A few decades ago, astronomers thought they had figured out how quasars operate. Now, a new study has thrown a wrench in the works.
For decades, astronomers have operated under a basic assumption: supermassive black holes are all basically the same. Sitting at the center of galaxies, these black holes gobble gas, and that gas relinquishes some of its energy as light before falling toward oblivion.
Whatever differences astronomers see in so-called active galaxies lies not in changes to their central black holes — or their attending retinues of gaseous disk, clouds, winds, and jets — but in how we view them. If our line of sight to a brilliant quasar passes through a haze of dust and gas, it’ll appear vastly different (in predictable ways) than if we see it face on.
But is there more to the story?
Many observations have supported this unification scenario. But recent studies have hinted that there might be some real differences between various active galaxies. Those hints have just become a lot stronger. Claudio Ricci (Pontificia Catholic University of Chile, Chinese Academy of Sciences South America Center for Astronomy, and Peking University) and colleagues surveyed 836 relatively nearby quasars and found that their supermassive black holes are not all the same.
Turns out that those quasars devouring a gaseous buffet do so out in the open, without anything hiding their accretion, while large amounts of dusty gas obscure those quasars that are reduced to nibbling on tidbits.
The key change occurs when there’s enough gas falling into the black hole that the radiation it emits begins to push back on gas that’s farther away. (Yes, it’s weird, but photons can actually exert pressure on gas!) So, the authors go on to argue, if a quasar accretes gas at a fast enough rate, it’ll produce enough radiation to blow away any obscuring dusty gas around it.
The results are published in the September 28th Nature.
Swift Spacecraft Offers Bird’s Eye View
Astronomers have found similar results previously, but this research is particularly convincing because of the quasars under study.
Ricci and colleagues used the Swift space telescope to pick out the quasars by their X-rays, emitted in the range between 14 and 195 kiloelectron volts (keV). Such high-energy radiation, which comes from areas very close to the black hole, ignores obscuring material on its way to Earth, passing through it in much the same way that X-rays from medical machines pass through your flesh and muscle to image your bones.
In previous studies it was difficult to make the case that faint, starving black holes had a lot of dusty gas around them, because it was just as easy to say that the obscuring material was masking the black hole, making it appear less luminous. But in this study, the high-energy X-rays give us a clear-eyed view of the quasars’ activity, so the results hold strong.
So what does this mean for quasar evolution? The authors put forth a simple scenario for how quasars might grow: First, a merger or some other event sends gas flowing to the center of a galaxy. There it both feeds the black hole and obscures it. But as the black hole eats more and more, its radiation expels the remaining gas. For a time, it exists as a brilliant quasar, its gas shining bright. But as the meal runs low, the black hole fades again into quiescence.
But let’s wait and see — there may turn out to be more to the story.