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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 self-absorption
(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
small-scale features which might otherwise be washed out. Our investiga-
tion is the rst to employ wide-eld 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 0 resolution with 0:8 km s 1 velocity channels over the region
[147:3 ф > ` > 74:2 ф ; 3:6 ф < b < +5:6 ф ].
This poster gives preliminary results for a full- edged analysis of the gas
properties and distribution of HISA features in the CGPS. We employ
several automated algorithms to identify self-absorption 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
gures shows some of the HISA mapped to date, while the bottom row
shows measured and derived HISA gas properties and an analysis of the
relation between HISA and CO emission.

Property 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 we determine
s from estimated characteristic angular scales
of features and likely distances based on velocity and a simple spiral arm
model (e.g., gas near ` = 140 ф ; v LSR
= 40 km s 1 is in the Perseus arm,
with a distance of  2 kpc, for which 1 0 corresponds to 0.6 pc). 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 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
lower-resolution 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 peaks at 0.8 and 1.6 km s 1 correspond to 1 and 2 chan-
nels, respectively; many of our features may be undersampled in ve-
locity. Likewise, most features have angular widths close to the 1 0
CGPS beam, suggesting there is ner structure we are missing.
 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.
 The H i column density is proportional to both T S
and  and drops
more than either for low f n . In the f n = 0:01 case, these columns
should be multiplied by  200 to obtain the full atomic + molecular
value. The total HISA mass in this 5 ф  5 ф sample is  10 5 M for
f n = 1.
 HISA strength shows no correlation with 12 CO brightness, but when
this apparent scatter is converted into a cumulative 2-D histogram of
HISA stronger than one threshhold matching CO stronger than an-
other threshhold, the resulting diagram shows a degree of HISA-CO
association which signicantly exceeds the random probability of asso-
ciation, especially for strong HISA and weak CO. Consequently, there
are many HISA clouds containing CO, but either the atomic/molecular
fraction is not constant in these objects, or the CO is not tracing all
the H 2 .

Figure 1: HISA Survey Map (`; b): Sky projection of detected HISA over a 28 ф section of the
CGPS, showing velocity-integrated T ON
T OFF values. Darker features have stronger absorption.
The overlay transparency shows 12 CO emission in the Perseus spiral arm, where most of the HISA
is found for this longitude range.
Figure 2: HISA Survey Map (`; v LSR ): Longitude-velocity projection of the HISA currently
mapped over the full CGPS, from ` = 147 ф to 74 ф ; data between ` = 114 ф and 87 ф have yet
to be processed. Contours of latitude-integrated HISA T ON T OFF contrast are shown on top of
H i emission from the Leiden-Dwingeloo survey. Red lines mark approximate velocity boundaries
between gas in the Local, Perseus, and Outer spiral arms.
Figure 3: HISA Survey Coverage and Completeness: A cumulative count of the number of
image voxels containing HISA, normalized by the number of voxels with bright enough H i for HISA
to be detected. About 8% of the voxels in which HISA might be found appear to have HISA at
some level, but there may be more HISA which is too faint to be seen in our data.
Figure 4: HISA Feature Properties: These histograms show the value distributions of a number
of properties. The top row of T ON T OFF ,
 and
v are all directly measured quantities. The two
lower rows give derived values for T S ,  , and N HI , using f n = 1 in the middle row and f n = 0:01 in
the bottom row.
Figure 5: HISA - CO Correlation: HISA T ON T OFF contrast and 12 CO brightness temperature
show no obvious correlation, in contradiction to the traditional expectation of HISA tracing a small,
xed fraction of atomic gas in molecular clouds. The intensity scale of the plot is logarithmic.
Figure 6: HISA - CO Association: Despite the lack of quantitative correlation, many HISA
clouds do contain CO at some level, and vice versa. Here we plot the number of image voxels with
HISA above a given contrast and CO above a given brightness, normalized by the number of voxels
which would be associated in a purely random distribution. The intensity scale is linear. The red
contours mark S=N levels of 3, 10, 30, and 100 for the normalized association statistic.