``Cold H I in the Outer Galaxy: Properties and Distribution''

S. J. Gibson, A. R. Taylor (University of Calgary);
L. A. Higgs, P. E. Dewdney, & C. M. Brunt (HIA)

2000, Bull. A.A.S., 32, 1401, #7.02

Electronic Poster Contents



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 (Taylor et al. 2001) 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. 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 illustrate this process over a small part of the survey, while Figures 5-6 plot the property analysis results.


A simple radiative transfer model representing a HISA cloud with foreground and background H I emission and background continuum emission is described by the expression

where TON and TOFF are observed brightnesses on and off the HISA feature, TS is the spin or excitation temperature of the HISA gas, tau is its optical depth, TC is the continuum intensity, and p is the fraction of H I emission lying behind the HISA feature. We measure TON, TOFF, and TC, and assume a likely value for p, but TS 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 TS 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 fn. With reasonable values applied to these new parameters, TS 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 delta-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 fn of 1.0 and 0.01.


The plots below give a number of results related to the properties we have begun to explore; these are taken from a 5° x 5° portion of the CGPS. We find most properties cluster around common values. Specifically:


Larger versions of each image below are available via links.

Figure 1: H I Brightness

Close view of a ``raw'' H I velocity channel with self-absorption. Brightness ranges from 40 K (black) to 120 K (white). ON and OFF H I spectra, from the cross and boxes respectively, are given for one strong but compact HISA feature marked in the intensity map. The upper spectrum panel compares ON (solid) with OFF (dashed), and the lower spectrum panel shows the ON-OFF temperature difference.


Figure 2: HISA Amplitude

ON-OFF temperature differences for all identified HISA voxels. The voxels have been assembled into contiguous 3-D groups whose non-HISA 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 self-absorb. The cooler sightlines frequently occur in cloud interiors, but this is not a general rule.


Figure 4: Optical Depth

tau values obtained simultaneously with TS. 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: Feature Properties

These histograms show the value distributions of a number of properties. TON - TOFF and delta-v are both measured quantities, independent of other assumptions (they are shown in both sets of plots). TS and tau are calculated for fn = 1 on the left and for fn = 0.01 on the right; NHI and MHI 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.



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