ALMA REVEALS THE MOLECULAR MEDIUM FUELING THE NEAREST NUCLEAR STARBURST

We use ALMA observations to derive mass, length, and time scales associated with NGC 253's nuclear starburst. This region forms similar to 2 M-circle dot yr(-1) of stars and resembles other starbursts in ratios of gas, dense gas, and star formation tracers, with star formation consuming the gas reservoir at a normalized rate 10 times higher than in normal galaxy disks. We present new similar to 35 pc resolution observations of bulk gas tracers (CO), high critical density transitions (HCN, HCO+, and CS), and their isotopologues. The starburst is fueled by a highly inclined distribution of dense gas with vertical extent < 100 pc and radius similar to 250 pc. Within this region, we identify 10 starburst giant molecular clouds (GMCs) that appear as both peaks in the dense gas tracer cubes and the HCN-to-CO ratio map. These are massive (similar to 10(7) M-circle dot) structures with sizes (similar to 30 pc) similar to GMCs in other systems, but compared to GMCs in normal galaxy disks, they have high line widths (sigma similar to 20-40 km s(-1), Mach number M similar to 90) and high surface and volume densities (Sigma(mol) similar to 6000 M-circle dot pc(-2), n(H2) similar to 2000 cm(-3)). The self gravity from such high densities can explain the high line widths and the short free fall time tau(ff) similar to 0.7 Myr in the clouds helps explain the more efficient star formation in NGC 253. Though the high inclination obscures the geometry somewhat, we show that simple models suggest a compact, clumpy region of high gas density embedded in a more extended, non-axisymmetric, bar-like distribution. Over the starburst, the surface density still exceeds that of a typical disk galaxy GMC and, as in the clouds, timescales in the disk as a whole are short compared to those in normal galaxy disks. The orbital time (similar to 10 Myr), disk free fall time (less than or similar to 3 Myr), and disk crossing time (less than or similar to 3 Myr) are each an order of magnitude shorter than in a normal galaxy disk. Finally, the CO-to-H-2 conversion factor implied by our cloud calculations is approximately Galactic, contrasting with results showing a low value for the whole starburst region. The contrast provides resolved support for the idea of mixed molecular ISM phases in starburst galaxies.