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Introduction
A skiagram is an image made up of shadows or outlines. Much of the cold
atomic hydrogen in the interstellar medium can only be studied skiagraph-
ically as H i self-absorption (HISA) shadows against background H i
21cm emission. Using the DRAO Synthesis Telescope (Landecker et al.
2000), the Canadian Galactic Plane Survey (CGPS; Taylor et al.
2002) has captured a multitude of HISA features, the majority of which
are invisible in prior surveys at lower resolution. We have developed au-
tomated methods of detecting and analyzing HISA that allow us to study
the cloud population systematically in the CGPS. Our HISA search al-
gorithms lter out all H i emission having large angular scales or broad
linewidths. The signicant negative features which remain are agged
as HISA. The brightness of the H i behind the HISA is estimated from
emission adjacent to the HISA features in space and velocity, and this
background intensity is then used to nd the amount of absorption and
various gas properties. Since the HISA gas temperature must be less than
the background brightness temperature, all the H i shown in this poster
has T<  100 K.

Panoramas
The two large images above provide panoramic views of the cold H i
visible as HISA in the CGPS. The upper panel is a longitude-latitude
projection integrated over all gas velocities, while the lower panel is a
longitude-velocity projection integrated over all latitudes. Both give an
integral of the quantity T ON
T OFF
, where darker colors indicate greater
absorption. The overlay transparencies show integrated 12 CO J = 1 0
emission from the FCRAO Outer Galaxy Survey (Heyer et al. 1998). The
HISA and CO line strengths correlate poorly even when HISA and CO
features coincide, which is also rare. This contradicts of the traditional
view of HISA arising from trace atomic gas in molecular clouds.
The strongest HISA appears to congregate in lamentary complexes. Most
of these are at Perseus spiral arm velocities near 40 km s 1 in the eastern
(left) half of the survey, and they track the velocity curve for H i gas just
downstream of the Perseus spiral shock (Roberts 1972) rather closely. By
contrast, the western (right) end of the survey is dominated by HISA in the
Local spiral arm. All our HISA detections are limited to areas with bright
H i backgrounds, so the absence of HISA in the maps does not necessarily
indicate an absence of cold H i gas. Since the brightest backgrounds are
found in the Perseus arm and along the Local arm tangent near `  80 ф ,
it is natural to see more HISA in these locations.

Strong vs. Weak Features
While strong HISA appears conned to discrete structures in space and
velocity, weaker HISA is found everywhere that the H i background is
suфciently bright. The two gures below illustrate this point. The panel
on the left shows integrated HISA optical depth, emphasizing the con-
centrated distributions of strong HISA. The panel on the right shows the
maximum optical depth at each position (among all latitudes along the
projected axis). The opacity of the weaker HISA does not drop o at the
edges of the areas where HISA is detectable: instead, the weak HISA has
a relatively constant strength wherever it can be seen.
We infer from this behavior that the weak HISA traces a low-level ambient
population of cold H i. This material may exist throughout the interstellar
medium, but it only appears as HISA where the viewing geometry is
favorable. Quite likely, the cold atomic gas revealed by HISA is only the
tip of the iceberg. The same may well be true for the strong HISA |
other large, organized, cold H i complexes may exist outside of the areas
we can examine skiagraphically.