393 research outputs found
Kinematics of Diffuse Interstellar Clouds :Recent GMRT Results
A high latitude HI 21 cm-line absorption survey toward extragalactic sources
was recently completed using the Giant Meterwave Radio Telescope (GMRT). A
total of 104 sources with mod(b) > 15 deg. and with a 21 cm flux density
greater than 1 Jy were observed for about 120 hours. With an optical depth
detection limit of about 0.01 this is the most sensitive high-latitude survey
as yet. Most of the detected HI 21 cm-line absorption features belong to a
population with a velocity dispersion of about 7.6 km/s. These are the
'standard' HI clouds and have been well-studied for decades. However, we also
detect a second population of absorbing clouds with a velocity dispersion of
about 21 km/s. About 20 percent of the total number of absorbing clouds belong
to this population. This new population of 'fast' clouds can be identified with
a similar velocity dispersion Ca II absorbing clouds and with the Halo clouds
recently detected in HI emission from Green Bank.Comment: 5 pages, 6 figures, uses basi.cls, to be published in Bull. Astr.
Soc. Indi
A High Galactic Latitude HI 21cm-line Absorption Survey using the GMRT: II. Results and Interpretation
We have carried out a sensitive high-latitude (|b| > 15deg.) HI 21cm-line
absorption survey towards 102 sources using the GMRT. With a 3-sigma detection
limit in optical depth of ~0.01, this is the most sensitive HI absorption
survey. We detected 126 absorption features most of which also have
corresponding HI emission features in the Leiden Dwingeloo Survey of Galactic
neutral Hydrogen. The histogram of random velocities of the absorption features
is well-fit by two Gaussians centered at V(lsr) ~ 0 km/s with velocity
dispersions of 7.6 +/- 0.3 km/s and 21 +/- 4 km/s respectively. About 20% of
the HI absorption features form the larger velocity dispersion component. The
HI absorption features forming the narrow Gaussian have a mean optical depth of
0.20 +/- 0.19, a mean HI column density of (1.46 +/- 1.03) X 10^{20} cm^{-2},
and a mean spin temperature of 121 +/- 69 K. These HI concentrations can be
identified with the standard HI clouds in the cold neutral medium of the
Galaxy. The HI absorption features forming the wider Gaussian have a mean
optical depth of 0.04 +/- 0.02, a mean HI column density of (4.3 +/- 3.4) X
10^{19} cm^{-2}, and a mean spin temperature of 125 +/- 82 K. The HI column
densities of these fast clouds decrease with their increasing random
velocities. These fast clouds can be identified with a population of clouds
detected so far only in optical absorption and in HI emission lines with a
similar velocity dispersion. This population of fast clouds is likely to be in
the lower Galactic Halo.Comment: 19 pages, 19 figures. Accepted for publication in Journal of
Astrophysics & Astronom
The interstellar clouds of Adams and Blaauw revisited: an HI absorption study - I
This investigation is aimed at clarifying the nature of the interstellar gas
seen in absorption against bright O and B stars. Towards this end we have
obtained for the first time HI absorption spectra towards radio sources very
close to the lines of sight towards 25 bright stars previously studied. In this
paper we describe the selection criteria, the details regarding our
observations, and finally present the absorption spectra. In the accompanying
paper we analyse the results and draw conclusions.Comment: 20 pages, 12 figures, accepted by Journal of Astrophysics and
Astronom
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