In the Large Magellanic Cloud a more intense UV radiation field coupled with a lower heavy-element abundance and a lower dust-to-gas ratio leads to an interstellar medium which is quite different from that in the Galaxy. The properties of molecular clouds are affected as well as those of the atomic gas. Studying the influence on the cool atomic phase is of special significance as cool atomic clouds can become the raw material for star formation and so determine the evolution of the whole galaxy.
The investigation of the cool atomic gas in the Magellanic Clouds started with a 21 cm absorption line survey using the ATCA toward continuum sources in or behind the H I halo around the Magellanic Clouds and toward the Magellanic Stream (Mebold et al. 1991). No H I absorption features were detected, indicating that all of the atomic hydrogen outside the optical boundaries of the Magellanic Clouds is in the warm phase. With lower spin temperature limits as high as 600 K, the diffuse gas in the halo of the Magellanic Clouds and in the Magellanic Stream appears warmer than that in the outer parts of our Galaxy.
In a second H I absorption line survey (Dickey et al. 1994, hereafter survey 2) the search for cool atomic gas has been extended to the inner part of the LMC. A large number of cool H I clouds were detected. Out of a sample of 30 compact sources we have identified 42 absorption features toward 19 sources. The comparison of the ATCA-H I -absorption spectra and Parkes-emission spectra near the sources yield spin temperatures between 4 K and 100 K. The mean temperature of 25 K is much below that found for H I clouds in the Milky Way ( 60 K, Kalberla et al. 1985). In a more detailed analysis of cloud temperatures near 30 Doradus, Mebold et al. (1997) compared ATCA absorption spectra with high resolution emission data near the sources and also derived low temperatures between 30 K and 40 K for the cool atomic gas. The energy balance of these cool clouds, some of which are located in warm surroundings, is still an open question. The mixture of the warm and cool phases depends critically on the gas pressure and on other physical parameters such as the heavy-element and dust abundance and the strength of the interstellar radiation field (Wolfire et al. 1995). The distribution of the cool H I gas in the LMC is in sharp contrast to the CO survey of Cohen et al. (1988) and the [C II] observations of Mochizuki et al. (1994), which show that only a few of the H I clouds have emission by heavy elements.
The unusually high fraction of cool H I near 30 Doradus and the detection of cool gas near LMC 4 and the eastern steep H I boundary raises the question whether cool gas is abundant in these regions due to a high pressure. Such a high pressure is expected near LMC 4, generated by the supernova driven shocks in this supergiant shell. Several H I shells have also recently been detected near 30 Doradus by Düsterberg et al. (in prep.). At the eastern steep H I boundary the pressure might be increased, because of the motion of the LMC through the halo of our Galaxy at rather high velocity (Mathewson & Ford 1984).
In the present third H I absorption survey toward 20 continuum sources in and behind the LMC we therefore studied the cool phase near LMC 4 and the eastern H I boundary in more detail and extended our investigation of cool H I to the vicinity of the Tarantula nebula. The physical properties of the cool H I clouds for these different regions of the LMC are determined and the spatial distribution of the abundance of the cool gas phase with regard to the locations within the LMC is investigated taking into account the results of survey 2. In two following papers we will study the relation between the cool atomic and molecular phases based on SEST-CO-observations (Paper II) and the heating/cooling balance using ISO-[C II ]-observations (Paper III).
In Sect. 2 the observing and data reduction procedures are described. The individual absorption spectra are presented in Sect. 3 and are discussed in relation to the local conditions along each line of sight through the LMC. The fraction of cool gas compared to warm, as revealed by the integrals of the absorption and emission spectra, is investigated in Sect. 4.1 and is compared with results from the Milky Way. In Sect. 4.2 the properties of individual clouds are examined and a comparison of cloud parameters for the different regions of the LMC (the 30 Doradus complex, LMC 4, the eastern steep H I boundary) is given.
© European Southern Observatory (ESO) 2000
Online publication: February 25, 2000