Research Note

More evidence for extinction of activity in galaxies

Toruń Centre for Astronomy, N. Copernicus University, 87-100 Toruń, Poland
e-mail: amr@astro.uni.torun.pl

Received: 4 March 2011
Accepted: 24 March 2011

Abstract

This Research Note amends an article in which we showed that radio-loud quasars can become radio-quiet. Exploring the analogy between galactic nuclei and X-ray binaries (XRB), we pointed out there that this transition in quasars could be identified with a switch from low/hard to high/soft state in microquasars. Here, we present the evidence that traces of past occurrences of this kind of phenomena can be found in normal but once active galaxies. Based on the properties of a few such “post-active” galaxies that are representative for a much wider group, it has been argued that they have reached the evolutionary stages when their nuclei, which were radio-loud in the past, now, mimicking the behaviour of XRBs, remain in the intermediate state on their way towards quiescence or even have already entered the quiescent state. It follows that the full evolutionary track of XRBs can be mapped onto the evolution of galaxies. The above findings are in line with those reported recently for IC 2497, a galaxy that 70 000 years ago or less hosted a quasar but now appears as a normal one. This scenario stems from the presence of Hanny’s Voorwerp, a nebulous object in its vicinity excited by that QSO in the epoch when IC 2497 was active. The post-active galaxies we deal with here are accompanied by extremely weak and diffuse relic radio lobes that were inflated during their former active period. These relics can be regarded as radio analogues of Hanny’s Voorwerp.