Molecular line emission in NGC 4945, imaged with ALMA

Abstract

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 ∼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