Milky Way Black Hole Could Reignite

sciencehabit sends us to Sciencemag.org for an account of a survey of nearby galaxies that points to the possibility that once-quiescent galactic nuclei could wake up and become active again. If the Milky Way's dormant black hole should become active, it could be bad news for life on Earth (and elsewhere in the neighborhood). The paper (PDF) is up on the arXiv.

Comments

20 to 40%? Reignite?

[Gat0r30y]

From TFA it seems like somewhere between 20-40% of galaxies surveyed have active nuclei, but how do they know they reignited?

It's not understood what is causing the black holes to become newly active, because in most cases there is no evidence of collisions or mergers. How could it be verified that despite the lack of a recent collision with another galaxy, these particular phenomenon were at some point dormant like ours, then reignited? How are they supporting this claim that these galactic nuclei must have spontaneously rekindled vs. they have been winding down from a collision very far in the past?

There's just one problem: Astronomers have found quasarlike centers--called active galactic nuclei (AGN)--in some relatively nearby galaxies, which should be far too old to generate such energies. So they should be too old for this sort of behavior, but how are they verifying the time since the last collision? Another possibility is that these galaxies had a collision more recently right?