``The Dark Side of the Galaxy: a Skiagraphic Panorama of Cold H I''

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

2002, 33rd Annual Meeting of the Canadian Astronomical Society

Electronic Poster Contents



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 skiagraphically 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 automated methods of detecting and analyzing HISA that allow us to study the cloud population systematically in the CGPS. Our HISA search algorithms filter out all H I emission having large angular scales or broad linewidths. The significant negative features which remain are flagged 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 find 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.


Figure 1 provides 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 TON - TOFF for the HISA, where darker blue colors indicate greater absorption. The pink is integrated 12CO J=1-0 emission from the FCRAO Outer Galaxy Survey (Heyer et al. 1998). Since the vast majority of HISA within the 102-141 degree longitude range of the FCRAO survey is in the Perseus arm, only Perseus CO is shown in the (l,v) projection. 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 filamentary complexes. Most of these are at Perseus spiral arm velocities near -40 km/s 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 L ~ 80 degrees, it is natural to see more HISA in these locations.


While strong HISA appears confined to discrete structures in space and velocity, weaker HISA is found everywhere that the H I background is sufficiently bright. Figure 2 illustrates this point. The panel on the left shows integrated HISA optical depth, emphasizing the concentrated 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 off 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.


Larger versions of each image below are available via links.

Figure 1: CGPS HISA + CO Panoramas

In the poster shown at the meeting, these two figures consisted of large, multipanel maps of the HISA (blue) with overlay transparencies of the CO (pink). Links are provided below for these PS panel files as well as single, lower-resolution GIFs. See text for further details.

(l,b) Projection:

GIFs: HISA + CO | HISA only | CO only + PS panels: HISA 1 | HISA 2 | HISA 3 | HISA 4 | CO 1 | CO 2

(l,v) Projection:

GIFs: HISA + CO | HISA only | CO only + PS panels: HISA 1 | HISA 2 | HISA 3 | HISA 4 | CO 1 | CO 2

Figure 2: HISA Optical Depth vs. Velocity

HISA distribution in velocity space for strong and weak features, with Roberts' (1972) spiral shock ridge shown again for comparison. See text for further details.

Total tau: Maximum tau:


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