In a galaxy 290 million light-years away, a cosmic fugitive has emerged from hiding, bearing clues about how galaxies and the supermassive black holes at their centers evolve.
The fugitive: the never-seen-before intermediate-sized black hole.
For three years, an international team of astronomers has had the elusive black hole under surveillance. The researchers now say they've found the strongest candidate yet for the mid-sized black hole, which may have been at the core of a dwarf galaxy that collided with a larger galaxy less than 200 million years ago.
If the analysis holds up, it would strengthen the notion that the black holes at the center of galaxies grow through collisions and mergers involving their host galaxies, just as the galaxies themselves are thought to grow. In the case of the fugitive, dubbed ESO 243-49 HLX-1, it's expected to eventually merge with the supermassive black hole at the center of its host galaxy.
A black hole is an object with gravity so strong that nothing can escape its grip, even when traveling at the speed of light. Astronomers have found convincing evidence that black holes, which form from the collapse of individual stars, exist. They've also found convincing evidence that supermassive black holes exist in the hearts of galaxies.
But questions remain about whether intermediate-mass black holes exist, let alone about how they formed, notes Mathieu Servillat, an astrophysicist and member of the team reporting the results in the journal Astrophysical Review Letters.
Although the team has been studying HLX-1 as if it were an intermediate-mass black hole, astronomers have yet to agree on any candidate as the real deal. In the past eight years, two other groups have each identified a candidate. But the evidence for each failed to convince other researchers.
“If intermediate black holes exist, they would have an impact at different stages of the evolution of the universe,” says Dr. Servillat, a researcher at France's National Center for Space Studies. “At the very beginning, you have to form intermediate-mass black holes. Is this a giant star that is collapsing? Is this a number of stellar-mass black holes accumulating to become an intermediate black hole?”
For now, the team's work may shed light on how a midsized black hole contributes to the growth of a supermassive black hole.
HLX-1 was discovered in 2009 by Sean Farrell, a researcher at the University of Sydney in Australia, and leader of the team reporting these latest results. It betrayed its presence through x-ray emissions that were extraordinarily bright. The emissions come from heating of dust and gas as it spirals in toward the black hole's point of no return.
HLX-1 was too bright to be a black hole formed from the collapse of a single star. The best match between what Dr. Farrell observed and theorists' models of black-hole behavior suggested a black hole with about 20,000 times the mass of the sun. Stellar black holes are thought to range from three to 80 solar masses. Supermassive black holes range from 1 million to more than 1 billion solar masses.
In addition, over the three years the team has monitored the object, it has brightened and dimmed roughly once a year. This suggests that a star may be swinging around the black hole in an elliptical orbit, losing matter to the black hole with each closest approach and causing it to flare.
But the team also wanted to know more about the black hole's environment. The galaxy associated with it appears edge on, and HLX-1 appears above the plane of the galaxy's disk. To the team, that strongly suggested the black hole wasn't locally grown. It also suggested that the object was embedded in a cluster of stars.
The remnants of a collision, perhaps?
Using the Hubble Space Telescope and NASA's SWIFT x-ray telescope simultaneously, the team observed HLX-1 in forms of light that range from near-infrared, visible, and ultraviolet to x-rays. This allowed them to associate any starlight that might surround the black hole with the black hole itself, which appears only at x-ray wavelengths.
Although Hubble can't spot individual stars in a galaxy 290 million light-years away, it can detect the range of wavelengths in the combined starlight reaching it. Again, the best match between models and the range of light the team observed indicated about 1 million solar masses worth of stars surrounding the black hole.
It will require more observations to get a better handle on the age of the stars. But the team says it thinks the stars are relatively young. If the stars were old – typical for so-called globular clusters of stars that populate space immediately around galaxies – that would imply some physically implausible processes taking place in the environment around the cluster.
A way to get young stars in a cluster is through a merger with a smaller galaxy, in this case, with a dwarf galaxy, since the size of a central black hole varies depending on the size of the galaxy it occupies.
During a collision or merger, the stars within the two galaxies are too far apart to smack together. But gravity from the larger galaxy can strip stars from the smaller, leaving the intermediate black hole surrounded with young stars from the dwarf's core as well as gravitationally disturbed clumps of dust and gas that can form new stars.
That's not to say globular clusters are devoid of mid-range black holes. If globular clusters are ancient relics of the centers of small galaxies merged with a main galaxy, as some suspect, they could harbor mid-sized black holes. But the clusters are devoid of the dust and gas whose heating would betray the black hole's presence. Some teams have been hunting for mid-sized black holes based on the effect the black hole's gravity would have on the motions of nearby stars in a cluster.
As for the new results, “through our observation, we have hints of this scenario of accreting a galaxy with an intermediate-mass black hole inside, which will slowly spiral inside the main galaxy and probably be accreted by the supermassive black hole,” Servillat says. “This shows us how supermassive black holes gain mass.”