More evidence for extinction of activity in galaxies
  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.
