A big step in solving the mystery of how the very biggest black holes grow has been taken by researchers using NASA’s Chandra X-ray Observatory. They identified a black hole containing about 200,000 times the mass of the Sun, buried in gas and dust in the dwarf galaxy Mrk 462. ”Future sensitive observations, using x-ray and the James Webb Space Telescope (JWST) for example, will be able to find signatures of direct collapse black holes over a wider range of masses and growth rates, and help determine whether they form the seeds of the supermassive black holes we see today,” wrote Dartmouth University astrophysicist Ryan Hickox in an email to The Daily Galaxy,
“This is important because it could help address a major question in astrophysics: How did black holes get so big so early in the universe?” these supermassive black holes could have formed during the formation of its host galaxy, and we wanted to turn these theoretical predictions into observational predictions that could be seen by the James Webb Space Telescope.”
James Webb Space Telescope Detection of the First Galaxies
“The big breakthrough with JWST is that it will begin detecting the early galaxies that would host these direct collapse black holes. If the black holes are massive enough, and accreting material rapidly enough, we may even be able to use JWST to see signatures of these direct collapse black hole growing,” Hickox explained in his email, referring to primordial black holes of the early universe that formed before the first stars.
JWST will be sensitive to infrared light, and such light is much less affected by dust and gas than is the optical light typically used by ground-based telescopes.
Mrk 462 contains only several hundred million stars. By contrast, our Milky Way is home to a few hundred billion stars. This is one of the first times that a heavily buried, or “obscured,” supermassive black hole has been found in a dwarf galaxy.
Notoriously Hard to Find
“This black hole in Mrk 462 is among the smallest of the supermassive, or monster, black holes,” said Jack Parker of Dartmouth College in New Hampshire, who led the study with colleague Ryan Hickox, also from Dartmouth. “Black holes like this are notoriously hard to find.”
In larger galaxies, astronomers often find black holes by looking for the rapid motions of stars in the centers of galaxies. However, dwarf galaxies are too small and dim for most current instruments to detect this. Another technique is to search for the signatures of growing black holes, such as gas being heated up to millions of degrees and glowing in X-rays as it falls towards a black hole.
The researchers in this study used Chandra to look at eight dwarf galaxies that had previously shown hints of black hole growth from optical data gathered by the Sloan Digital Sky Survey. Of those eight, only Mrk 462 showed the X-ray signature of a growing black hole.
The unusually large intensity of high energy X-rays compared to low energy X-rays, along with comparisons to data at other wavelengths, indicates that the Mrk 462 black hole is heavily obscured by gas.
Problem of Buried Black Holes
“Because buried black holes are even harder to detect than exposed ones, finding this example might mean there are a lot more dwarf galaxies out there with similar black holes,” said Hickox. “This is important because it could help address a major question in astrophysics: How did black holes get so big so early in the universe?”
Previous research has shown that black holes can grow to a billion solar masses by the time the universe is less than a billion years old, a small fraction of its current age. One idea is that these huge objects were created when massive stars collapse to form black holes that weighed only about 100 times the mass of the Sun. Theoretical work, however, struggles to explain how they could pack on weight quickly enough to reach the sizes seen in the early universe.
An alternative explanation is that the early universe was seeded with black holes containing tens of thousands of solar masses when they were created — perhaps from the collapse of gigantic clouds of gas and dust.
Astonishing Growth of Black Hole Seeds
A large fraction of dwarf galaxies with supermassive black holes favors the idea that small black hole seeds from the earliest generation of stars grew astonishingly quickly to form the billion solar mass objects in the early universe. A smaller fraction would tip the scales to favor the idea that black holes began life weighing tens of thousands of Suns.
These expectations apply because the conditions necessary for the direct collapse from a giant cloud to a medium-sized black hole should be rare, so it is not expected that a large fraction of dwarf galaxies would contain supermassive black holes. Stellar-mass black holes, on the other hand, are expected in every galaxy.
“We can’t make strong conclusions from one example, but this result should encourage much more extensive searches for buried black holes in dwarf galaxies,” said Parker. “We’re excited about what we might learn.”