Alma Reveals the Molecular Medium Fueling the Nearest Nuclear Starburst

Abstract

We use new ALMA observations to derive the mass, length, and time scales associated with the disk and the star-forming clouds in the starburst at the heart of the nearby spiral galaxy NGC 253. This region forms ∼ 2 M yr −1 of stars and resembles other starburst systems in star formation-CO-HCN scaling relations, with star formation consuming the gas reservoir at a normalized rate 10 times higher than in normal galaxy disks. We present sensitive ∼ 35 pc resolution observations of the bulk gas tracers CO and C 17 O and the high critical density transitions HCN (1-0), HCO + (1-0), and CS (2-1) 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 ten starburst giant molecular clouds that appear as both peaks in the dense gas tracer cubes and enhancements in the HCN-to-CO ratio map. These clouds appear as massive (∼ 10 7 M ) structures with sizes (∼ 30 pc) similar to GMCs in other systems, but when compared to a large literature compilation they show very high line widths (σ ∼ 20-40 km s −1 ) given their size, with implied Mach numbers as high as M ∼ 90. The clouds also show very high surface densities (∼ 6, 000 M pc −2 ) and volume densities (n H2 ∼ 2, 000 cm −3 ). The self gravity from such high densities is sufficient to explain the high line widths and the short free fall time τ ff ∼ 0.7 Myr in the clouds may explain the more efficient star formation in NGC 253. We also consider the starburst region as a whole. The geometry is confused by the high inclination, but we show that simple models support a non-axisymmetric, bar-like geometry with a compact, clumpy region of high gas density embedded in an extended CO distribution. Even when considering the region as a whole, the surface density still vastly exceeds that of a typical disk galaxy GMC. 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 spiral galaxy disk. We compare to simple models with mixed success, showing that some but not all aspects of the structure correspond to the predictions from assuming vertical dynamical equilibrium or a marginally stable thin gas disk. Finally, the CO-to-H 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.

Energy Budget of Forming Clumps in Numerical Simulations of Collapsing Clouds

“…These clumps are seen to occupy a region in the diagram that extends from the virial and free-fall lines to over one order of magnitude in σ v /R 1/2 above those lines, at roughly constant column density. We note that this is precisely the kind of behavior displayed in observational clump surveys (see, e.g., Figure 10 of Keto & Myers (1986) and Figure 13 of Leroy et al (2015)). …”

Effects of galactic disc inclination and resolution on observed GMC properties and Larson's scaling relations

“…Interestingly, the weak to no correlations of PPV L edge and PPV L face are in line with the latest unresolved observations of GMCs (Colombo et al 2014;Rebolledo et al 2015;Utomo et al 2015;Leroy et al 2015).…”

ALMA Observations of a Quiescent Molecular Cloud in the Large Magellanic Cloud

“…against Σ, which is analogous to the "boundedness" plot discussed by Heyer et al (2009) (see also Keto & Myers 1986, Field et al 2011, Leroy et al 2015. This plot removes much of the intrinsic correlation in the L-M plot and allows loci of pressure-bounded equilibrium to be distinguished (at least in principle) from virialization.…”