Astronomers are edging closer to discovering how the universe’s most supermassive black holes form — by studying the smallest galaxies.

It’s easy to make a black hole a few times the mass of the Sun: a massive star loses its battle against gravity and collapses under its own weight. But no star contains millions or even billions of Suns, which is where the most massive black holes weigh in.

There are two main ideas on how to grow these monsters. One camp holds that supermassive black holes could be the remnants of the first generation of stars. Unpolluted by heavier elements, these stars could have grown to tremendous sizes, with masses up to hundreds of times that of the Sun.

These stars then would have collapsed into “seed” black holes that then grew to supermassive sizes by sucking in more material. Even better, says another camp, would be to collapse a black hole directly out of a gas cloud. That’s tricky to do, but with the right conditions it might be possible to make black holes thousands of times the mass of the Sun. Observers have detected black holes a billion times the mass of the Sun just 1 billion years after the Big Bang, so this theory is a tempting one. To find the answer, astronomers are looking at dwarf galaxies.

Dwarfs haven’t changed much since the beginning of the universe, so they should retain the memory of black holes’ seeding scenario, says Amy Reines (NRAO). If supermassive black holes formed from the first stars, then every dwarf should have one.

On the other hand, because the direct-collapse process is more difficult, it would probably leave most dwarf galaxies without a supermassive black hole to call their own. Until recently, astronomers had only found a few cosmic heavyweights in dwarf galaxies, such as the one in NGC 4395, which is a few hundred thousand times the mass of the Sun.

Reines and her colleagues have expanded that sample. Presenting at the winter AAS meeting, Reines’s team pulled 25,000 dwarf galaxies from the Sloan Digital Sky Survey and searched their visible-light spectra for telltale signs of a massive, feeding black hole.

They found those signals in 151 dwarf galaxies — not even 1% of the full sample. Reines cautions that this result is only a first step, because the team could only detect feeding black holes. Dormant ones — and most of these objects would be dormant most of the time — would go undetected. Furthermore, a dwarf galaxy with lots of star formation would drown out the signal from its black hole.

The team next plans to observe with the Very Large Array and NASA’s Chandra X-ray Observatory. Black holes emit in radio and X-rays even when not chowing down, so the expanded survey should reveal quieter beasts that are invisible to SDSS.

Echoes of an aging star

