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Published March 3, 2003 | Published + Submitted
Journal Article Open

The shroud around the twin radio jets in NGC 1052


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 v_(app) ~ 0.26c(H_0km s^(-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 18 cm 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 H_2O masing gas seen elsewhere. In the high velocity system we have also detected the 18 cm 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.

Additional Information

© 2003 ESO. Received: 7 August 2002. Accepted: 25 November 2002. We thank the staff of the VLBA and the WSRT for their extensive support. The VLBA is an instrument of the NRAO, a facility of the National Science Foundation of the USA, operated under cooperative agreement by Associated Universities, Inc. The WSRT is operated by ASTRON (The Netherlands Foundation for Research in Astronomy) with support from the Netherlands Foundation for Scientific Research (NWO). This work is also based on observations with the 100-m telescope of the MPIfR (Max-Planck-Institut für Radioastronomie) at Effelsberg. This work was initially supported in part by the US National Science Foundation under grant AST-9420018, and has made use of NASA's Astrophysics Data System (ADS), and the NASA/IPAC Extragalactic Database (NED) which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA. We are grateful to Frodo Wesseling for assistance with the data reduction, and to Dan Homan, Matthias Kadler and Matt Lister for helpful discussions and suggestions. We thank Gabriele Giovannini for prompt and helpful comments as referee.

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