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

摘要

We present the first interferometer map of the water maser emission in theactive nucleus of NGC 4945. Although the declination of the galaxy is about -49degrees, we were able to make the observations with the southernmost antennasof the Very Long Baseline Array. Strong maser emission is present in threevelocity ranges, one near the systemic velocity and two shifted roughlysymmetrically by +/-(100-150) km/s. This is the first detection of highlyblue-shifted water emission in NGC 4945. We determined the position of themaser to be RA(B1950)= 13 02 32.28 +/- 0.02 ; Dec(B1950)= -49 12 01.9 +/- 0.1.The uncertainties in earlier estimates are at least several arcseconds. Themaser lies within 2'' (36 pc at a distance of 3.7 Mpc) of the peaks in 1.4 GHzcontinuum and 1.6 micron emission from the nucleus. The mappable maser emissionis distributed roughly linearly over about 40 milliarcseconds (0.7 pc) at aposition angle of about 45 degrees, which is close to the 43 +/- 2 degreeposition angle of the galactic disk. The red and blue-shifted emissionsymmetrically stradle the systemic emission on the sky, which suggests materialin edge-on circular motion around a central object. The position-velocitystructure indicates a binding mass of about one million Suns, within a volumeof radius about 0.3 pc. This implies that the central engine radiates on theorder of 10% of its Eddington luminosity.