ALMA REVEALS THE MOLECULAR MEDIUM FUELING THE NEAREST NUCLEAR STARBURST

Abstract

We use ALMA to derive the mass, length, and time scales associated with the\nnuclear starburst in NGC 253. This region forms ~2 M_sun/yr of stars and\nresembles other starbursts in scaling relations, with star formation consuming\nthe gas reservoir 10 times faster than in galaxy disks. We present observations\nof CO, the high effective density transitions HCN(1-0), HCO+(1-0), CS(2-1), and\ntheir isotopologues. We identify ten clouds that appear as peaks in line\nemission and enhancements in the HCN-to-CO ratio. These clouds are massive\n(~10^7 M_sun) structures with sizes (~30 pc) similar to GMCs in other systems.\nCompared to disk galaxy GMCs, they show high line widths (~20-40 km/s) given\ntheir size, with implied Mach numbers ~90. The clouds also show high surface\n(~6,000 M_sun/pc^2) and volume densities (n_H2~2,000 cm^-3). Given these,\nself-gravity can explain the line widths. This short free fall time (~0.7 Myr)\nhelps explain the more efficient star formation in NGC 253. We also consider\nthe starburst region as a whole. The geometry is confused by the high\ninclination, but simple models support a non-axisymmetric, bar-like geometry\nwith a compact, clumpy region of high gas density embedded in an extended CO\ndistribution. Even for the whole region, the surface density still exceeds that\nof a disk galaxy GMC. The orbital time (~10 Myr), disk free fall time (<~ 3\nMyr), and disk crossing time (<~ 3 Myr) are each much shorter than in a normal\nspiral galaxy disk. Some but not all aspects of the structure correspond to\npredictions from assuming vertical dynamical equilibrium or a marginally stable\nrotating disk. Finally, the CO-to-H2 conversion factor implied by our cloud\ncalculations is approximately Galactic, contrasting with results showing a low\nvalue for the whole starburst region. The contrast provides resolved support\nfor the idea of mixed molecular ISM phases in starburst galaxies.\n