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Astron. Astrophys. 359, 1111-1116 (2000)

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1. Introduction

The molecular hydrogen (H2) is by far the most abundant molecule in the interstellar medium (ISM) and thus plays a key role for our understanding of the molecular gas in the ISM of the Milky Way and other galaxies. Despite its large abundance, H2 in the ISM is difficult to measure because it is not seen in radio emission, in striking contrast to the second most abundant interstellar molecule, carbon monoxide (CO). H2 emission lines are seen in the near infrared (NIR), but unfortunately they are weak (quadropole transitions) and thus can not be used to study the overall interstellar abundance of H2. Molecular hydrogen in the diffuse ISM can only be studied by way of absorption spectrosopy in the far ultraviolet (FUV) toward stars or other bright UV background sources. During the seventies, considerable effort was put into the investigation of H2 absorption lines with the Copernicus satellite. Savage et al. (1977; hereafter S77) summarized Copernicus H2 measurements of 102 lines of sight toward nearby stars in the Milky Way. One of the most striking results was the correlation between the H2 column density N(H2) and colour excess [FORMULA], representative of the dust amount along a sight line. This relation has been interpreted in terms of the self-shielding effect of H2 (Federman et al. 1979). S77 showed that the transition from low to high molecular fractions in the local Galactic gas is found at total hydrogen column densities near [FORMULA] cm-2.

Measurements with Copernicus , however, were limited to the very local interstellar gas of the Milky Way. For more distant background sources, Copernicus was not sensitive enough. Later UV satellites, such as IUE and HST , had better sensitivity, but these instruments do not cover the wavelength range below 1150 Å where the transitions of H2 are seen. The analysis of extragalactic H2 gas is of great importance since the abundance of H2 can be studied in environments very different from those of the Milky Way. The Magellanic Clouds, the most nearby satellite galaxies of the Milky Way, are ideal hunting grounds for extragalactic H2 measurements, because they provide many bright stars suitable as UV background sources for absorption spectroscopy. Moreover, H2 has been detected from warm regions in both galaxies in the near-IR emission lines (Koornneef & Israel 1985, Israel & Koornneef 1988, 1991). It has been suggested that, due to the lower metallicity and the lower dust content, the amount of H2 in the diffuse interstellar medium of the Magellanic Clouds is significantly lower than in the Milky Way (Clayton et al. 1995).

The ORFEUS telescope, launched for its second mission in 1996, was the first instrument able to measure H2 absorption lines in the LMC (de Boer et al. 1998) and SMC (Richter et al. 1998). In addition, the spectrum of LH 10:3120 was used to detemine an upper limit for the H2/CO ratio in the LMC gas along this line of sight (Richter et al. 1999a). Together with the observations presented here, these ORFEUS spectra provide the first opportunity to investigate the relations between [FORMULA]H I [FORMULA], [FORMULA]H[FORMULA] and [FORMULA] in diffuse interstellar gas of the Magellanic Clouds in comparison to the Milky Way.

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© European Southern Observatory (ESO) 2000

Online publication: July 13, 2000