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Introduction
Though a major constituent of the interstellar medium, cold atomic gas,
with T # 100 K, is elusive. Maps of 21cm emission are dominated by
warm H i, and most observations of H i absorption against continuum
sources are limited to discrete points. However, H i self­absorption
(HISA) against warmer background H i can give a better view of the
structure and distribution of cold H i clouds in the Galaxy.
A systematic HISA study of cold Galactic H i requires broad angular
coverage to remain unbiased, as well as high angular resolution to detect
small­scale features which might otherwise be washed out. Our investiga­
tion is the first to employ wide­field synthesis imaging to these ends. We
are using Canadian Galactic Plane Survey (Taylor et al. 2001)
maps taken with the DRAO Synthesis Telescope. Our CGPS images
have # 1 # resolution with 0.8 km s -1 velocity channels over the region
[147.3 # > # > 74.2 # , -3.6 # < b < +5.6 # ].
This poster gives preliminary results for a full­fledged analysis of the gas
properties and distribution of HISA features in the CGPS. We employ
several automated algorithms to identify self­absorption within the H i
data, estimate the background brightness being absorbed, and compute
its physical properties from a limited set of assumptions. The top row of
figures illustrates this process over a small part of the survey, while the
lower figures plot the property analysis results.

Method
A simple radiative transfer model representing a HISA cloud with fore­
ground and background H i emission and background continuum emission
is described by the expression
T ON - T OFF
=  T S - T C - p T OFF   1 - e -#  , (1)
where T ON
and T OFF
are observed brightnesses on and o# the HISA feature,
T S
is the spin or excitation temperature of the HISA gas, # is its optical
depth, T C
is the continuum intensity, and p is the fraction of H i emission
lying behind the HISA feature. We measure T ON
, T OFF
, and T C
, and
assume a likely value for p, but T S
and # remain unknown. To constrain
these two variables, we make use of line integral and ideal gas relations to
derive a second equation
T S
=
v u u u u t
P f n C #s
k # 0 #v
, (2)
which gives T S
in terms of the line center opacity # 0 , linewidth #v, the
physical thickness of the HISA feature along the sightline #s, and the
partial pressure of the atomic gas, P · f n . With reasonable values applied
to these new parameters, T S
and # can be obtained by solving the two
equations together (see Gibson et al., 2000 ApJ 540, 851 for details).
For this poster, we assume p = 1, the most favorable value for seeing
HISA, and #s = 0.6 pc, which corresponds to 1 # in the Perseus arm, and
may serve as a rough average scale of ``solidity'' for the HISA we detect.
We use a canonical ISM pressure of 4000 cm -3 K and consider two extreme
values for f n of 1.0 and 0.01.

Results
The plots below give a number of results related to the properties we have
begun to explore; these are taken from a 5 # â 5 # portion of the CGPS. We
find most properties cluster around common values. Specifically:
. The HISA features have a temperature contrast of only 10% against
typical H i backgrounds of # 100 K. Most are too subtle to be seen in
lower­resolution searches; the crowding of features near zero contrast
suggests many may be missed by our survey as well.
. Most features have very narrow linewidths, indicating quiescent gas.
The strong peak at 1.6 km s -1 corresponds to 2 channels in the CGPS;
many of our features may be undersampled in velocity.
. If the cold gas traced by HISA is purely atomic (f n = 1), it is at
the cold end of the normal H i temperature range, but warmer than
molecular gas. If however the gas is mostly H 2 (f n = 0.01), it has
typical molecular cloud temperatures.
. Though it often spatially anticorrelates with T S
, the optical depth
also drops significantly with T S
for low f n . The low­# tail of the
distribution represents the warmer, barely­discernable HISA.
. The H i column density is proportional to both T S
and # and drops
more than either for low f n ; the gas mass is similarly a#ected. The
second mass peak represents Local arm gas, with the main peak cor­
responding to Perseus arm gas. In the f n = 0.01 case, these columns
and masses should be multiplied by # 200 to obtain the full atomic
+ molecular value. The total HISA mass in this 5 # â 5 # sample is
# 10 5 M# for f n = 1.
. The spatial and velocity distribution of # HISA
tracks the H i back­
ground, but incompletely; most, but not all, HISA may be mixed
homogeneously with H i emission. Our investigation of the spatial
distribution of HISA is ongoing.

Figure 1: H i Brightness: Close view of a ``raw'' H i velocity channel with self­absorption.
Brightness ranges from 40 K (black) to 120 K (white). ON and OFF H i spectra, from the cross
and boxes respectively, are given below for one strong but compact HISA feature. The upper panel
compares ON (solid) with OFF (dashed), and the lower panel shows the ON­OFF temperature
di#erence.
Figure 2: HISA Amplitude: ON­OFF temperature di#erences for all identified HISA voxels. The
voxels have been assembled into contiguous 3­D groups whose non­HISA spatial and velocity edges
are used to obtain the best estimate of H i brightness behind the HISA feature. This is essential
for determining its absorption properties and mass.
Figure 3: HISA Temperature: Spin temperatures obtained at all points of the map containing
HISA; for cold H i, these are similar to the gas kinetic temperature. The intensity scale ranges
from 30 K (black) to 110 K (white), at which point the gas is too warm to self­absorb. The cooler
sightlines frequently occur in cloud interiors, but this is not a general rule.
Figure 4: HISA Optical Depth: # values obtained simultaneously with T S . The intensity scale
ranges from 0 (white) to 1.4 (black). Optical depth and temperature anticorrelate to a large degree;
the more opaque regions tend to be colder.
Figure 5: HISA Feature Properties: These histograms show the value distributions of a number
of properties. T ON -T OFF
and #v are both measured quantities. T S
and # are calculated for f n = 1
on the left and for f n = 0.01 on the right; N HI and M HI are derived from these values. HISA in the
Local spiral arm is generally less massive than that in the Perseus arm at this longitude.
Figure 6: HISA and H i Emission Velocities: This longitude ­ velocity projection shows the
relative distributions of HISA and H i emission over a larger area. The background image is H i
emission intensity, with contours of weak (yellow) and strong (red) HISA opacity overlaid. Both
weak and strong HISA trace the background H i to a limited extent, but also exhibit a degree of
independence indicating the HISA gas is not homogeneously mixed with the ambient H i.