Star Cluster Surrounds Wayward Black Hole in Cannibal Galaxy

Astronomers using NASA's Hubble Space Telescope may have found evidence for a cluster of young, blue stars encircling one of the first intermediate-mass black holes ever discovered. Astronomers believe the black hole may once have been at the core of a now-disintegrated unseen dwarf galaxy. The discovery of the black hole and the possible star cluster has important implications for understanding the evolution of supermassive black holes and galaxies.

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.

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