460 research outputs found
Powering The Intra-cluster Filaments in Cool-Core Clusters of Galaxies
The first radio surveys of the sky discovered that some large clusters of
galaxies contained powerful sources of synchrotron emission. Optical images
showed that long linear filaments with bizarre emission-line spectra permeated
the intra-cluster medium. Recent observations in the infrared and radio show
that these filaments have very strong emission lines of molecular hydrogen and
carbon monoxide. The mass of molecular material is quite large, the gas is
quite warm, and the filaments have not formed stars despite their ~Gyr age. I
will discuss the general astrophysical context of large clusters of galaxies
and how large masses of molecular gas can be heated to produce what we observe.
The unique properties of the filaments are a result of the unique
environment. Magnetically confined molecular filaments are surrounded by the
hot intra-cluster medium. Thermal particles with keV energies enter atomic and
molecular regions and produce a shower of secondary nonthermal electrons. These
secondaries collisionally heat, excite, dissociate, and ionize the cool gas.
While ionization is dominated by these secondary particles, recombination is
controlled by charge exchange, which produces the unusual optical emission line
spectrum. I will describe some of the physical processes that are unique to
this environment and outline some of the atomic physics issues.Comment: Atomic processes in plasmas - Proceedings of the 17th International
Conference on Atomic processes in plasmas (2011) Edited by: KM Aggarwal and
SFC Shearer (AIP
Magnetic fields and the location of the PDR
I review recent studies of the emission-line regions in Orion and M17. Both
have similar geometries, a bubble of hot shocked gas surrounding the central
star cluster, with H^+, H^0, and H_2 regions, often referred to as H II
regions, PDRs, and molecular clouds, forming successive shells on the surface
of a molecular cloud. The magnetic fields in the H^0 regions have been measured
with 21 cm Zeeman polarization and are found to be 1 -- 2 dex stronger than the
field in the diffuse ISM. The regions appear to be in rough hydrostatic
equilibrium. The H^+ region is pushed away from the star cluster by starlight
radiation pressure. Since most starlight is in ionizing radiation, most of its
outward push will act on the H^+ region and then on to the H^0 region. The
magnetic pressure in the H^0 region balances the momentum in starlight and
together they set the location of the H^0 region. The picture is that, when the
star cluster formed, it created a bubble of ionized gas which expanded and
compressing surrounding H^0 and H_2 regions. The magnetic field was amplified
until its pressure was able to support the momentum in starlight. This offers a
great simplification in understanding the underlying physics that establishes
parameters for PDR models
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