In the quest to discover how massive black holes form, astronomers have spotted several of these objects in some of the smallest galaxies yet found to host them.

Astronomers have detected 13 massive black holes in distant dwarf galaxies, and more than half of them wander far from their galaxies’ centers.

Giant black holes generally sit in the hearts of their host galaxies. Nearly every large galaxy has a central supermassive black hole, in fact. But when galaxies interact or black holes merge, it can kick the little spacetime monsters out of their galactic nuclei, leaving them to maraud in the outskirts.

Galaxies the size of our Milky Way may have many such stowaways. Smaller, dwarf galaxies are particularly vulnerable to the gravitational tumult that can wrench a black hole from its normal stomping grounds. Recent simulations predict that roughly half of all the big black holes in dwarf galaxies wander far from the center.

For several years, Amy Reines (Montana State University) and her colleagues have been scouring runt galaxies for signs of massive black holes, those smaller than supermassive ones but formed (probably) the same way. They’re doing so in hopes that they can find out how small a massive black hole can be, and how often they’re that small. This information may tell astronomers more about how the first giant black holes formed.

The problem is, these teeny massive black holes are tough to find. At visible and infrared wavelengths, they’re often hidden by star formation and dust. Reines’s team previously found 151 dwarf galaxies with active galactic nuclei (AGN), black holes chomping down on hot gas. But such observations only pick up the brightest black holes, those that are the most voracious eaters.

The team has now taken a different approach and turned to radio observations. AGN emit radio waves that easily cut through dust, and the Karl G. Jansky Very Large Array in New Mexico provides a sharp view of where these radio waves come from in a galaxy. Culling from a much larger sample, the astronomers looked at 111 dwarf galaxies that pop up in both radio and optical data. The heftiest of the dwarfs are about the mass of the Large Magellanic Cloud, the most massive dwarf galaxy in the Milky Way’s environs. On average, the galaxies lie 230 million light-years away.

Of these 111, 35 contain compact regions emitting radio waves. And of those, 13 have radio sources far too bright to be from star formation or supernovae. They are almost certainly AGN.

Reines presented the work January 7th at the winter American Astronomical Society meeting in Honolulu. The study appears in the January 1st Astrophysical Journal.

The team doesn’t know the black holes’ masses. But if they assume that the masses relate to the host galaxies’ total mass in stars the same way that supermassive black holes do, then the black holes range from 13,000 to 630,000 solar masses, with an average around 400,000. That decidedly puts them among the smallest big black holes astronomers have found.

The majority of the newbies sit well away from their galaxies’ centers. Several of these galaxies are extended or distended, probably due to past interactions with other galaxies. Intriguingly, the AGN found in dwarf galaxies at visible wavelengths all appear to sit in their galaxies’ centers — evidence that radio observations are giving astronomers access to a population of black holes they couldn’t detect before.

How Did the First Big Black Holes Form?

At the moment, astronomers have found roughly a couple hundred candidates for big black holes in dwarf galaxies, with various degrees of certainty, Reines says. Among these are results presented by Igor Chilingarian (Smithsonian Astrophysical Observatory) during the same session at the AAS meeting. His team has identified signs of 14 massive black holes using new and archival X-ray observations. Assuming the sources are indeed black holes, the objects have masses between 30,000 and 200,000 Suns, he estimates.

There are a couple of possibilities for making a massive black hole. The first stars likely formed big, and in dying they could have made black holes of about 100 solar masses, which then would grow by merging and eating gas. That takes times, however. Conversely, if conditions were just right in the early universe (and it’s quite the delicate balance), clouds of gas could have collapsed directly to make black holes of about 10,000 to 100,000 Suns.

If stars were the seed, the thinking goes, we’d expect to see a bunch of teeny massive black holes in dwarf galaxies. But if the direct-collapse approach was more prevalent, we’d expect only a few.

Solving this question motivates work like Reines’s and Chilingarian’s. We likely won’t have the observations to decide the question any time soon, though. Observers are not yet able to find large numbers of black holes with masses around 10,000 Suns and test these scenarios in earnest, Chilingarian says. Furthermore, in an upcoming review Jenny Greene (Princeton, and one of Reines’s collaborators) and her colleagues worry that too many things could happen to black holes as they grow to say for sure which way they were made. But counting up the number of off-center black holes like those Reines’s team found might prove key, they note, because they essentially would have been in deep freeze.

References:

Amy Reines et al. “Abstract 344.01. Wandering Massive Black Holes in Dwarf Galaxies.” American Astronomical Society 235 Meeting.

Igor Chilingarian et al. “Abstract 344.07. Active galactic nuclei powered by intermediate-mass black holes and properties of their host galaxies.” American Astronomical Society 235 Meeting.

Amy Reines et al. “A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies from High-resolution Radio Observations.” Astrophysical Journal. January 1, 2020.