502 research outputs found
NH3 in the Central 10 pc of the Galaxy I: General Morphology and Kinematic Connections Between the CND and GMCs
New VLA images of NH3 (1,1), (2,2), and (3,3) emission in the central 10
parsecs of the Galaxy trace filamentary streams of gas, several of which appear
to feed the circumnuclear disk (CND). The NH3 images have a spatial resolution
of 16.5''x14.5'' and have better spatial sampling than previous NH3
observations. The images show the ``southern streamer,'' ``50 km/s cloud,'' and
new features including a ``western streamer'', 6 parsecs in length, and a
``northern ridge'' which connects to the CND. NH3(3,3) emission is very similar
to 1.2 mm dust emission indicating that NH3 traces column density well. Ratios
of the NH3(2,2) to (1,1) line intensities give an estimate of the temperature
of the gas and indicate high temperatures close to the nucleus and CND. The new
data cover a velocity range of 270 km/s, including all velocities observed in
the CND, with a resolution of 9.8 km/s. Previous NH3 observations with higher
resolution did not cover the entire range of velocities seen in the CND. The
large-scale kinematics of the CND do not resemble a coherent ring or disk. We
see evidence for a high velocity cloud within a projected distance of 50'' (2
pc) which is only seen in NH3(3,3) and is likely to be hot. Comparison to 6 cm
continuum emission reveals that much of the NH3 emission traces the outer edges
of Sgr A East and was probably pushed outward by this expanding shell. The
connection between the northern ridge (which appears to be swept up by Sgr A
East) and the CND indicates that Sgr A East and the CND are in close proximity
to each other. Kinematic evidence for these connections is presented in this
paper, while the full kinematic analysis of the central 10 pc will be presented
in Paper II.Comment: 16 pages (containing 6 figures), 8 additional JPEG figures. Accepted
for publication in ApJ. For full resolution images, see
http://cfa-www.harvard.edu/~rmcgary/SGRA/nh3_figures.htm
NH3 in the Central 10 pc of the Galaxy. II. Determination of Opacity for Gas with Large Linewidths
The 23 GHz emission lines from the NH3 rotation inversion transitions are
widely used to investigate the kinematics and physical conditions in dense
molecular clouds. The line profile is composed of hyperfine components which
can be used to calculate the opacity of the gas (Ho & Townes 1983). If the
intrinsic linewidth of the gas exceeds one half of the separation of these
quadrupole hyperfine components (~5-10 km/s) these lines blend together and the
observed linewidths greatly overestimate the intrinsic linewidths. If
uncorrected, these artificially broad linewidths will lead to artificially high
opacities. We have observed this effect in our NH3 data from the central 10 pc
of the Galaxy where uncorrected NH3 (1,1) linewidths of ~30 km/s exaggerate the
intrinsic linewidths by more than a factor of two (Genzel & Townes 1987).
Models of the effect of blending on the line profile enable us to solve for the
intrinsic linewidth and opacity of NH3 using the observed linewidth and
intensity of two NH3 rotation inversion transitions. We present the result of
the application of this method to our Galactic Center data. We successfully
recover the intrinsic linewidth and opacity of the gas. Clouds close to the
nucleus in projected distance as well as those that are being impacted by Sgr A
East show the highest intrinsic linewidths. The cores of the ``southern
streamer'' (Ho et al. 1991; Coil & Ho 1999, 2000) and the ``50 km/s'' giant
molecular cloud have the highest opacities.Comment: 27 pages, 10 figures, accepted for publication in Ap
Gas infall towards Sgr A* from the clumpy circumnuclear disk
We present the first large-scale mosaic performed with the Submillimeter
Array (SMA) in the Galactic center. We have produced a 25-pointing mosaic,
covering a ~2' x 2' area around Sgr A*. We have detected emission from two
high-density molecular tracers, HCN(4-3) and CS(7-6), the latter never before
reported in this region. The data have an angular resolution of 4.6" x 3.1",
and the spectral window coverage is from -180 km/s to 1490 km/s for HCN(4-3)
and from -1605 km/s to 129 km/s for CS(7-6). Both molecular tracers present a
very clumpy distribution along the circumnuclear disk (CND), and are detected
with a high signal-to-noise ratio in the southern part of the CND, while they
are weaker towards the northern part. Assuming that the clumps are as close to
the Galactic center as their projected distances, they are still dense enough
to be gravitationally stable against the tidal shear produced by the
supermassive black hole. Therefore, the CND is a non-transient structure. This
geometrical distribution of both tracers suggests that the southern part of the
CND is denser than the northern part. Also, by comparing the HCN(4-3) results
with HCN(1-0) results we can see that the northern and the southern parts of
the CND have different excitation levels, with the southern part warmer than
the northern. Finally, we compare our results with those obtained with the
detection of NH3, which traces the warmer and less dense material detected in
the inner cavity of the CND. We suggest that we are detecting the origin point
where a portion of the CND becomes destabilized and approaches the dynamical
center of the Milky Way, possibly being impacted by the southern streamer and
heated on its way inwards.Comment: 35 pages, 25 figures, 2 tables, accepted for publication in ApJ,
emulate-apj styl
The Nature of the Molecular Environment within 5 pc of the Galactic Center
We present a detailed study of molecular gas in the central 10pc of the
Galaxy through spectral line observations of four rotation inversion
transitions of NH3 made with the VLA. Updated line widths and NH3(1,1)
opacities are presented, and temperatures, column densities, and masses are
derived. We examine the impact of Sgr A East on molecular material at the
Galactic center and find that there is no evidence that the expansion of this
shell has moved a significant amount of the 50 km/s GMC. The western streamer,
however, shows strong indications that it is composed of material swept-up by
the expansion of Sgr A East. Using the mass and kinematics of the western
streamer, we calculate an energy of E=(2-9)x10^{51} ergs for the progenitor
explosion and conclude that Sgr A East was most likely produced by a single
supernova. The temperature structure of molecular gas in the central ~20pc is
also analyzed in detail. We find that molecular gas has a ``two-temperature''
structure similar to that measured by Huttemeister et al. (2003a) on larger
scales. The largest observed line ratios, however, cannot be understood in
terms of a two-temperature model, and most likely result from absorption of
NH3(3,3) emission by cool surface layers of clouds. By comparing the observed
NH3 (6,6)-to-(3,3) line ratios, we disentangle three distinct molecular
features within a projected distance of 2pc from Sgr A*. Gas associated with
the highest line ratios shows kinematic signatures of both rotation and
expansion. The southern streamer shows no significant velocity gradients and
does not appear to be directly associated with either the circumnuclear disk or
the nucleus. The paper concludes with a discussion of the line-of-sight
arrangement of the main features in the central 10pc.Comment: 51 pages, 16 figures, accepted for publication in ApJ. Due to size
limitations, some of the images have been cut from this version. A complete,
color PS or PDF version can be downloaded from
http://www.astro.columbia.edu/~herrnstein/NH3/paper
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