THE AUSTRALIA TELESCOPE SURVEY OF 21-CM ABSORPTION IN THE MAGELLANIC SYSTEM .2. THE COOL ATOMIC GAS IN THE LMC

We have used the Australia Telescope Compact Array to survey 21-cm abs orption toward compact continuum sources in and behind the Large Magel lanic Cloud. We find many absorption lines: out of a sample of 30 line s of sight observed we detect absorption in 19. This shows that the co ol atomic phase of the interstellar medium is abundant in the LMC, mor e so relative to the warm neutral medium than in the Milky Way. This a bundance of cool atomic clouds contrasts with the relative scarcity of molecular clouds in the LMC, suggesting a difference in the cloud pop ulation compared with what is found in the Milky Way. An alternative i nterpretation which is consistent with our results is that the tempera ture of the cool atomic clouds is much lower in the LMC than in the Mi lky Way, perhaps 40 K compared to 60 K in the solar neighborhood, but with a similar abundance. Further observations will resolve this ambig uity. The spatial distribution of the absorption suggests that the abu ndance of cool-phase gas in the LMC decreases with distance away from the region of intense star formation in the vicinity of 30 Dor at the north end of the giant molecular cloud. Within about 1 kpc of 30 Dor a ll lines of sight show cool gas, whereas beyond about 6 kpc none do. T his region of intense star formation and abundant molecular gas around and south of 30 Dor weighs so heavily in the cool gas statistics that it accounts for all the difference between the LMC and the Milky Way. Considering only our lines of sight that do not pass within one kpc o f 30 Dor, we find exactly the same distribution of warm and cool H I p hases as in the solar neighborhood. Most of the absorption lines detec ted are quite narrow, with half power widths of only 2 to 3 kms(-1). T hey thus allow us to probe the kinematics of the cloud population in d etail. We find random (one-dimensional) velocity dispersion of about 1 2 km s(-1) relative to the large scale velocity field as mapped in 21- cm emission surveys. Some of this velocity dispersion may be due to th e finite thickness of the H I distribution coupled with the velocity g radient along each line of sight through the LMC. Assuming cylindrical rotation for the disk, the low velocity H I emission, the so called ' 'L'' component of Luks and Rohlfs (1992), must be located on the far s ide of the LMC disk. The strong star formation activity close to and s outh of 30 Dor has to be even further away on the back side of the ''L '' component.