25 research outputs found
A Neutral Hydrogen Self-Absorption Cloud in the SGPS
Using data from the Southern Galactic Plane Survey (SGPS) we analyze an HI
self-absorption cloud centered on l = 318.0 deg, b = -0.5 deg, and velocity, v
= -1.1 km/s. The cloud was observed with the Australia Telescope Compact Array
(ATCA) and the Parkes Radio Telescope, and is at a near kinematic distance of
less than 400 pc with derived dimensions of less than 5 x 11 pc. We apply two
different methods to find the optical depth and spin temperature. In both
methods we find upper limit spin temperatures ranging from 20 K to 25 K and
lower limit optical depths ~ 1. We look into the nature of the HI emission and
find that 60-70% originates behind the cloud. We analyze a second cloud at the
same velocity centered on l = 319 deg and b = 0.4 deg with an upper limit spin
temperature of 20 K and a lower limit optical depth of 1.6. The similarities in
spin temperature, optical depth, velocity, and spatial location are evidence
the clouds are associated, possibly as one large cloud consisting of smaller
clumps of gas. We compare HI emission data with 12CO emission and find a
physical association of the HI self-absorption cloud with molecular gas.Comment: 33 pages, 17 figures, 5 tables; Accepted for publication in ApJ. A
version with higher quality images availabe at
http://www.astro.umn.edu/~dkavars/ms.p
The VLA Galactic Plane Survey
The VLA Galactic Plane Survey (VGPS) is a survey of HI and 21-cm continuum
emission in the Galactic plane between longitude 18 degrees 67 degr. with
latitude coverage from |b| < 1.3 degr. to |b| < 2.3 degr. The survey area was
observed with the Very Large Array (VLA) in 990 pointings. Short-spacing
information for the HI line emission was obtained by additional observations
with the Green Bank Telescope (GBT). HI spectral line images are presented with
a resolution of 1 arcmin x 1 arcmin x 1.56 km/s (FWHM) and rms noise of 2 K per
0.824 km/s channel. Continuum images made from channels without HI line
emission have 1 arcmin (FWHM) resolution. VGPS images are compared with images
from the Canadian Galactic Plane Survey (CGPS) and the Southern Galactic Plane
Survey (SGPS). In general, the agreement between these surveys is impressive,
considering the differences in instrumentation and image processing techniques
used for each survey. The differences between VGPS and CGPS images are small, <
6 K (rms) in channels where the mean HI brightness temperature in the field
exceeds 80 K. A similar degree of consistency is found between the VGPS and
SGPS. The agreement we find between arcminute resolution surveys of the
Galactic plane is a crucial step towards combining these surveys into a single
uniform dataset which covers 90% of the Galactic disk: the International
Galactic Plane Survey (IGPS). The VGPS data will be made available on the World
Wide Web through the Canadian Astronomy Data Centre (CADC).Comment: Accepted for publication in The Astronomical Journal. 41 pages, 13
figures. For information on data release, colour images etc. see
http://www.ras.ucalgary.ca/VGP
A Synthetic 21-cm Galactic Plane Survey of an SPH Galaxy Simulation
We have created synthetic neutral hydrogen (HI) Galactic Plane Survey data
cubes covering 90 degrees < l < 180 degrees, using a model spiral galaxy from
SPH simulations and the radiative transfer code TORUS. The density, temperature
and other physical parameters are fed from the SPH simulation into TORUS, where
the HI emissivity and opacity are calculated before the 21-cm line emission
profile is determined. Our main focus is the observation of Outer Galaxy
`Perseus Arm' HI, with a view to tracing atomic gas as it encounters shock
motions as it enters a spiral arm interface, an early step in the formation of
molecular clouds. The observation of HI self-absorption features at these shock
sites (in both real observations and our synthetic data) allows us to
investigate further the connection between cold atomic gas and the onset of
molecular cloud formation.Comment: MNRAS accepted; 11 pages, 12 figure
The structure of HI in galactic disks: Simulations vs observations
We generate synthetic HI Galactic plane surveys from spiral galaxy
simulations which include stellar feedback processes. Compared to a model
without feedback we find an increased scale height of HI emission (in better
agreement with observations) and more realistic spatial structure (including
supernova blown bubbles). The synthetic data show HI self-absorption with a
morphology similar to that seen in observations. The density and temperature of
the material responsible for HI self-absorption is consistent with
observationally determined values, and is found to be only weakly dependent on
absorption strength and star formation efficiency.Comment: 12 pages, 7 figures. Accepted for publication in MNRA
Compact HI clouds at high forbidden velocities in the inner Galaxy
The VLA Galactic Plane Survey (VGPS) of the first Galactic quadrant was
searched for HI emission with velocities well above the maximum velocity
allowed by Galactic rotation. A sample of 17 small fast-moving clouds was
identified. The distribution of the ensemble of clouds in longitude and
velocity indicates that the clouds are part of the Galactic disk, despite their
large forbidden velocity. The median angular diameter of the clouds detected in
the VGPS is 3.4 arcminutes. These clouds would not be noticed in previous low
resolution surveys because of strong beam dilution. Assuming each cloud is
located at the tangent point, a median cloud has a diameter of 10 pc, HI mass
of 60 M_sun, and a velocity more than 25 km/s beyond the local terminal
velocity derived from 12CO observations. Three clouds in the sample have a
velocity between 50 and 60 km/s in excess of the local terminal velocity. The
longitude distribution of the sample peaks near l = 30 degrees, while the
latitude distribution of the clouds is nearly flat. The observed longitude and
latitude distributions are compared with simulated distributions taking into
account the selection criteria of the cloud search. It is found that the number
of clouds declines with distance from the Galactic center, with an exponential
scale length 2.8 - 8 kpc at the 99% confidence level. We find a lower limit to
the scale height of the clouds of 180 pc (HWHM), but the true value is likely
significantly higher.Comment: Accepted for publication in The Astrophysical Journal. 28 pages, 10
figures of which 3 as separate jpg file
"Missing Link" Clouds in the Southern Galactic Plane Survey
We present an automated routine to search for HI self-absorption features
within the Southern Galactic Plane Survey (SGPS). The data were taken with the
Australia Telescope Compact Array (ATCA) and the Parkes Radio Telescope and
encompass 3 deg by 105 deg of sky in the Galactic plane. We apply our routine
to this entire region and derive spin temperatures and column densities for 70
of the larger HISA complexes, finding spin temperatures ranging from 6-41 K
with HI number densities of a few cm^-3. These `missing link' clouds fill in
the spin temperature and density gaps between dense molecular clouds and
diffuse atomic clouds. We compare the HI emission with 12CO emission and find
that 60% of detected HI self-absorption is correlated in space and in velocity
with a molecular counterpart. This is potentially due to a molecular/atomic gas
transition. We also compare HI self-absorption with Galactic spiral arms and
discuss the possibility of using it as a spiral arm tracer.Comment: 37 pages, 13 figures, 2 tables; Accepted for publication in Ap
MALT90 Kinematic Distances to Dense Molecular Clumps
Using molecular-line data from the Millimetre Astronomy Legacy Team 90 GHz Survey (MALT90), we have estimated kinematic distances to 1905 molecular clumps identified in the ATLASGAL 870 ÎŒm continuum survey over the longitude range 295° < l < 350°. The clump velocities were determined using a flux-weighted average of the velocities obtained from Gaussian fits to the HCO+, HNC, and N2H+ (1â0) transitions. The near/far kinematic distance ambiguity was addressed by searching for the presence or absence of absorption or self-absorption features in 21 cm atomic hydrogen spectra from the Southern Galactic Plane Survey. Our algorithm provides an estimation of the reliability of the ambiguity resolution. The Galactic distribution of the clumps indicates positions where the clumps are bunched together, and these locations probably trace the locations of spiral arms. Several clumps fall at the predicted location of the far side of the ScutumâCentaurus arm. Moreover, a number of clumps with positive radial velocities are unambiguously located on the far side of the Milky Way at galactocentric radii beyond the solar circle. The measurement of these kinematic distances, in combination with continuum or molecular-line data, now enables the determination of fundamental parameters such as mass, size, and luminosity for each clump
Very Long Baseline Interferometer measurements of turbulence in the inner solar wind
Very Long Baseline Interferometry (VLBI) observations were made of six sources in
September and 1998. The observations were made at a time when the solar
elongations of five of those sources ranged from 4\fdg 4 to 6\fdg8, so that the
lines of sight to these sources passed at distances of closest approach to the Sun of 16.5 to
25.3Â . The solar elongation of the sixth source was 16\fdg6, corresponding
to a distance of .
The observations were made with the Medicina and Noto antennas of the
Istituto di Radioastronomia, the Matera (Italy) antenna of the Agenzia Spaziale
Italiana, and the Wettzell (Germany) antenna of the Bundesamt fĂŒr Kartographie und
GeodÀsie. On two of the three days of observation (September 24 and
October 15, 1998) simultaneous observations were made at 8.380 (XÂ band) and
2.257 (SÂ band) GHz. In the third observing session (September 25, 1998) observations
were made at 4.981 GHz (CÂ band). Temporal fluctuations in the interferometer phase, induced
by the solar wind, were analysed to yield information on plasma turbulence in the
inner heliosphere. The results of this investigation are as follows.
(1) The measured interferometer phase power spectra are consistent
with a theoretical expression which incorporates independent information on solar wind
turbulence. (2) The values for , the normalization constant of the density power
spectrum, are reasonably consistent with a previously-determined expression for
, where r is the heliocentric distance, although the new values for are
systematically high with respect to this prior expression. (3) Our estimates for
the speed at which the irregularities move with respect to
the antennas, and in the radial direction from the Sun, are in the range of
190â340Â kmâs-1 for heliocentric distances of 16â26Â . These values are in
good agreement with observational estimates of the solar wind flow speed in this part of
space based on coronagraph observations. The speeds are lower than the sum of the estimated
solar wind flow speed and the local Alfvén speed. The potential significance of this
result is discussed