The shroud around the twin radio jets in NGC 1052

Abstract

We discuss multiple Very Long Baseline Interferometry (VLBI) continuum and spectral line imaging observations and Westerbork Synthesis Radio Telescope spectroscopy of the compact variable nuclear radio jet source in the elliptical galaxy NGC 1052. Absorption and emission signatures reveal ionised, atomic, and molecular components of the surrounding medium. Ten epochs of Very Long Baseline Array (VLBA) data at 15 GHz, spanning almost six years, show bi-symmetric jets, in which multiple sub-parsec scale features display outward motions of typically vapp ∼ 0.26c (H0 = 65 kms-1 Mpc-1) on each side. The jets are most likely oriented near the plane of the sky. Multi-frequency VLBA observations at seven frequencies between 43 and 1.4 GHz show free-free absorption in the inner parsec around the nucleus, probably together with synchrotron self-absorption. The free-free absorption is apparently due to a structure which is geometrically thick and oriented roughly orthogonal to the jets, but which is patchy. The western jet is covered more deeply and extensively, and hence is receding. HI spectral line VLBI observations reveal atomic gas in front of the approaching as well as the receding jet. There appear to be three velocity systems. Broad, shallow absorption asymmetrically straddles the systemic velocity spanning -35 to 85 km s-1. This gas could be local to the AGN environment, or distributed on galactic scales. Superimposed in the range 25 to 95 km s-1 are several sharper (3-15 km s-1) features, each detectable over a few tenths of a pc at various places along the inner 2 pc of the approaching jet. The third, deepest system is at "high velocities", which is receding by 125 to 200 km s-1 with respect to the systemic velocity of NGC 1052. It may have a continuous velocity gradient across the nucleus of some 10 km s-1 pc-1. This atomic gas seems restricted to a shell 1-2 pc away from the core, within which it might be largely ionised. Westerbork Synthesis Radio Telescope spectroscopy has revealed the 18cm OH main lines (1667 and 1665 MHz) in absorption along the full velocity span of -35 to 200 km s-1, with their line ratio varying roughly from 1:1 to 2:1. They are deepest in the high velocity system, where the OH profiles are similar to HI, suggesting co-location of that atomic and molecular gas, and leaving unclear the connection to the H2O masing gas seen elsewhere. In the high velocity system we have also detected the 18cm OH satellite lines: 1612 MHz in absorption, and 1720 MHz in emission. The conjugate behaviour of the satellite line profiles, and the variable main line ratio resemble the situation in Cen A and NGC 253.