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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 investigation is the first to employ wide-field synthesis imaging to these ends. We are using Canadian Galactic Plane Survey (A. R. Taylor et al., in prep) maps taken with the DRAO Synthesis Telescope. Our CGPS images have ~ 1' resolution with 0.8 km/s velocity channels over the region [147.3° > l > 74.2°, -3.6° < b < +5.6°].

This poster gives preliminary results for a full-fledged analysis of the gas properties and distribution of HISA features in the CGPS (S. J. Gibson et al., in prep). 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. Figures 1-4 show some of the HISA mapped to date, while Figures 5-8 show measured and derived HISA gas properties and an analysis of the relation between HISA and CO emission.

where T_ON and T_OFF are observed brightnesses on and off the HISA feature, T_S is the spin or excitation temperature of the HISA gas, tau 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 tau remain unknown. To constrain these two variables, we make use of line integral and ideal gas relations to derive a second equation

which gives T_S in terms of the line center opacity tau₀, linewidth delta-v, the physical thickness of the HISA feature along the sightline delta-s, and the partial pressure of the atomic gas, P f_n. With reasonable values applied to these new parameters, T_S and tau 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 delta-s from estimated characteristic angular scales of features and likely distances based on velocity and a simple spiral arm model (e.g., gas near L=140, V=-40 is in the Perseus arm, with a distance of ~ 2 kpc, for which 1 arcminute 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.

• 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 correspond to 1 and 2 channels, respectively; many of our features may be undersampled in velocity. Likewise, most features have angular widths close to the 1' CGPS beam, suggesting there is finer 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 H₂ (f_n = 0.01), it has typical molecular cloud temperatures.

• Since the H I column density is proportional to both T_S and tau, it drops precipitously for low f_n. In the f_n = 0.01 case, the H I columns should be multiplied by ~200 to obtain the full atomic + molecular value. The total HISA mass in this 5 x 5 degree sample is ~10^5 M_sun for f_n = 1.

• HISA strength shows no correlation with ¹²CO brightness, but when this apparent scatter is converted into a cumulative 2-D histogram of HISA stronger than one threshhold matching CO stronger than another threshhold, the resulting diagram shows a degree of HISA-CO association which significantly exceeds the random probability of association, 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 H₂.

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Bottom: ¹²CO emission in the Perseus spiral arm, where most of the HISA is found for this longitude range. Note: In the AAS poster, this CO map was a transparency overlaid on the HISA map.

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While some property values are preferred over others, there are no obvious correlations (the linear features are sampling artifacts), and in fact there a general scatter over the full range of parameter values which are permissible by the limits of the telescope and the image processing software. No size vs. linewidth relation is seen in the HISA (perhaps indicating the clouds are not gravitationally bound), and the bimodality in the characteristic temperatures and optical depths results from Local arm gas being systematically closer than Perseus arm gas, which translates to smaller delta-s sightline dimensions in the property solution method (the HISA we detect has similar angular size regardless of distance). In effect, this survey is seeing the gas which it is able to see (which is colder in the local arm), but by no means the full range of what is actually there.

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