The Distribution of H2O Maser Emission in the Nucleus of NGC 4945

We present the first interferometer map of the H2O maser emission in the active nucleus of NGC 4945, which exhibits both starburst and Seyfert qualities. Although the declination of the galaxy is -49°, we were able to make the observations with the southernmost antennas of the Very Long Baseline Array. Strong maser emission is present in three velocity ranges, one near the systemic velocity and two shifted roughly symmetrically by ±(100-150) km s-1. This is the first detection of highly blueshifted water emission in NGC 4945. We also report a marginal detection of previously unreported redshifted emission at a velocity of about +200 km s-1 with respect to the systemic velocity. From fringe rate analysis we determined the position of the maser to be α1950 = 13h02m32.ˢ28 ± 0.ˢ02; δ1950 = -49°12'01.″9 ± 0.″1. The uncertainties in earlier estimates are at least several arcseconds. The maser lies within 2'' (36 pc at a distance of 3.7 Mpc) of the peaks in 1.4 GHz continuum and 1.6 μm emission from the nucleus. The mappable maser emission is distributed roughly linearly over ≈ 40 milliarcseconds (0.7 pc) at a position angle of ≈ 45°, which is close to the 43° ± 2° position angle of the galactic disk. The red- and blueshifted emission symmetrically straddle the systemic emission on the sky, which suggests material in edge-on circular motion around a central object. The position-velocity structure indicates a binding mass of ~1 × 106 M☉, within a volume of radius ≈ 0.3 pc, which in combination with estimates of the AGN luminosity, implies that the central engine radiates on the order of 10% of its Eddington luminosity.