Discovery of young stars surrounding a black hole of intermediate mass

A cluster of young stars sheltering an intermediate-mass black hole: this is the astonishing discovery of an international team including several researchers from IRAP1 (CNRS/University Toulouse 3) and AIM2 (University Paris-Diderot /CEA/CNRS). This result allows a better understanding of the origin of black holes of this type, still poorly known, but also of super massive black holes and more widely of galaxies. It is published online on the website of the scientific journal Astrophysical Journal.

The ESO 243-49 galaxy, where black hole HLX-1 is located, as seen by Hubble. NASA/HST

Black holes are objects where matter is so densely concentrated that gravity is strong enough to prevent even light from escaping. Astronomers classify them in several categories, ranging from “stellar black holes” (with a mass of a few times that of our Sun, even a few dozen times for the most massive) to “super-massive black holes” (with a mass of several million or billion times that of the Sun).

Halfway there are the intermediate-mass black holes, which are poorly known. It was only in 2009 that an international team led by Sean Farrell discovered the first of them in the galaxy ESO 243-49, named HLX-1, with about 20 000 solar masses.

It is around this black hole that the same team has just detected the presence of a very young massive cluster of stars, using Hubble and Swift, two NASA space telescopes, as well as new modelling techniques.

Sean Farrell and his team wanted to take a closer look at “their” black hole, to better understand its environment and to observe its probable fusion with the galaxy that houses it. Surprise: they then detected the presence of a very young and massive cluster of stars around the black hole. This suggests that HLX-1 would be the central black hole of a dwarf galaxy of very low mass, which then collided with the massive galaxy in which it is currently located.

Before this discovery, astrophysicists did not know for sure where intermediate-mass black holes could come from. This result also allows us to better understand the formation of super-massive black holes, which play a key role in the formation of galaxies and most likely result from the fusion of intermediate-mass black holes. The study of the latter and their environment is therefore of major importance.


1 Institut de Recherche en Astrophysique et Planétologie
 2 Laboratoire Astrophysique, Instrumentation et Modélisation

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