Positional Coincidence of H2O Maser and a Plasma-Obscuring Torus in Radio Galaxy NGC 1052

摘要

We present multifrequency simultaneous VLBA observations at 15, 22, and 43 GHz toward the nucleus of the nearby radio galaxy NGC 1052. These three continuum images reveal a double-sided jet structure whose relative intensity ratios imply that the jet axis is oriented close to the sky plane. The steeply rising spectra at 15-43 GHz at the inner edges of the jets strongly suggest that synchrotron emission is absorbed by foreground thermal plasma. We detected H2O maser emission in the velocity range of 1550-1850 km s?1, which is redshifted by 50-350 km s?1 with respect to the systemic velocity of NGC 1052. The redshifted maser gas appears projected against both sides of the jet, in the same manner as the H I seen in absorption. The H2O maser gas is located where the free-free absorption opacity is large. This probably implies that the masers in NGC 1052 are associated with a circumnuclear torus or disk as in the nucleus of NGC 4258. Such circumnuclear structure could be the source of accretion onto the central engine.