There is something strange about the black holes discovered to date. We have found many smaller black holes, with masses less than 100 times that of the Sun, and many huge black holes, with masses millions or even billions of times that of the sun. But we’ve barely found black holes in the intermediate mass range, possibly not enough to confirm that they exist, and it’s not really clear why.
Now astronomers are using the Hubble Space Telescope to search for these lost black holes. Hubble has previously found some evidence for black holes in this intermediate range, and it is now being used to search for examples within a few thousand light-years of Earth.
It is difficult to detect these intermediate black holes because the effect they have on the stars around them is more modest than that of the huge supermassive black holes that astronomers usually observe. Hubble has been observing targets like Messier 4, a globular cluster thought to contain a black hole with a mass about 800 times that of the Sun. The black hole cannot be directly observed, but its presence can be inferred by observing its subtle effects on nearby stars.
The researchers also used data from Gaia, a project to create a 3D map of stars in the Milky Way, which helped provide information about the shape of the globular cluster. However, even with these two powerful telescopes, it is still difficult for researchers to tell whether they are looking at a black hole or a less dense group of objects like neutron stars or white dwarfs.
“Using the latest data from Gaia and Hubble, it was not possible to distinguish between a dark population of stellar remnants and a single larger point source,” explained the lead author of the research, Eduardo Vitral, from the Space Telescope Science Institute, in a statement. a release. “So one of the possible theories is that instead of being a bunch of separate small dark objects, this dark mass could be a medium-sized black hole.”
If there were a bunch of objects together, they would have to be piled up in an unstable formation. The most likely explanation is that there is only one black hole with an intermediate mass.
“We have good confidence that we have a very small region with a lot of concentrated mass,” Vitral said. “It is about three times smaller than the densest dark mass we have found before in other globular clusters. The region is more compact than we can reproduce with numerical simulations when we take into account a collection of black holes, neutron stars, and white dwarfs segregated at the center of the cluster. They are not capable of forming such a compact mass concentration.”
That means researchers can’t be entirely sure they’ve found one of the elusive intermediate black holes, but it’s a definite possibility. And that means there’s more exciting research to come. “Science is rarely about discovering something new in a single moment,” said Gaia mission scientist Timo Prusti. “It’s about being more certain of a step-by-step conclusion, and this could be a step to make sure intermediate-mass black holes exist.”
The research is published in the journal Monthly Notices of the Royal Astronomical Society.