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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 720134.0, MPI für Astronomie / Publikationen_mpia
ALMA reveals the molecular medium fueling the nearest nuclear starburst
Authors:Leroy, A. K.; Bolatto, A. D.; Ostriker, E. C.; Rosolowsky, E.; Walter, F.; Warren, S. R.; Donovan Meyer, J.; Hodge, J.; Meier, D. S.; Ott, J.; Sandstrom, K.; Schruba, A.; Veilleux, S.; Zwaan, M.
Date of Publication (YYYY-MM-DD):2015
Title of Journal:The Astrophysical Journal
Issue / Number:1
Start Page:id. 25 (28 pp)
Audience:Not Specified
Abstract / Description: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 (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 (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.
Free Keywords:galaxies: ISM; galaxies: starburst; galaxies: star formation; ISM: molecules; radio lines: galaxies; radio lines: ISM; stars: formation
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
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