A team of astronomers using the Hubble telescope has studied a mid-sized black hole in the spectacular edge-on galaxy ESO 243-49 and found that it is surrounded by a cluster of young stars.
Circle identifies the black hole HLX-1 in the galaxy ESO 243-49 (NASA / ESA / S. Farrell et al)
The discovery, published in the Astrophysical Journal Letters, also suggests that the black hole, called HLX-1, formed in the core of a now-disintegrated dwarf galaxy.
HLX-1 was discovered in 2009 by Dr. Sean Farrell of the Sydney Institute for Astronomy in Australia and the University of Leicester, UK. It weighs in around 20 000 times the mass of the Sun and lies towards the edge of galaxy ESO 243-49, which is 290 million light-years from Earth.
Now, Dr. Farrell and his team have studied this black hole in ultraviolet, visible and infrared light using Hubble, and simultaneously in X-rays using the Swift spacecraft.
“For a unique source we needed a unique telescope,” said Dr. Mathieu Servillat, a co-author on the study. “Hubble provided such precision in its images that it helped us understand the origin and environment of this intermediate-mass black hole.”
Hubble’s images of the region show an excess of red light, which cannot be explained by emissions from the accretion disc alone. This light, the team has concluded, is evidence of a cluster of hot stars surrounding the black hole as the brightness and color of the light is similar to that from star clusters in nearby galaxies.
“What we can definitely say with our Hubble data is that we require both emission from an accretion disc and emission from a stellar population to explain the colors we see,” Dr. Farrell said.
The existence of a star cluster around the black hole in turn gives clues about where the intermediate mass black hole may have come from, and why it lies in its present location in ESO 243-49.
“The fact that there’s 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,” Dr. Farrell concluded. “The dwarf galaxy was then swallowed by the more massive galaxy.”
