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A study of the cool gas in the Large Magellanic Cloud
I. Properties of the cool atomic phase - a third H I absorption survey
M. Marx-Zimmer 1,
U. Herbstmeier 2,
J.M. Dickey 3,
F. Zimmer 1,
L. Staveley-Smith 4 and
U. Mebold 1
Received 16 November 1998 / Accepted 20 April 1999
The cool atomic interstellar medium of the Large Magellanic Cloud (LMC) seems to be quite different from that in the Milky Way. In a series of three papers we study the properties of the cool atomic hydrogen in the LMC (Paper I), its relation to molecular clouds using SEST-CO-observations (Paper II) and the cooling mechanism of the atomic gas based on ISO-[C II ]-investigations (Paper III).
In this paper we present the results of a third 21 cm absorption line survey toward the LMC carried out with the Australia Telescope Compact Array 1 (ATCA). 20 compact continuum sources, which are mainly in the direction of the supergiant shell LMC 4, toward the surroundings of 30 Doradus and toward the eastern steep H I boundary, have been chosen from the 1.4 GHz snapshot continuum survey of Marx et al. We have identified 20 absorption features toward nine of the 20 sources. The properties of the cool H I clouds are investigated and are compared for the different regions of the LMC taking the results of Dickey et al. (survey 2) into account.
We find that the cool H I gas in the LMC is either unusually abundant compared to the cool atomic phase of the Milky Way or the gas is clearly colder ( 30 K) than that in our Galaxy ( 60 K). The properties of atomic clouds toward 30 Doradus and LMC 4 suggest a higher cooling rate in these regions compared to other parts of the LMC, probably due to an enhanced pressure near the shock fronts of LMC 4 and 30 Doradus. The detected cool atomic gas toward the eastern steep H I boundary might be the result of a high compression of gas at the leading edge.
Key words: galaxies: Magellanic Clouds ISM: general galaxies: ISM radio lines: galaxies
Send offprint requests to: M. Marx-Zimmer
Online publication: February 25, 2000