NASA astronomers using the Hubble Space Telescope have found a cluster of young, blue stars encircling the first intermediate-mass black hole ever discovered.
According to Mathieu Servillat of the Harvard-Smithsonian Center for Astrophysics, the presence of the star cluster suggests the black hole was once at the core of a now-disintegrated dwarf galaxy.
"The discovery of the black hole and the star cluster has important implications for understanding the evolution of supermassive black holes and galaxies," explained Servillat.
"[And now], for the first time, we have evidence on the environment, and thus the origin, of this middle-weight black hole."
Indeed, astronomers know how massive stars collapse to form stellar-mass black holes (which weigh about 10 times the mass of our sun), but it remains unclear how supermassive black holes (like the four million solar-mass monster at the center of the Milky Way) form in the cores of galaxies.
One current theory? Supermassive black holes may build up through the merger of smaller, intermediate-mass black holes weighing hundreds to thousands of suns.
The intermediate-mass black hole - dubbed HLX-1 (Hyper-Luminous X-ray source 1) - was first discovered by 2009 by Sean Farrell of the Sydney Institute for Astronomy in Australia, who used the European Space Agency's XMM-Newton X-ray space telescope. The black-hole, which weighs in at 20,000 solar masses, is located towards the edge of the galaxy ESO 243-49, approximately 290 million light-years from Earth.
Farrell and his team subsequently observed HLX-1 with NASA's Swift observatory in X-ray and Hubble in near-infrared, optical and ultraviolet wavelengths. The intensity and the color of the light shows a cluster of young stars, 250 light-years across, encircling the black hole. Although Hubble can't resolve the stars individually because the suspected cluster is too far away, the brightness and color are consistent with other clusters of young stars seen in other galaxies.
The astronomers also detected blue light from hot gas in the accretion disk swirling around the black hole. However, they also identified red light produced by much cooler gas, which would most likely originate from stars. To be sure, computer models suggest the presence of a young, massive cluster of stars encircling the black hole.
"What we can definitely say with our Hubble data is that we require both emission from an accretion disk and emission from a stellar population to explain the colors we see," said Farrell.
Such young clusters of stars are typically observed in nearby galaxies, but not outside the flattened starry disk, as found with HLX-1. The best explanation thus far? The HLX-1 black hole was the central black hole in a dwarf galaxy. The larger host galaxy then captured the dwarf, while most of the dwarf's stars were stripped away through the collision between the galaxies. Concurrently, new young stars were formed in the encounter, as the interaction that compressed the gas around the black hole also triggered star formation.
Farrell and Servillat have determined the star cluster is less than 200 million years old. This means the bulk of the stars were formed following the dwarf's collision with the larger galaxy (the age of the stars tells how long ago the two galaxies crashed into each other).
The future of the black hole is uncertain at this stage, and remains contingent on its trajectory, which is currently unknown. It's entirely possible the black hole may spiral in to the center of the big galaxy and eventually merge with the supermassive black hole there. Alternately, the black hole could settle into a stable orbit around the galaxy. Either way, it's likely to fade away in X-rays as it depletes its supply of gas.
"This black hole is unique in that it's the only intermediate-mass black hole we've found so far. Its rarity suggests that these black holes are only visible for a short time," added Servillat.