7,559 research outputs found
Active galactic nucleus feedback in clusters of galaxies
Observations made during the last ten years with the Chandra X-ray
Observatory have shed much light on the cooling gas in the centers of clusters
of galaxies and the role of active galactic nucleus (AGN) heating. Cooling of
the hot intracluster medium in cluster centers can feed the supermassive black
holes found in the nuclei of the dominant cluster galaxies leading to AGN
outbursts which can reheat the gas, suppressing cooling and large amounts of
star formation. AGN heating can come in the form of shocks, buoyantly rising
bubbles that have been inflated by radio lobes, and the dissipation of sound
waves.Comment: Refereed review article published in Chandra's First Decade of
Discovery Special Feature edition of the Proceedings of the National Academy
of Science
Hysteresis and Post Walrasian Economics
Macroeconomics, hysteresis The “new consensus” dsge (dynamic stochastic general equilibrium) macroeconomic model has microfoundations provided by a single representative agent. In this model shocks to the economic environment do not have any lasting effects. In reality adjustments at the micro level are made by heterogeneous agents, and the aggregation problem cannot be assumed away. In this paper we show that the discontinuous adjustments made by heterogeneous agents at the micro level mean that shocks have lasting effects, aggregate variables containing a selective, erasable memory of the shocks experienced. This hysteresis framework provides foundations for the post-Walrasian analysis of macroeconomic systems
Chandra Observation of the Radio Source / X-ray Gas Interaction in the Cooling Flow Cluster Abell 2052
We present a Chandra observation of Abell 2052, a cooling flow cluster with a
central cD that hosts the complex radio source 3C 317. The data reveal
``holes'' in the X-ray emission that are coincident with the radio lobes. The
holes are surrounded by bright ``shells'' of X-ray emission. The data are
consistent with the radio source displacing and compressing, and at the same
time being confined by, the X-ray gas. The compression of the X-ray shells
appears to have been relatively gentle and, at most, slightly transonic. The
pressure in the X-ray gas (the shells and surrounding cooler gas) is
approximately an order of magnitude higher than the minimum pressure derived
for the radio source, suggesting that an additional source of pressure is
needed to support the radio plasma. The compression of the X-ray shells has
speeded up the cooling of the shells, and optical emission line filaments are
found coincident with the brightest regions of the shells.Comment: accepted for publication in ApJ Letters; for high-resolution color
figures, see http://www.astro.virginia.edu/~elb6n/abell2052.htm
Time-Series Ensemble Photometry and the Search for Variable Stars in the Open Cluster M11
This work presents the first large-scale photometric variability survey of
the intermediate age (~200 Myr) open cluster M11. Thirteen nights of data over
two observing seasons were analyzed (using crowded field and ensemble
photometry techniques) to obtain high relative precision photometry. In this
study we focus on the detection of candidate member variable stars for
follow-up studies. A total of 39 variable stars were detected and can be
categorized as follows: 1 irregular (probably pulsating) variable, 6 delta
Scuti variables, 14 detached eclipsing binary systems, 17 W UMa variables, and
1 unidentified/candidate variable. While previous proper motion studies allow
for cluster membership determination for the brightest stars, we find that
membership determination is significantly hampered below V=15,R=15.5 by the
large population of field stars overlapping the cluster MS. Of the brightest
detected variables that have a high likelihood of cluster membership, we find
five systems where further work could help constrain theoretical stellar
models, including one potential W UMa member of this young cluster.Comment: 38 pages, 13 figures, accepted for December 2005 AJ, high-resolution
version available upon reques
The Detectability of AGN Cavities in Cooling-Flow Clusters
Chandra X-ray Observatory has revealed X-ray cavities in many nearby cooling
flow clusters. The cavities trace feedback from the central active galactic
nulceus (AGN) on the intracluster medium (ICM), an important ingredient in
stabilizing cooling flows and in the process of galaxy formation and evolution.
But, the prevalence and duty cycle of such AGN outbursts is not well
understood. To this end, we study how the cooling is balanced by the cavity
heating for a complete sample of clusters (the Brightest 55 clusters of
galaxies, hereafter B55). In the B55, we found 33 cooling flow clusters, 20 of
which have detected X-ray bubbles in their ICM. Among the remaining 13, all
except Ophiuchus could have significant cavity power yet remain undetected in
existing images. This implies that the duty cycle of AGN outbursts with
significant heating potential in cooling flow clusters is at least 60 % and
could approach 100 %, but deeper data is required to constrain this further.Comment: 4 pages, 2 figures; to appear in the proceedings of "The Monsters'
Fiery Breath", Madison, Wisconsin 1-5 June 2009, Eds. Sebastian Heinz & Eric
Wilcots; added annotation to the figur
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