Astron. Astrophys. 363, 451-454 (2000)

1. Introduction

Within galaxies such as the Milky Way and the Magellanic clouds, star formation is thought to occur within molecular clouds (Shu et al. 1987). The formation of these clouds is driven by many factors, including large-scale gravitational instabilities, shock waves from supernova explosions, random cloud-cloud coalescence and, within spirals, compression due to spiral density waves (Brosch et al. 1998). At some point after molecular cloud creation, a high density core is formed which collapses to form stars once it exceeds the Jeans mass (Vanhala & Cameron 1998).

Within environments where the gas density is lower, it is possible that collisions between HI cloudlets shock the gas and lead to star formation (Dyson & Hartquist 1983). One such region where this may be occurring is the region between the Large and Small Magellanic Clouds, the Magellanic Bridge. Even though the HI gas density in the bulk of the Bridge is relatively low (generally atoms cm^-2; McGee & Newton 1986), a number of blue stellar associations have still been formed within it (Irwin et al. 1990; Bica & Schmitt 1995). Spectroscopic analyses of individual stars in the Bridge yield stellar ages that are as young as 7 Myrs (Hambly et al. 1994). Such objects must have been formed within the intercloud medium, and not simply moved there from another birthplace.

In order to investigate the likely star formation mechanism within the Bridge, we have recently embarked upon a HI -line synthesis study, whose aim is to obtain the sizes, densities and velocities of Bridge gas cloudlets. The results will be compared to the predictions of Christodoulou et al. (1997) to see whether cloud-cloud collisions are a viable star formation mechanism within the Bridge. In parallel with the above work, the present study aims to search for molecular gas within the Bridge out of which stars may form. The recent detection of cold atomic hydrogen in the ICM by Kobulnicky & Dickey (1999, hereafter KD99) has fueled interest in this subject because within the Galaxy, cold HI is often associated with molecular clouds (Garwood & Dickey 1989). Similarly, within the LMC, Marx-Zimmer et al. (1998) found that 8/25 of pointings towards regions showing HI absorption also show CO in emission.

To our knowledge, no previous search for CO within the Bridge has been published. This is probably due to the perceived difficulty in observing any CO, caused by the ratio of molecular to atomic gas being much lower in the SMC and LMC than in the Milky Way (by factors of 15 and 4, respectively), combined with a conversion factor `X' for I(CO)-H₂ which is some 20 and 6 times higher in the SMC and LMC than in the Galaxy (Israel et al. 1993 and references therein). As there is evidence that metallicities in the Bridge are even lower than in the SMC (Rolleston et al. 1999), and that fractional CO contents decrease with metallicity (Israel et al. 1993), CO studies in this region will inevitably be challenging.

The current paper describes a search for CO within the Bridge at one of the positions (0311-7651) where cold HI gas was detected by KD99. As the 0311-7651 region observed by KD99 lies just to the south of Bridge regions searched by Irwin et al. (1990) and Battinelli & Demers (1992), we have also undertaken a search for a stellar association towards this region using BV CCD photometry.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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