Starship Asterisk*

New research explains how galaxy centers light up

Galaxies like our own were built billions of years ago from a deluge of giant clouds of gas, some of which continue to rain down. Now new calculations tie the rain of giant clouds of gas to active galactic nuclei (AGN), the extremely bright centers of some galaxies. If a gas cloud with millions of times more mass than our Sun wanders too close to the center of a galaxy, it can either be consumed by the supermassive black hole that lurks there or, through shocks and collapse, give birth to new stars.

"For a while, people have known that gas clouds are falling onto galaxies, and they've also known that active galactic nuclei are powered by gas falling onto supermassive black holes," says Barry McKernan, a research associate in the Department of Astrophysics at the American Museum of Natural History and an assistant professor at the Borough of Manhattan Community College (BMCC), City University of New York. "But no one put the two ideas together until now and said, 'Hey, maybe one is causing the other!'"

All galaxies are believed to host a supermassive black hole at their center, yet only a fraction of galactic centers show signs of brighter activity due to black hole feeding. The new research provides an explanation for the apparent conundrum: galactic centers which have sustained recent cloud impacts have enough fuel to light up by giving birth to hundreds of stars and feeding the central black hole. Galactic centers that have not been hit for a while (in cosmic terms, for more than about 10 million years) will be relatively inactive and their cores will appear normal.

"It's interesting that only some galaxies are active, even though we think every galaxy contains a supermassive black hole," says K. E. Saavik Ford, a research associate at the Museum and an assistant professor at BMCC. "The cloud bombardment idea provides an explanation: it's just random luck."

• Astrophysical Journal Letters, 718 L83 doi: 10.1088/2041-8205/718/2/L83

ABSTRACT We present optical spectra and high-resolution multiwavelength radio observations of the compact steep-spectrum radio source MRC B1221-423 (z = 0.1706). MRC B1221-423 is a very young (~105yr), powerful radio source which is undergoing a tidal interaction with a companion galaxy. We find strong evidence of interaction between the active galactic nucleus (AGN) and its environment. The radio morphology is highly distorted, showing a dramatic interaction between the radio jet and the host galaxy, with the jet being turned almost back on itself. HI observations show strong absorption against the nucleus at an infall velocity of ~250kms-1 compared to the stellar velocity, as well as a second, broader component which may represent gas falling into the nucleus. Optical spectra show that star formation is taking place across the whole system. Broad optical emission lines in the nucleus show evidence of outflow. Our observations confirm that MRC B1221-423 is a young radio source in a gas-rich nuclear environment, and that there was a time delay of a few times 100 Myr between the onset of star formation and the triggering of the AGN.