A New View of Cold H I Clouds in the Milky Way
S. J. Gibson, A. R. Taylor, L. A. Higgs, P. E. Dewdney;
ApJ, 2000, 540, 851
ABSTRACT:We reveal cold Galactic clouds of neutral hydrogen in unprecedented detail.
Our 21 cm synthesis maps, taken from the Canadian Galactic Plane Survey,
show a numerous and diverse population of H I self-absorption (HISA)
features in gas outside the solar circle.
These objects vary in size, shape, and
contrast against the background H I.
All display a high level of angular and
velocity structure, and most would appear significantly diluted, if not
invisible, in lower resolution H I surveys.
A number of Perseus arm features
remain unresolved by the 1' beam of our survey, with apparent diameters less
than 0.6 pc at 2 kpc distance.
The majority of HISA features we detect have no
obvious 12CO emission counterparts.
This suggests that either
HISA is not found predominantly in molecular clouds, as has often been
presumed in the past, or that CO is not a good tracer of H2.
Some HISA
lacking CO shows far-infrared dust emission, though whether this arises from
shielded molecular gas or from diffuse atomic clouds is not
clear.
Constraining the gas properties of HISA remains a difficult problem, but we
introduce a new method that aids this process.
Our approach relates a number of
physical parameters via gas law and line integral relationships and should
prove powerful if the input variables are sufficiently well known.
We
explore the current allowed parameter ranges for three sample features of very
different appearance.
We find spin temperatures <~50 K and densities
>~102 cm-3.
KEYWORDS: ism: clouds, ism: globules, ism: structure, methods: analytical, radiative transfer, radio lines: ism
CODE: gibson2000