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DARK HYDROGEN IN THE GALACTIC PLANE
Steven J. Gibson, 1 A. Russell Taylor, 1 Jeroen M. Stil, 1 Lloyd A. Higgs, 2 Peter
E. Dewdney, 2 & Christopher M. Brunt 2
1 University of Calgary 2 Herzberg Institute of Astrophysics
Abstract New high­resolution surveys reveal an abundance of cold H i features in the
Galactic plane. These frequently trace spiral arm structure while failing to trace
CO features as well as they should if the cold H i is primarily in molecular clouds.
1. The Cold H i Phase
Cold atomic hydrogen gas with T < 10 2 K is an important component of
Galactic interstellar matter. Though it occupies only a small fraction of the ISM
volume, cold H i contains  30% of the total gas mass near the Sun [9]. It also
has abundant small­scale structure in 21cm line data, probably from turbulent
and magnetic processes, and like molecular gas, it is often found in quiescent
regions. The detailed relationship between cold H i and H 2 is of great interest,
since classical ``onion­skin'' static cloud models require an association of the
two phases that is not always seen [2, 11]. Evolution from one phase to the
other may explain such disagreements, especially in the context of large­scale
events like spiral density waves, whose structure may be probed on a Galactic
scale by the radiative transfer of the 21cm line itself.
Despite its importance, cold H i is difficult to observe [3]. Its 21cm emission
mixes with that of warmer gas, while its absorption against bright continuum
sources is limited by their angular extents. 21cm H i self­absorption (HISA)
against warm H i emission is much better for mapping cold H i, but it re­
quires high angular resolution and broad sky coverage in order to measure the
absorption properly and to chart the cloud population in an unbiased way.
2. HISA in the Galactic Plane
Detailed mapping of cold H i has become possible with the advent of the
International Galactic Plane Survey, a collection of multiwavelength surveys of
the ionized, atomic, and molecular gas and dust emission at arcminute scales
over most of the Galactic disk. 21cm line data from the Canadian [12], Southern

2
Figure 1. Top: VLA Galactic Plane Survey H i channel map of a fan­shaped  1 ô HISA
complex in the inner Galaxy. Contours show 12 CO 1 0 emission [1] for b  +1 ô with T b =1,
2, and 3 K. The dark spot at ` = 39:2 ô ; b = 0:3 ô is H i absorption against the continuum
source 3C 396. Two asterisks mark spectral sight lines. Lower left: H i and CO spectra where
HISA and CO coincide. The brightness scale is for the H i, with the CO scale exactly 10% of
this. The vertical line marks the map LSR velocity. Lower right: HISA without CO.
[8], and VLA [13] Galactic Plane Survey components of the IGPS reveal a rich
and subtle population of HISA features, many of which are invisible at lower
resolutions. Analyses of several CGPS and SGPS features [4, 6, 7] find H i
temperatures of a few tens of Kelvins and densities of order 10 2 cm 3 . Some
have obvious counterparts in CO emission, while others do not; this is also the
case with the VGPS HISA in Figure 1. While most inner­Galaxy HISA has

Dark Hydrogen in the Galactic Plane 3
Figure 2. HISA line strength integrated over latitude for a 25 ô section of the VGPS, with
darker features being stronger. Lines of constant Galactocentric radius R are overplotted for a
flat rotation curve with R0 = 8:5 kpc and v0 = 220 km s 1 .
associated CO, most outer Galaxy HISA does not [3, 4, 7]. Since inner Galaxy
sight lines are more likely to have the bright H i backgrounds needed for HISA,
more frequent association of HISA with CO is likely in the inner Galaxy. But
HISA without CO is not easy to explain: either the HISA coexists with H 2
untraced by CO, or the HISA exists outside molecular clouds, where its cold
temperature is hard to reconcile with stable gas phase models.
A systematic study using algorithms to identify and analyze HISA features
in the CGPS is underway [5]. Because these algorithms are sensitive only to the
most obvious HISA features, which are in turn biased by the need for adequate
background H i fields, they detect only a small fraction of the total cold H i
mass; however, this fraction is still very useful for studying the structure and
distribution of cold H i clouds in the Galaxy. Preliminary results indicate that,
while faint HISA occurs wherever H i backgrounds are bright, strong HISA
is concentrated in cloud complexes, many of which lie in longitude­velocity
structures tracing spiral arms [3].
Both populations require explanation, since simple differential rotation pre­
dicts only one distance for each radial velocity in the outer Galaxy, and HISA
needs a background. In this case, the weak, ubiquitous HISA probably arises
from ambient temperature fluctuations in the ISM revealed by turbulent eddies
in the H i velocity field. The strong HISA requires a more organized process:

4
its distribution is consistent with an origin in the Perseus arm's velocity reversal
[10]. Rapid cooling downstream of the spiral shock may also be the source of
the cold H i appearing as strong HISA, though this is difficult to prove directly.
The longitude­velocity distribution of HISA in the VGPS is even more striking,
as Figure 2 demonstrates. Many prominent HISA structures lie nearly parallel
to curves of constant Galactocentric radius in a simple model, much as spiral
features might appear. These HISA structures appear more concentrated and
organized than either the H i emission or the CO emission in the same region.
Acknowledgments
T. Bania and R.­J. Dettmar raised useful points that have been incorporated in
the main text due to a lack of space for a separate discussion section. Information
on CGPS and VGPS HISA can be found at www.ras.ucalgary.ca/gibson/hisa.
An overview of the IGPS is available at www.ras.ucalgary.ca/IGPS. Figures
were generated using Karma visualization software. This work is supported by
a grant from the Natural Sciences and Engineering Research Council of Canada.
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