In many galaxies the average surface density of molecular gas is much greater than the surface densities of individual GMCs shown in Fig. 8 (right; Helfer et al., 2003). These regions of high surface density can be as much as a kiloparsec in extent. Indeed, about half of the galaxies in the BIMA SONG survey (Helfer et al., 2003) have central surface densities in excess of 100 M pc-2. Moreover, regions with high surface densities of molecular gas are invariably associated with dramatically enhanced star formation rates (Kennicutt, 1998). In regions of such high surface density, are there even individual, identifiable GMCs? If so, do they obey the same relations shown in Figs. 6 - 8?
Several recent studies have begun to attack these questions. The only such molecule-rich region in the Local Group is in the vicinity of the Galactic Center where cloud properties were analyzed by Oka et al. (2001). They found that clouds in the Galactic center were smaller, denser and had larger line widths than the GMCs in the Galactic disk. For targets beyond the Local Group, achieving the requisite spatial resolution to study individual GMCs requires significant effort. To date, only a few extragalactic, molecule-rich regions have been studied. Keto et al. (2005) show clouds in M82 to be roughly in virial equilibrium. At the high surface densities of molecular gas observed in M82, this requires clouds to be smaller and denser than those found in the Galactic disk. Similarly, Rosolowsky and Blitz (2005) observed the inner region of the Galaxy M64, which has a surface density of ~ 100 M pc-2 over the inner 300 pc of the galaxy. They found 25 GMCs with densities 2.5 times higher, and are 10 times more massive, on average, than typical disk GMCs. This conclusion is quite robust against differences in cloud decomposition because if some of the clouds they identify are in fact blends of smaller clouds, then the derived densities are lower limits, reinforcing their conclusions. In M64, Rosolowsky and Blitz (2005) examine many of the relationships shown in Figs. 7 and 8 and find that all are significantly different.
5.1. GMC Formation in Galactic Centers
The peak H2 surface density in the central 1 kpc of M64 is about 20 times the H I surface density (Braun et al., 1994; Rosolowsky and Blitz, 2005), which is typical of many galaxies (Helfer et al., 2003). In such regions, the formation of GMCs cannot take place by first collecting atomic hydrogen into filaments and then turning the gas molecular. If the gas is cycled between the atomic and molecular phases, as is required by the presence of H2 regions in the central regions of M64, then continuity requires that the amount of time that the gas remains in each phase is roughly equal to the ratio of surface densities at each particular radius. Thus, gas ionized by the O stars must quickly return to the molecular phase, which is catalyzed by the very large pressures in the central region (Section 4). More than likely, the GMCs are formed and destroyed without substantially leaving the molecular phase, unlike what happens in the disks. Indeed, Rosolowsky and Blitz (2005) present evidence for a diffuse molecular component that is not bound into GMCs. Thus it seems likely that, as in galactic disks, the formation of structure (filaments?) in galactic nuclei occurs before the formation of the GMCs. The gas, though, is largely molecular prior to the formation of the clouds.
Measuring the properties of individual GMCs in more distant molecule-rich galaxies will rely upon future improvements in angular resolution and sensitivity. At present, some information can be gleaned from single-dish spectra of the regions in multiple tracers of molecular gas. The observations of Gao and Solomon (2004) and Narayanan et al. (2005) show that the star formation rate is linearly proportional to the mass of molecular gas found at high densities ( 105 cm-3), and that the fraction of dense gas increases with the amount of molecular mass in the system. Since the fraction of molecular mass found at high densities is relatively small in Galactic GMCs, this implies there are substantial differences in GMC properties in these starburst systems.