Since they don’t emit anything that we can detect, there are many things about black holes that we just don’t know.
Among the most mysterious supermassive black holes, the mass of which is millions and billions of times greater than the mass of our Sun. As black holes form from the cores of massive, dead individual stars, how do supermassive behemoths get so dangerous and huge? A new discovery can help us find answers to such sought-after questions.
In a tiny dwarf galaxy 110 million light-years away, astronomers have just discovered one of the smallest supermassive black holes ever seen. The mass of the “monster” in the heart of the galaxy Mrk 462 is only 200,000 times the mass of the Sun.
The results were presented at the 239th Virtual Meeting of the American Astronomical Society.
“This black hole in Mrk 462 is one of the smallest supermassive or monstrous black holes,” said astronomer Jack Parker of Dartmouth College. “Black holes like this are notoriously hard to find.”
His discovery suggests that supermassive black holes – at least some of them – are growing from the seeds of stellar mass less than 100 times the mass of the Sun, rather than becoming larger and larger.
One thorn in the stellar mass model is the discovery of many supermassive black holes in the early universe. It is difficult to reconcile the growth of a tiny seed of a stellar core with a giant supermassive black hole in the short period of time in which these objects appeared after the Big Bang.
Another possibility is that in the early universe, huge, dense clouds of gas and dust directly collapsed into larger black holes, tens of thousands of times the mass of the Sun, creating a larger starting point from which supermassive black holes can grow.
And that gives us a method for determining the likelihood of each scenario, because the direct collapse model is a rare process. This means that we can expect a relatively small number of dwarf galaxies containing supermassive black holes compared to the starseed model.
However, there is one catch. In dwarf galaxies, it is really difficult to see black holes in the core. In larger galaxies, astronomers can use the orbit of the stars in the center to infer a central black hole, but dwarf galaxies are too small and dim for that.
Another method is to look for extremely bright, high-energy radiation such as X-rays. This suggests a black hole actively accreting material, heating it to such insane temperatures that it emits high-energy light.
The researchers did just that, using the Chandra X-ray Observatory to study eight dwarf galaxies that, according to optical data, may contain an active supermassive black hole. Mrk 462 alone showed the X-ray signature of a supermassive black hole feeding about 200,000 times the mass of the Sun.
But there was also something special. The ratio of high-energy X-rays to low-energy X-rays suggests that the black hole was heavily obscured or “buried” by a thick cloud of dust.
This could tip the scales towards the starseed scenario.
“Because hidden black holes are even more difficult to detect than open ones, finding this example could mean that there are many more dwarf galaxies with similar black holes,” said astronomer Ryan Hickox of Dartmouth College.
Recent research suggests that this may be the case. In the past few years, astronomers have found elusive medium-weight black holes in dwarf galaxies and dwarf galactic remnants. It could also support the idea that supermassive black holes can grow fairly quickly from the seeds of stellar mass.
As our technologies and techniques improve, perhaps we can expect to find more babies of black holes that have previously eluded detection.
“We cannot draw unambiguous conclusions from one example, but this result should stimulate a broader search for hidden black holes in dwarf galaxies,” said Parker. “We’re thrilled about what we might find out.”
The study was presented virtually at the 239th meeting of the American Astronomical Society.