Molecular line emission in NGC 4945, imaged with ALMA

Abstract

NGC 4945 is one of the nearest (D approximate to 3.8 Mpc; 1” approximate to 19 pc) starburst galaxies. To investigate the structure, dynamics, and composition of the dense nuclear gas of this galaxy, ALMA band 3 (lambda approximate to 3-4mm) observations were carried out with approximate to 2” resolution. Three HCN and two HCO+ isotopologues, CS, C3H2, SiO, HCO, and CH3C2H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10” x 2” reaching out to a galactocentric radius of r 100 pc with position angle PA = 45 degrees +/- 2 degrees, inclination i = 75 degrees +/- 2 degrees and an unresolved bright central core of less than or similar to 2” size The continuum source, representing mostly free free radiation from star forming regions, is more compact than the nuclear disk by a linear factor of two but shows the same position angle and is centered 0”.39 +/- 0”.14 northeast of the nuclear accretion disk defined by H2O maser emission. Near the systemic velocity but outside the nuclear disk, both HCN J = 1 -> 0 and CS J = 2 -> 1 delineate molecular arms of length greater than or similar to 15” (greater than or similar to 285 pc) on opposite sides of the dynamical center. These are connected by a (deprojected) approximate to 0.6 kpc sized molecular bridge, likely a dense gaseous bar seen almost ends -on, shifting gas from the front and back side into the nuclear disk. Modeling this nuclear disk located farther inside (r less than or similar to 100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojected velocity of approximate to 50 km s(-1). All our molecular lines, with the notable exception of CH3C2H, show significant absorption near the systemic velocity (approximate to 571 km s(-1)), within the range approximate to 500-660 km s(-1). Apparently, only molecular transitions with low critical H-2 density (n(crit) less than or similar to 10(4) cm(-3)) do not show absorption. The velocity field of the nuclear disk, derived from CH3C2H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M-tot = (2.1 +/- 0.2) x 10(8) M-circle dot inside a galactocentric radius of 2”.45 (approximate to 45 pc), with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to approximate to 1”.5 (approximate to 30 pc) southwest of the continuum peak, presumably due to absorption, which appears to be most severe slightly northeast of the nuclear maser disk. A nitrogen isotope ratio of N-14/N-15 approximate to 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because N-15 is less abundant than expected, the question for strong N-15 enrichment by massive star ejecta in starbursts still remains to be settled.