The complex radio and X-ray structure in the nuclear regions of the active galaxy NGC 1365

Abstract

ABSTRA C T We present a multiwavelength analysis of the prominent active galaxy NGC 1365, in particular looking at the radio and X-ray properties of the central regions of the galaxy. We analyse ROSAT (PSPC and HRI) observations of NGC 1365, and discuss recent ASCA results. In addition to a number of point sources in the vicinity of NGC 1365, we find a region of extended X-ray emission extending along the central bar of the galaxy, combined with an emission peak near the centre of the galaxy. This central X-ray emission is centred on the optical/radio nucleus, but is spatially extended. The X-ray spectrum can be well fitted by a thermal plasma model, with a temperature of kT a 0:6‐0:8 keV and a very low local absorbing column. The thermal spectrum is suggestive of starburst emission rather than emission from a central black hole. The ATCA radio observations show a number of hotspots, located in a ring around a weak radio nucleus. Synchrotron emission from electrons accelerated by supernovae and supernova remnants (SNRs) is the likely origin of these hotspots. The radio nucleus has a steep spectrum, indicative perhaps of an active galactic nucleus (AGN) or SNRs. The evidence for a jet emanating from the nucleus (as has been previously claimed) is at best marginal. The extent of the radio ring is comparable to that of the extended central X-ray source. We discuss the nature of the central activity in NGC 1365 in the light of these observations. The extended X-ray emission and the thermal spectra strongly suggest that at soft X-ray energies we are not seeing emission predominantly from a central black hole, although the presence of Fe K line emission at higher energies does suggest the presence of an AGN. Consequently, a black hole is probably not the dominant contributor to the energetics of the central regions of NGC 1365 at radio, optical or soft X-ray wavelengths. Activity associated with a starburst is likely the dominant explanation for the observed properties of NGC 1365.