Alma reveals the molecular medium fueling the nearest nuclear starburst

Adam K. Leroy, Alberto D. Bolatto, Eve Charis Ostriker, Erik Rosolowsky, Fabian Walter, Steven R. Warren, Jennifer Donovan Meyer, Jacqueline Hodge, David S. Meier, Jürgen Ott, Karin Sandstrom, Andreas Schruba, Sylvain Veilleux, Martin Zwaan

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156 Scopus citations


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 ) and high surface and volume densities (Σmol 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 τff 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 (≲ 3 Myr), and disk crossing time (≲ 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.

Original languageEnglish (US)
Article number25
JournalAstrophysical Journal
Issue number1
StatePublished - Mar 1 2015

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: molecules
  • galaxies: ISM
  • galaxies: star formation
  • galaxies: starburst
  • radio lines: ISM
  • radio lines: galaxies
  • stars: formation


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