Hubble finds cluster of blue stars around 'massive' black hole in cannibal galaxy

Washington, Feb 18 (ANI): Astronomers have found possible evidence for a cluster of young, blue stars encircling one of the first intermediate-mass black holes ever discovered, thanks to NASA's Hubble Space Telescope.

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 believe the black hole may once have been at the core of a now-disintegrated unseen dwarf galaxy.

This spectacular edge-on galaxy, called ESO 243-49, is home to an intermediate-mass black hole that may have been stripped off of a cannibalized dwarf galaxy.

The estimated 20,000-solar-mass black hole lies above the galactic plane. This is an unlikely place for such a massive back hole to exist, unless it belonged to a small galaxy that was gravitationally torn apart by ESO 243-49.

The circle identifies a unique X-ray source that pinpoints the black hole. The X-rays are believed to be radiation from a hot accretion disk around the black hole. The blue light not only comes from a hot accretion disk, but also from a cluster of hot young stars that formed around the black hole.

The galaxy is 290 million light-years from Earth. Hubble can't resolve the stars individually because the suspected cluster is too far away. Their presence is inferred from the colour and brightness of the light coming from the black hole's location.

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 colon of the light may indicate the presence of a young, massive cluster of blue stars, perhaps 250-light-years across, encircling 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," he explained.

Farrell theorizes that the possible star cluster may be less than 200 million years old. This means that the bulk of the stars 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.

Farrell proposed for more observations this year.

The new findings have been recently published in the Astrophysical Journal. (ANI)