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Astron. Astrophys. 354, 787-801 (2000)

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5. Conclusions

We have used the ATCA to measure H I absorption toward 20 continuum sources in and behind the LMC. The lines of sight are mainly in directions of the 30 Doradus complex, toward the supergiant shell LMC 4 and toward the eastern steep H I boundary. We have identified 20 absorption features toward nine of the 20 sources with a 3 [FORMULA] detection threshold between 0.33 and 0.75. The derived spin temperatures for the cool atomic clouds range from 8 K to 69 K. The cool gas parameters i.e. the optical depth [FORMULA], the "equivalent width" EW, the spin temperature [FORMULA] and the cool gas fraction [FORMULA] have been compared for the different regions of the LMC taking the results of the previous H I absorption survey of Dickey et al. (1994, survey 2) into account.

The cool gas clouds toward 30 Doradus and LMC 4 differ from clouds far from star forming regions and shock fronts in having higher values of EW and [FORMULA] and in a higher fraction of cool gas. The 30 Doradus complex shows an unusually large amount of cool H I about half of the atomic neutral hydrogen. We know of no other galaxy which possesses regions with that high fraction of cool H I gas compared to the warm. The cool gas in the vicinity of 30 Doradus shows a complex dynamic structure even beyond the optical part of this giant star forming region. Whereas all lines of sight toward the 30 Doradus complex show cool H I clouds, only about half of the lines of sight toward LMC 4 reveal H I absorption features. Cool H I clouds have been even less frequently detected in the direction of the eastern steep H I boundary. The number of detected cool H I clouds and their properties suggests a higher cooling rate near LMC 4 and 30 Doradus compared to the reference positions, caused by an enhanced density near shock fronts. We do not find a statistically significant enhancement of cool clouds toward the eastern boundary of the LMC, in spite of an expected compression zone due to the motion of the galaxy through the halo of the Milky Way. But the detected cool H I clouds at the leading edge differ from atomic clouds at reference positions by higher values of EW and [FORMULA], which seems to indicate a higher cooling rate of gas behind a large shock on the east side. The distribution of cool H I suggests a higher compression of gas on the north end of this boundary.

From the present H I absorption studies we conclude that the fraction of cool gas in the LMC is determined by local conditions of the ISM (e.g. H II regions, SNRs), rather than by the distance from the gravitational centre.

The new data corroborate the earlier suggestion that the H I clouds in the LMC are either unusually cool ([FORMULA] [FORMULA] compared to the cool phase in the Milky Way ([FORMULA] = 60 K), or that the cool atomic phase of the interstellar medium is more abundant in the LMC ([FORMULA] = 35[FORMULA] for [FORMULA] = 60 K) relative to the warm neutral medium than in our Galaxy ([FORMULA] = 24[FORMULA] for [FORMULA] = 60 K). Even, if we exclude lines of sight toward the 30 Doradus complex and toward LMC 4 we find, assuming [FORMULA] = 60 K, a similar mixture of warm and cool interstellar phases compared to that in the Milky Way, despite the completely different radiation field, heavy element abundance and dust-to-gas ratio among these galaxies.

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© European Southern Observatory (ESO) 2000

Online publication: February 25, 2000