NASA’s Hubble Space Telescope discovers Black Hole surrounded by Star Cluster

Astronomers know how massive stars collapse to form black holes but it is not clear how supermassive black holes, which can weigh billions of times the mass of our sun, form in the cores of galaxies. One idea is that supermassive black holes may build up through the merger of smaller black holes.

Sean Farrell of the Sydney Institute for Astronomy in Australia discovered a middleweight black hole in 2009 using the European Space Agency’s XMM-Newton X-ray space telescope. Known as HLX-1 (Hyper-Luminous X-ray source 1), the black hole has an estimated weight of about 20,000 solar masses. It lies towards the edge of the galaxy ESO 243-49, 290 million light-years from Earth.

Farrell then observed HLX-1 simultaneously 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 may indicate the presence of a young, massive cluster of blue stars, perhaps 250-light-years across, encircling the black hole. Hubble can’t resolve the stars individually because the suspected cluster is too far away. The brightness and color is consistent with other clusters of stars seen in other galaxies, but some of the light may be coming from the gaseous disk around the black hole.

“Before this latest discovery, we suspected that intermediate-mass black holes could exist, but now we understand where they may have come from,” Farrell said. “The fact that there seems to be a very young cluster of stars indicates that the intermediate-mass black hole may have originated as the central black hole in a very-low-mass dwarf galaxy. The dwarf galaxy might then have been swallowed by the more massive galaxy, just as happens in our Milky Way.”

From the signature of the X-rays, Farrell’s team knew there would be some blue light emitted from the high temperature of the hot gas in the disk swirling around the black hole. They couldn’t account for the red light coming from the disk. It would have to be produced by a much cooler gas, and they concluded this would most likely come from stars.

Small distant galaxies host supermassive black holes

Using the Hubble Space Telescope to probe the distant universe, astronomers have found supermassive black holes growing in surprisingly small galaxies. The findings suggest that central black holes formed at an early stage in galaxy evolution.

"It's kind of a chicken or egg problem: Which came first, the supermassive black hole or the massive galaxy? This study shows that even low-mass galaxies have supermassive black holes," said Jonathan Trump, a postdoctoral researcher at the University of California, Santa Cruz. Trump is first author of the study, which has been accepted for publication in the Astrophysical Journal and is currently available online.

All massive galaxies host a central supermassive black hole, which may shine brightly as an active galactic nucleus if the black hole is pulling in nearby gas clouds. In the local universe, however, active black holes are rarely seen in small "dwarf" galaxies. The galaxies studied by Trump and his coauthors are about 10 billion light-years away, giving astronomers a view of galaxies as they appeared when the universe was less than a quarter of its current age.

"When we look 10 billion years ago, we're looking at the teenage years of the universe. So these are very small, young galaxies," Trump said.

The study, part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), used a powerful new instrument on the Hubble Space Telescope.

The "slitless grism" on Hubble's WFC3 infrared camera provided detailed information about different wavelengths of light coming from the galaxies. Spectroscopy allows researchers to spread out the light from an object into its component colors or wavelengths. With Hubble's high spatial resolution, the researchers were able to get separate spectra from the center and the outer part of each galaxy. This enabled them to identify the tell-tale emissions from a central black hole.

"This is the first study that is capable of probing for the existence of small, low-luminosity black holes back in time," said coauthor Sandra Faber, University Professor of astronomy and astrophysics at UC Santa Cruz and CANDELS principal investigator. "Up to now, observations of distant galaxies have consistently reinforced the local findings--distant black holes actively accreting in big galaxies only. We now have a big puzzle: What happened to these dwarf galaxies?"