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 ~2 M _⊙ 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 ~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 ~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 (~10⁷ M _⊙) structures with sizes (~30 pc) similar to GMCs in other systems, but compared to GMCs in normal galaxy disks, they have high line widths (σ ~ 20-40 km s^-1, Mach number {M} ∼ 90) and high surface and volume densities (Σ_mol ~ 6000 M _⊙ pc^-2, n _H2 ~ 2000 cm^-3). The self gravity from such high densities can explain the high line widths and the short free fall time τ_ff ~ 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 (~10 Myr), disk free fall time (lsim 3 Myr), and disk crossing time (lsim 3 Myr) are each an order of magnitude shorter than in a normal galaxy disk. Finally, the CO-to-H₂ 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.