How to Grow a Supermassive Black Hole

A new study says most black holes may grow by snacking instead of collision-induced feeding frenzies.

Supermassive black holes lurk deep inside the central bulges of galaxies throughout the universe. These are no “ordinary” black holes (if black holes could ever said to be ordinary) — these monsters have millions to billions of times the mass of the Sun. No exploding star can produce a black hole like these, so how did they grow to be such heavy beasts?

“Galaxy collisions” has long been the mantra of theorists and observers alike. According to theorists, as two galaxies merge, the chaos of collision should send gas flowing to the central black holes. As an added bonus, the galaxies’ two black holes themselves will eventually merge. Observers testing this theory have seen signs of smashed galaxies around some supermassive black holes. But a recent study, accepted for publication in Monthly Notices of the Royal Astronomical Society, suggests that most black holes don’t need a collision-induced feeding frenzy in their past to explain their girth.

The study examines 28 Dust-Obscured Galaxies, or DOGs for short. (Yes, you read that right — astronomers never fail to find offbeat acronyms to suit their needs.) The DOGs exist 8 billion to 12 billion years ago, when the universe was only a fraction of its current age Most of the DOGs host a hungry supermassive black hole feeding off a steady stream of gas. Black holes themselves never shine, but gas heats up and blazes brilliantly as it falls down close to the gaping mouth of the event horizon, before it’s swallowed forever. Astronomers see the glowing gas, rather than the black hole itself. The most powerful black hole-powered feasts are known as quasars. But in DOGs, a dust shroud hides most of the glowing gas, and thus the black hole, from our view. Only the faintest of X-ray signals makes its way through the dust, letting us know that the black holes are really there.

Luckily for astronomers, the dust itself glows at infrared wavelengths. So by imaging the dust-enshrouded galaxies with the Spitzer Space Telescope, Kevin Schawinski and his colleagues can take a closer look at the structure of the galaxies themselves.

According to the standard picture of quasar evolution, galaxy mergers are what should have set loose the streams of dust and gas that both hide and feed the black hole. Yet most of the DOGs show no signs of mergers. Only one of the 28 DOGs shows clear signs of collision with another galaxy. Two more DOGs show signs of potential disruption, but it’s unclear whether a collision really happened. The rest of the DOGs are spiral-type galaxies, apparently undisturbed by intruding neighbors. Even the most conservative estimate says that only a quarter of DOGs have been through a recent collision.

“The brilliant quasars born of galaxy mergers get all the attention because they are so bright and their host galaxies are so messed up,” says Schawinski. “But the typical bread-and-butter quasars are actually where most of the black-hole growth is happening. They are the norm, and they don’t need the drama of a collision to shine.”

Other studies have also found that mergers may not always play a major role in black hole evolution, particularly in fainter quasars. But the DOGs represent typical quasars, not faint ones, even though most of the activity is hidden by dust.

Still, Phil Hopkins (University of California, Berkeley) urges caution. "At these [large distances], it's very hard to see the detailed tidal features that normally distinguish mergers," he explains. "People are very much still arguing about whether some of the very nearest quasars are actually mergers or not!"

"It may be that we won't be able to settle the question until, say, the James Webb Space Telescope comes online and can get deeper images of these objects," he adds.

So what’s feeding the black holes so heavily if it's not mergers? The authors suggest that the gas could come from many sources, anything from random mixing of gas, to supernovae explosions near the galactic center, to instabilities in the galactic structure. Small snacks like these, they say, are capable of sustaining most of the black hole growth over the ages.