ALMA reveals the molecular medium fueling the nearest nuclear starburst
- National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903 (United States)
- Department of Astronomy, Laboratory for Millimeter-Wave Astronomy and Joint Space Institute, University of Maryland, College Park, MD 20742 (United States)
- Department of Astrophysical Sciences, Princeton University, Princeton, NJ 08544 (United States)
- Department of Physics, University of Alberta, Edmonton, AB T6G 2R3 (Canada)
- Max Planck Institute für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany)
- New Mexico Institute of Mining and Technology, 801 Leroy Place, Socorro, NM 87801 (United States)
- National Radio Astronomy Observatory, P.O. Box O, 1003 Lopezville Road, Socorro, NM 87801 (United States)
- Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721 (United States)
- Max-Planck-Institut für Extraterrestrische Physik (MPE), Giessenbachstr., D-85748 Garching (Germany)
- European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching (Germany)
We use ALMA observations to derive mass, length, and time scales associated with NGC 253's nuclear starburst. This region forms ∼2 M {sub ☉} yr{sup –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{sup +}, 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 (∼10{sup 7} M {sub ☉}) 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{sup –1}, Mach number M∼90) and high surface and volume densities (Σ{sub mol} ∼ 6000 M {sub ☉} pc{sup –2}, n {sub H2} ∼ 2000 cm{sup –3}). The self gravity from such high densities can explain the high line widths and the short free fall time τ{sub 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 (≲ 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-H{sub 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.
- OSTI ID:
- 22882667
- Journal Information:
- Astrophysical Journal, Vol. 801, Issue 1; Other Information: Country of input: International Atomic Energy Agency (IAEA); Since 2009, the country of publication for this journal is the UK.; ISSN 0004-637X
- Country of Publication:
- United Kingdom
- Language:
- English
Similar Records
STAR-FORMING CLOUD COMPLEXES IN THE CENTRAL MOLECULAR ZONE OF NGC 253
Hubble space telescope imaging of decoupled dust clouds in the ram pressure stripped Virgo spirals NGC 4402 and NGC 4522