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Dynamo.
The bright nucleus of galaxy NGC5806 reveals the presence of an active supermassive black hole.

Credit: The Sloan Digital Sky Survey

A Bang, a Whimper, and Another Bang?

By Phil Berardelli
ScienceNOW Daily News
9 April 2008

The relatively quiet black hole at the center of our Milky Way galaxy could one day reignite, spewing forth so much radiation that the sky would never darken. That grim scenario has become more likely based on a new survey of galaxies hosting active black holes at their centers.

Nearly all galactic cores contain black holes weighing as much as millions or even billions of suns. In the early universe, galaxies collided relatively often and their black holes sometimes merged, growing more massive in the process and sometimes birthing hugely energetic objects known as quasars. Over time, astronomers think, the quasars dissipated as they ran out of fuel, and the black holes eventually quieted down. 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 a team of researchers decided to survey similarly aged galaxies to see just how many black holes have second winds.

Astrophysicist Paul Westoby and colleagues from Liverpool John Moores University in the U.K. used data from the Sloan Digital Sky Survey to survey spectral lines from 360,000 relatively nearby galaxies. As Westoby reported last week at the National Astronomy Meeting in Belfast, some 70,000 of those galaxies--nearly 20%--definitely showed active nuclei, and 65,000 more were listed as "probable AGN," Westoby says. 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. The answers have more than scientific relevance. The Milky Way's central black hole is currently quiet, but if for some reason it roared to life again, it could spew out deadly gamma rays and x-rays, possibly in our direction. "We need to understand the physics behind AGN triggering to know if it will reignite," Westoby notes.

It's an interesting find, says astrophysicist Christopher Reynolds of the University of Maryland, College Park, but he's not convinced that all of the active nuclei spotted by Westoby's team are abnormally energetic. "Every galactic center is active at some level," he says. And Reynolds isn't losing any sleep over the Milky Way's black hole reigniting. "We're pretty far from the galactic center," he says, so we're safe "unless the galactic center ramped up to its full power."