Molecular line emission in NGC 4945, imaged with ALMA

NGC 4945 is one of the nearest (D ≈ 3.8 Mpc; 1⁰⁰ ≈ 19 pc) 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 ≈2⁰⁰ resolution. Three HCN and two HC⁺ isotopologues, CS, C₃H₂, SiO, HCO, and CH₃C₂H were measured. Spectral line imaging demonstrates the presence of a rotating nuclear disk of projected size 10⁰⁰ × 2⁰⁰ reaching out to a galactocentric radius of r ≈ 100 pc with position angle PA = 45^◦ ± 2^◦, inclination i = 75^◦ ± 2^◦ and an unresolved bright central core of size^<_∼2⁰⁰. 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 0⁰⁰ . 39 ± 0⁰⁰ . 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^>_∼15⁰⁰ (^>_∼285 pc) on opposite sides of the dynamical center. These are connected by a (deprojected) ≈ 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^<_∼100 pc) with tilted rings provides a good fit by inferring a coplanar outflow reaching a characteristic deprojected velocity of ≈50 km s⁻¹. All our molecular lines, with the notable exception of CH₃C₂H, show significant absorption near the systemic velocity (≈571 km s⁻¹), within the range ≈500-660 km s⁻¹. Apparently, only molecular transitions with low critical H₂ density (n_crit^<_∼ 10⁴ 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) × 10⁸ M inside a galactocentric radius of 2⁰⁰ . 45 (≈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 ≈1⁰⁰ . 5 (≈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 of¹⁴N/¹⁵N ≈ 200-450 is estimated. This range of values is much higher then previously reported on a tentative basis. Therefore, because¹⁵N is less abundant than expected, the question for strong¹⁵N enrichment by massive star ejecta in starbursts still remains to be settled.