Astronomers have found the best evidence for a black hole of an elusive class known as "intermediate-mass," which betrayed its existence by tearing apart a wayward star that passed too close.
Though there have been a few other intermediate-mass black holes (IMBH) candidates, researchers consider these new observations the strongest evidence yet for mid-sized black holes in the universe.
So far, scientists knew only of two sizes of black holes: "stellar" black holes, with masses about 10 times our Sun, and "supermassive" black holes located at the centers of galaxies, with masses ranging from millions to billions times our Sun. But eventually, evidence began to emerge that a mid-sized, "intermediate-mass" black hole also exists.
These so-called IMBHs are a long-sought "missing link" in black hole evolution. Weighing in at about 50,000 times the mass of our Sun, the black hole is smaller than the supermassive black holes that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of a massive star, the researchers said.
"Intermediate-mass black holes are very elusive objects, and so it is critical to carefully consider and rule out alternative explanations for each candidate. That is what Hubble has allowed us to do for our candidate," said Dacheng Lin of the University of New Hampshire, principal investigator of the study.
Lin and his team used Hubble Space Telescope to follow up on leads from NASA's Chandra X-ray Observatory and the European Space Agency's X-ray Multi-Mirror Mission (XMM-Newton).
In 2006 these satellites detected a powerful flare of X-rays, but they could not determine whether it originated from inside or outside of our galaxy.
Researchers attributed it to a star being torn apart after coming too close to a gravitationally powerful compact object, like a black hole.
The X-ray source, named 3XMM J215022.4-055108, was not located in a galaxy's centre, where massive black holes normally would reside.
The study, which has been published in the Astrophysical Journal Letters, raised hopes that an IMBH was the culprit, the researchers said.
However, another possible source of the X-ray flare had to be ruled out: a neutron star in our own Milky Way galaxy, cooling off after being heated to a very high temperature, they said.
Neutron stars are the crushed remnants of an exploded star.
Hubble was pointed at the X-ray source to resolve its precise location. The Hubble Space Telescope is a project of international cooperation between NASA and ESA (European Space Agency).
Deep, high-resolution imaging provides strong evidence that the X-rays emanated not from an isolated source in our galaxy, but instead in a distant, dense star cluster on the outskirts of another galaxy -- just the type of place astronomers expected to find an IMBH.
Previous Hubble research has shown that the mass of a black hole in the centre of a galaxy is proportional to that host galaxy's central bulge.
The star cluster that is home to 3XMM J215022.4-055108 may be the stripped-down core of a lower-mass dwarf galaxy that has been gravitationally and tidally disrupted by its close interactions with its current larger galaxy host.
IMBHs have been particularly difficult to find because they are smaller and less active than supermassive black holes, the researchers said.
They do not have readily available sources of fuel, nor as strong a gravitational pull to draw stars and other cosmic material which would produce telltale X-ray glows, they said.
Astronomers essentially have to catch an IMBH red-handed in the act of gobbling up a star.
Lin and his colleagues combed through the XMM-Newton data archive, searching hundreds of thousands of observations to find one IMBH candidate.
The X-ray glow from the shredded star allowed astronomers to estimate the black hole's mass of 50,000 solar masses.