Although a great deal of progress has been made on the topic of star and planet formation since the last Protostars and Planets conference in Santa Barbara, little work has been done to connect what we know about star formation in the Milky Way to star formation in the Universe as a whole. Fundamental limitations include only a weak understanding of how the massive stars form, how clusters and associations form, and the constancy of the IMF. After all, in external galaxies, we generally observe only the effects of massive star formation and the formation of star clusters. Furthermore, knowledge of the initial conditions for star formation at all masses remains elusive both within and outside of the Milky Way.
Since nearly all stars form in Giant Molecular Clouds (GMCs), one way to make progress is to examine the properties of GMCs in a number of different extragalactic environments to see how they differ. From the similarities and differences, it might be possible to make some general conclusions about how star formation varies throughout the Universe. Although individual, extragalactic GMCs had been observed previously at high enough resolution to at least marginally resolve them (e.g., Vogel et al., 1987; Lada et al., 1988), the first attempts to do this in a systematic way were by Christine Wilson (Wilson and Scoville, 1990; Wilson and Reid, 1991; Wilson and Rudolph, 1993; Wilson, 1994) using the OVRO and BIMA interferometers. Her efforts were hampered by small survey areas in a few galaxies, so general conclusions could only be made by extrapolation. Numerous other authors subsequently studied one or a few extragalactic GMCs, both in the Local Group and beyond. An exhaustive list of their efforts is beyond the scope of the present article.
The situation has changed in the last five years as a result of the construction of the NANTEN telescope in the Southern Hemisphere and the completion of the 10-element BIMA Array. The former made it possible to map the Magellanic Clouds completely with high enough spatial resolution and signal-to-noise to identify all of the GMCs with masses > 3 × 104 M; the completion of the BIMA interferometer made it possible to identify GMCs in other, more distant galaxies in the Local Group. Because of their relatively large fields of view, these two telescopes could completely survey nearby galaxies. Thus, the first complete survey of GMCs in any galaxy was of the LMC (Fukui et al., 1999; Mizuno et al., 2001b) and not the Milky Way (MW). Although the molecular gas in the MW has been essentially completely mapped, velocity crowding in many directions makes it impossible to generate a full catalog of GMCs. Similarly, the first complete CO surveys of the Magellanic Clouds were by Cohen et al. (1988) and Rubio et al. (1991), but the resolution was too poor to determine the properties of individual molecular clouds.
In this paper, we review the recent surveys of CO in Local Group galaxies that (1) have sufficient resolution to study individual molecular clouds and (2) span all or most of the target galaxy. We compare the results of observations of GMCs in the four external Local Group galaxies that have been mapped in their entirety in CO: the Large Magellanic Cloud (LMC, Fukui et al., 2001; Fukui et al., 2006), the Small Magellanic Cloud (SMC, Miznuo et al., 2001a; Mizuno et al., 2006), IC 10 (Leroy et al., 2006), and M33 (Engargiola et al., 2003). We have also made observations in a small strip in M31 (Rosolowsky, 2006), and we compare the properties of the GMCs in all of these galaxies to clouds in the outer MW (from Dame et al., 2001) using a uniform set of analytic techniques. The LMC and SMC observations were made with the single-dish NANTEN telescope in Chile, the remaining galaxies were observed with the BIMA millimeter-wave interferometer at Hat Creek, California (combined with obsevations from the Caltech OVRO millimeter interferometer for IC 10). A tabulation of the galaxies we observed, their metallicities and the resolution used to observe them is given in Table 1.
|LMC||NANTEN||0.33 Z||40 pc||1||SMC||NANTEN||0.1 Z||48 pc||2||IC 10||OVRO/BIMA||0.25 Z||14 - 20 pc||3||M33||BIMA||0.1 - 1.0 Z||20 - 30 pc||4||M31||BIMA||0.5 Z||26 - 36 pc||5|
|References. (1) Fukui et al. (2006); (2) Mizuno et al. (2006); (3) Leroy et al. (2006); (4) Engargiola et al. (2003); (5) Rosolowsky (2006)|