2018A&A...615A.155H

NGC 4945 is one of the nearest (D~=3.8Mpc; 1''~=19pc) starburst galaxies. To investigate the structure, dynamics, and composition of the dense nuclear gas of this galaxy, ALMA band 3 (λ~=3-4 mm) observations were carried out with ~=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''x2'' reaching out to a galactocentric radius of r~=100pc with position angle PA=45°±2°, inclination i=75°±2° and an unresolved bright central core of size ≤2''. 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 ≥15'' (≥285pc) on opposite sides of the dynamical center. These are connected by a (deprojected) ~=0.6kpc 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≤100pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojected velocity of ~=50km/s. All our molecular lines, with the notable exception of CH₃ C₂H, show significant absorption near the systemic velocity (~=571km/s), within the range ~=500-660km/s. Apparently, only molecular transitions with low critical H₂ density (n_crit≤10⁴cm^–3) do not show absorption. The velocity field of the nuclear disk, derived from CH₃ C₂H, provides evidence for rigid rotation in the inner few arcseconds and a dynamical mass of M_tot=(2.1±0.2)x10⁸M_☉ inside a galactocentric radius of 2.45" (~=45pc), with a significantly flattened rotation curve farther out. Velocity integrated line intensity maps with most pronounced absorption show molecular peak positions up to ~=1.5" (~=30pc) 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 ¹⁴N/¹⁵N~=200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because ¹⁵N is less abundant than expected, the question for strong ¹⁵N enrichment by massive star ejecta in starbursts still remains to be settled.