Using the combined power of two X-ray observatories and the keen vision of NASA's Hubble Space Telescope, astronomers have found the best evidence for a black hole of an elusive class known as "intermediate-mass".
Weighing in at about 50,000 times the mass of our Sun, the black hole is smaller than the supermassive black holes (at millions or billions of solar masses) that lie at the cores of large galaxies, but larger than stellar-mass black holes formed by the collapse of a massive star, according to the study published in The Astrophysical Journal Letters.
These so-called intermediate-mass black holes (IMBHs) are a long-sought "missing link" in black hole evolution.
Though there have been a few other IMBH candidates, researchers consider these new observations the strongest evidence yet for mid-sized black holes in the universe.
"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 principal investigator of the study Dacheng Lin of the University of New Hampshire in the US.
Lin and his team used Hubble to follow up on leads from NASA's Chandra X-ray Observatory and ESA's (the European Space Agency) 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.
Surprisingly, the X-ray source, named 3XMM J215022.4-055108, was not located in a galaxy's centre, where massive black holes normally would reside.
This raised hopes that an intermediate-mass black holes was the culprit, but first 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.
Neutron stars are the crushed remnants of an exploded star.
Hubble was pointed at the X-ray source to resolve its precise location. Deep, high-resolution imaging provided 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.
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, said the study.