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Closing in on Intermediate-sized Black Holes

artist rendition

© H. Sagerud

Artist's rendition of the recently discovered HLX-1 source (blue spot above the galactic bulb), located in the ESO 243-49 spiral galaxy.

Using the ESA's XMM-Newton X-ray space telescope, Olivier Godet1 and his colleagues at CESR2 believe they have found the “missing link” in black hole formation.3 A black hole is a region of space so dense that nothing–not even light–can escape it. To date, two types of black holes have been observed: stellar ones, formed by the collapse of a star and which are relatively small (3-20 solar masses), and supermassive ones (millions to billions of solar masses) that can be found at the center of galaxies like our own. Yet the formation of the latter have stumped astrophysicists for years. A theory is that they are the result of smaller black holes colliding and continuously accreting more matter. But if this is the case, why have we never been able to detect intermediate-sized black holes?
There are at least two ways of identifying–and “weighing”–black holes: by observing stars that orbit an area of space that seems empty, (common for stellar mass black holes), or by using powerful X-ray or Gamma-ray telescopes to scan the universe for the energy emitted by a black hole's accretion disc, as gas and dust spin faster and faster before being swallowed up. If the latter works well for estimating the size of supermassive black holes, intermediate-sized ones are more treacherous to identify because they are far away, and they have a more limited impact on their immediate surroundings.
Yet Godet and his colleagues are confident that the object that they have discovered at the edge of a galaxy called ESO 243-49, more than 290 million light-years away, is an intermediate-sized black hole. Dubbed “Hyper-Luminous X-ray Source 1” (HLX-1), it has an estimated size of more than 500 solar masses. HLX-1 was first observed by XMM-Newton in 2004, but the researchers needed to rescan the area in 2008 to confirm its nature. This is the only black hole that has been observed in this galaxy so far, “but we are planning a battery of multiwavelength observations to find out if this source is part of a star cluster, which would have all the gas necessary to make it grow,” he concludes.

Saman Musacchio

Notes :

1. Department of Physics and Astronomy, University of Leicester, UK (now back at CESR).
2. Centre d'Etude Spatiale des Rayonnements (CNRS / Université Paul Sabatier).
3. S. Farrell et al., “An Intermediate Mass Black Hole of Ž 500 Solar Masses in the Galaxy ESO 243-49,” Nature, 2009. 460: 73-5.

Contacts :

Olivier Godet
CESR, Toulouse.


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