Statistics
Scientific paper
Jun 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997mnras.288..457p&link_type=abstract
Monthly Notices of the Royal Astronomical Society, Volume 288, Issue 2, pp. 457-469.
Statistics
2
Quasars: Absorption Lines, Galaxies: Statistics
Scientific paper
A study is made of the properties of a randomly orientated distribution of neutral hydrogen clouds, whose densities are constant on concentric similar spheroids. It is found that the distribution of column densities is relatively insensitive to the flattening of the clouds except at the highest column densities. If the clouds have an exponential surface density profile, the theoretical column density distribution for a single type of cloud can agree with the observed distribution over many orders of magnitude in column density. For a spherical cloud, a surface density profile can be found which gives complete agreement with observations. This implies that the column density distribution alone provides no strong constraints on the shapes and sizes of clouds. For rotating flattened clouds, the velocity width and velocity profile of the absorbing atoms are calculated. If broadening mechanisms are ignored, the velocity profiles can be highly asymmetric in a minority of cases. Although such asymmetric profiles could be a signature of rotation, the inclusion of Doppler broadening at a level observed in clouds removes the asymmetry, and it seems unlikely that rotation could be deduced from line profiles. However, the velocity at the line centre may differ from that of the disc centre by a significant fraction of the maximum rotation speed.
Pitts E.
Tayler Roger John
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