal emission from dust. Implications
for cosmic background studies are discussed.Comment: 13 pages, 3 eps figures, uses aaspp4.sty . Accepted by Ap.J.Letters
97/12/09. Corrected typos and added 1 referenc
Halo White Dwarfs and the Hot Intergalactic Medium
We present a schematic model for the formation of baryonic galactic halos and
hot gas in the Local Group and the intergalactic medium. We follow the
dynamics, chemical evolution, heat flow and gas flows of a hierarchy of scales,
including: protogalactic clouds, galactic halos, and the Local Group itself.
Within this hierarchy, the Galaxy is built via mergers of protogalactic
fragments. We find that early bursts of star formation lead to a large
population of remnants (mostly white dwarfs), which would reside presently in
the halo and contribute to the dark component observed in the microlensing
experiments. The hot, metal-rich gas from early starbursts and merging
evaporates from the clouds and is eventually incorporated into the
intergalactic medium. The model thus suggests that most microlensing objects
could be white dwarfs (m \sim 0.5 \msol), which comprise a significant
fraction of the halo mass. Furthermore, the Local Group could have a component
of metal-rich hot gas similar to, although less than, that observed in larger
clusters. We discuss the known constraints on such a scenario and show that all
local observations can be satisfied with present data in this model. The
best-fit model has a halo that is 40% baryonic, with an upper limit of 77%.Comment: 15 pages, LaTex, uses aas2pp4.sty, 7 postscript figures.
Substantially revised and enlarged to a full-length article. Somewhat
different quantitative results, but qualitative conclusions unchange
Cluster Evolution in the ROSAT North Ecliptic Pole Survey
The deepest region of the ROSAT All-Sky Survey, at the North Ecliptic Pole,
has been studied to produce a complete and unbiased X-ray selected sample of
clusters of galaxies. This sample is used to investigate the nature of cluster
evolution and explore potential implications for large-scale structure models.
The survey is 99.6% optically identified. Spectroscopic redshifts have been
measured for all the extragalactic identifications. In this Letter, first
results on cluster evolution are presented based on a comparison between the
number of the observed clusters in the North Ecliptic Pole survey and the
number of expected clusters assuming no-evolution models. At z>0.3 there is a
deficit of clusters with respect to the local universe which is significant at
> 4.7sigma. The evolution appears to commence at L_{0.5-2.0} > 1.8x10^{44} erg
s^{-1} in our data. The negative evolution goes in the same direction as the
original EMSS result, the results from the 160 deg^{2} survey by Vikhlinin et
al. (1998) and the recent results from the RDCS (Rosati et al. 2000). At lower
redshifts there is no evidence for evolution, a result in agreement with these
and other cluster surveys.Comment: 17 pages, 3 figures. Accepted for publication in ApJ Letter
Cosmological Effects of Powerful AGN Outbursts in Galaxy Clusters: Insights from an XMM-Newton Observation of MS0735+7421
We report on the results of an analysis of XMM-Newton observations of
MS0735+7421, the galaxy cluster which hosts the most energetic AGN outburst
currently known. The previous Chandra image shows twin giant X-ray cavities
(~200 kpc diameter) filled with radio emission and surrounded by a weak shock
front. XMM data are consistent with these findings. The total energy in
cavities and shock (~6 \times 10^{61} erg) is enough to quench the cooling flow
and, since most of the energy is deposited outside the cooling region (~100
kpc), to heat the gas within 1 Mpc by ~1/4 keV per particle. The cluster
exhibits an upward departure (factor ~2) from the mean L-T relation. The boost
in emissivity produced by the ICM compression in the bright shells due to the
cavity expansion may contribute to explain the high luminosity and high central
gas mass fraction that we measure. The scaled temperature and metallicity
profiles are in general agreement with those observed in relaxed clusters.
Also, the quantities we measure are consistent with the observed M-T relation.
We conclude that violent outbursts such as the one in MS0735+7421 do not cause
dramatic instantaneous departures from cluster scaling relations (other than
the L-T relation). However, if they are relatively common they may play a role
in creating the global cluster properties.Comment: 69 pages, 30 figures, accepted for publication in ApJ Main Journa
Low Frequency Radio Observations of X-ray Ghost Bubbles in Abell 2597: A History of Radio Activity in the Core
A previous analysis of the Chandra X-ray image of the center of the cooling
core cluster Abell 2597 showed two ``ghost holes'' in the X-ray emission to the
west and northeast of the central radio galaxy PKS 2322-123. Previous radio
observations did not detect any radio emission coming from the interior of the
X-ray holes. We present new low frequency radio observations of Abell 2597. At
330 MHz, radio emission extends into the interior of the western ghost bubble,
but not the northeast one. Our re-analysis of the archival Chandra data shows
evidence for an X-ray tunnel (elongated region of reduced X-ray emission)
extending from near the center of the cD out to the west ghost bubble. We also
detect a smaller X-ray hole to the northeast of the center of the cD and closer
than the outer ghost bubbles. Radio observations at 1.3 GHz show extensions to
the west along the X-ray tunnel toward the west ghost bubble, to the northeast
into the new X-ray hole, and to the northwest. All of these structures are much
larger than the two inner radio lobes seen previously at 8 GHz. The X-ray
tunnel suggests that the west ghost bubble is part of a continuous flow of
radio plasma out from the active galactic nucleus, rather than a detached
buoyant old radio lobe, and thus it may be an intermediate case between an
active radio galaxy and a buoyant lobe.Comment: ApJ accepted, 5 page
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