39 research outputs found
Role of Electon Excitation and Nature of Molecular Gas in Cluster Central Elliptical Galaxies
We present observations in CO(3-2) that, combined with previous observations
in CO(2-1), constrain the physical properties of the filamentary molecular gas
in the central 6.5 kpc of NGC 1275, the central giant elliptical galaxy
of the Perseus cluster. We find this molecular gas to have a temperature
K and a density -, typically
warmer and denser than the bulk of Giant Molecular Clouds (GMCs) in the Galaxy.
Bathed in the harsh radiation and particle field of the surrounding
intracluster X-ray gas, the molecular gas likely has a much higher ionization
fraction than that of GMCs. For an ionization fraction of ,
similar to that of Galactic diffuse ()
partially-molecular clouds that emit in HCN(1-0) and HCO(1-0), we show that
the same gas traced in CO can produce the previously reported emissions in
HCN(3-2), HCO(3-2), and CN(2-1) from NGC 1275; the dominant source of
excitation for all the latter molecules is collisions with electrons. To
prevent collapse, as evidenced by the lack of star formation in the molecular
filaments, they must consist of thin strands that have cross-sectional radii
0.2-2 pc if supported solely by thermal gas pressure; larger radii
are permissible if turbulence or poloidal magnetic fields provide additional
pressure support. We point out that the conditions required to relate CO
luminosities to molecular gas masses in our Galaxy are unlikely to apply in
cluster central elliptical galaxies. Rather than being virialized structures
analogous to GMCs, we propose that the molecular gas in NGC 1275 comprises
pressure-confined structures created by turbulent flows.Comment: 41 pages, 1 table, 12 figures; accepted by Ap
Tomography of X-ray Nova Muscae 1991: Evidence for ongoing mass transfer and stream-disc overflow
We present a spectroscopic analysis of the black hole binary Nova Muscae 1991
in quiescence using data obtained in 2009 with MagE on the Magellan Clay
telescope and in 2010 with IMACS on the Magellan Baade telescope at the Las
Campanas Observatory. Emission from the disc is observed in H alpha, H beta and
Ca II (8662 A). A prominent hotspot is observed in the Doppler maps of all
three emission lines. The existence of this spot establishes ongoing mass
transfer from the donor star in 2009-2010 and, given its absence in the
1993-1995 observations, demonstrates the presence of a variable hotspot in the
system. We find the radial distance to the hotspot from the black hole to be
consistent with the circularization radius. Our tomograms are suggestive of
stream-disc overflow in the system. We also detect possible Ca II (8662 A)
absorption from the donor star.Comment: 10 pages, 11 figures, 1 table. Accepted for publication in MNRA
The Complete Local Volume Groups Sample - I. Sample Selection and X-ray Properties of the High-Richness Subsample
We present the Complete Local-Volume Groups Sample (CLoGS), a statistically
complete optically-selected sample of 53 groups within 80 Mpc. Our goal is to
combine X-ray, radio and optical data to investigate the relationship between
member galaxies, their active nuclei, and the hot intra-group medium (IGM). We
describe sample selection, define a 26-group high-richness subsample of groups
containing at least 4 optically bright (log L_B>=10.2 LBsol) galaxies, and
report the results of XMM-Newton and Chandra observations of these systems. We
find that 14 of the 26 groups are X-ray bright, possessing a group-scale IGM
extending at least 65kpc and with luminosity >10^41 erg/s, while a further 3
groups host smaller galaxy-scale gas halos. The X-ray bright groups have masses
in the range M_500=0.5-5x10^13 Msol, based on system temperatures of 0.4-1.4
keV, and X-ray luminosities in the range 2-200x10^41 erg/s. We find that
~53-65% of the X-ray bright groups have cool cores, a somewhat lower fraction
than found by previous archival surveys. Approximately 30% of the X-ray bright
groups show evidence of recent dynamical interactions (mergers or sloshing),
and ~35% of their dominant early-type galaxies host AGN with radio jets. We
find no groups with unusually high central entropies, as predicted by some
simulations, and confirm that CLoGS is in principle capable of detecting such
systems. We identify three previously unrecognized groups, and find that they
are either faint (L_X,R500<10^42 erg/s) with no concentrated cool core, or
highly disturbed. This leads us to suggest that ~20% of X-ray bright groups in
the local universe may still be unidentified.Comment: Accepted for publication by MNRAS, 25 Manuscript pages with 6 tables
and 10 figures, plus 30 pages of appendices. v2 corrects minor typographical
errors identified at proof stag
Cavities and shocks in the galaxy group HCG 62 as revealed by Chandra, XMM and GMRT data
We report on the results of an analysis of Chandra, XMM-Newton and new GMRT
data of the X-ray bright compact group of galaxies HCG 62, which is one of the
few groups known to possess clear, small X-ray cavities in the inner regions.
This is part of an ongoing X-ray/low-frequency radio study of 18 groups,
initially chosen for the availability of good-quality X-ray data and evidence
for AGN/hot gas interaction. At higher frequency (1.4 GHz), the HCG 62 cavity
system shows minimal if any radio emission, but the new GMRT observations at
235 MHz and 610 MHz clearly detect extended low-frequency emission from radio
lobes corresponding to the cavities. By means of the synergy of X-ray and
low-frequency radio observations, we compare and discuss the morphology,
luminosity and pressure of the gas and of the radio source. We find that the
radio source is radiatively inefficient, with a ratio of radio luminosity to
mechanical cavity power of , and that the radio pressure of the
lobes is about one order of magnitude lower than the X-ray pressure of the
surrounding thermal gas. Thanks to the high spatial resolution of the Chandra
surface brightness and temperature profiles, we also identify a shock front
located at 36 kpc to the south-west of the group center, close to the southern
radio lobe, with a Mach number and a total power which is about one
order of magnitude higher than the cavity power. Such a shock may have heated
the gas in the southern region, as indicated by the temperature map. The shock
may also explain the arc-like region of enriched gas seen in the iron abundance
map, as this may be produced by a non-Maxwellian electron distribution near its
front.Comment: 14 pages, 8 figures, accepted for publication in ApJ. Revised version
including minor comments and expanded discussion (version with full
resolution figures available at
http://hea-www.harvard.edu/~mgitti/hcg62-gitti.pdf