Molecular line emission in NGC 4945, imaged with ALMA
  NGC 4945 is one of the nearest (~3.8 Mpc; 1" ~ 19 pc) starburst galaxies.
ALMA band 3 (3--4\,mm) observations of HCN, HCO+, CS, C3H2, SiO, HCO, and
CH3C2H were carried out with ~2" resolution. The lines reveal a rotating
nuclear disk of projected size 10" x 2" with position angle ~45 deg,
inclination ~75 deg and an unresolved bright central core of size <2.5". The
continuum source (mostly free-free radiation) 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. Outside the nuclear disk, both HCN and CS delineate molecular
arms 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 <100 pc) with tilted
rings indicates a coplanar outflow reaching a characteristic deprojectd
velocity of ~50 km/s. All our molecular lines, with the notable exception of
CH3C2H, show significant absorption near the systemic velocity (~571 km/s),
within a range of ~500-660 km/s. Apparently, only molecular transitions with
low critical H2-density 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 = (2.1+_0.2) x 10^8 Mo inside a
galactocentric radius of 2.45", 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" southwest of the continuum
peak, presumably due to absorption, which appears to be most severe slightly
northeast of the nuclear maser disk.
