``A New View of Cold Atomic Clouds in the Outer Galaxy''

S. J. Gibson, A. R. Taylor (University of Calgary), & L. A. Higgs (HIA)

1999, Bull. A.A.S., 31, 1475, #72.06

Press Release

A press release on the Canadian Galactic Plane Survey was associated with this poster.

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 HI, and most observations of HI absorption against continuum sources are limited to discrete points. However, HI self-absorption (HISA) against warmer background HI can give a better view of the structure and distribution of cold HI clouds in the Galaxy.

A systematic HISA study of cold Galactic HI 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. 2000; English et al. 1998) maps taken with the DRAO Synthesis Telescope (Landecker et al. 1999). 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°].


Below are maps of HI emission, HISA, and 12CO emission in the eastern 25% of the survey, given as projections along the velocity (Figure 1) and latitude (Figure 2) axes. We find:


English, J., et al., 1998, Pub. Ast. Soc. Aust., 15, 56
Heyer, M. H., Brunt, C., Snell, R. L., Howe, J. E., Schoerb, F. P., & Carpenter,
     J. M., 1998, Ap. J. Supp., 115, 241
Landecker, T. L., et al., 1999, A. & A., submitted
Roberts, W. W., 1972, Ap. J., 173, 259
Taylor, A. R., et al., 2000, A. J., submitted


Larger versions of each image below are available via links. PostScript copies of Figures 1 & 2 are in many pieces to permit printing on regular letter-sized pages; they are collected in sizeable gzipped tarfiles. Please note assembling these panels after printing is a somewhat tedious job!

Figure 1: HI Survey Map (l,b)

HI 21cm brightness integrated over all velocity channels; white is bright and black is faint. Blue contours show HISA column density (8 × 1019 cm-2), computed for an assumed HISA spin temperature of 30 K. Red contours show FCRAO 12CO J=1-0 brightness integrated over the Perseus arm, from the Heyer et al. (1998) survey.

Full-size version: JPEG | GIF | PS panels

Figure 2: HI Survey Map (l,v)

As above, except integrated over latitude instead of velocity; HISA column contours are for 0.5 and 1.0 × 1021 cm-2. Local gas occurs near 0 km/s, and Perseus arm gas near -40 km/s. The green line marks the position of the Roberts (1972) model HI spiral shock ridge in the Perseus arm.

Full-size version: JPEG | GIF | PS panels

Figure 3: HI Brightness

Close view of a ``raw'' HI velocity channel with self-absorption. Brightness ranges from 40 K (black) to 130 K (white). ON and OFF HI spectra, from the cross and boxes respectively (visible in the full-scale image), are plotted for one strong but compact HISA feature. The upper panel compares ON (solid) with OFF (dashed), and the lower panel shows the ON-OFF temperature difference.


Figure 4: Velocity Search

We are developing automated methods of locating HISA in velocity cubes by removing large-scale emission in space and velocity and flagging small-scale dark residuals. This image shows (negative) temperatures of the dark residuals found with our spectral search, ranging from 0 to -40 K.

Full JPEG | GIF | PS

Figure 5: Spatial Search

Here are residual (negative) temperatures from our spatial search. We combine these results with those of the spectral search to flag suspected HISA voxels (3-D pixels).

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Figure 6: HISA Amplitude

Final 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 HI brightness behind the HISA feature. This is essential for determining its absorption properties and mass.

Full JPEG | GIF | PS


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