Two new intermediate-mass black holes (and 16 new possible candidates in the category) have been discovered by the research group coordinated by Ivan Zolotukhin from the Institut de Recherche en Astrophysique et Planétologie (IRAP) in Toulouse, France, as reported in an article recently published in The Astrophysical Journal.
A large number of black holes are known today, both in our galaxy and beyond.
The largest ones, called supermassive black holes, have masses up to billions of times that of our Sun and are found at the center of most known galaxies, including ours. A few dozen much smaller ones (10-20 times the mass of the Sun) are also known, mainly in our galaxy, and are thought to form from the collapse of single massive stars at the end of their lives.
Much harder to find are the so-called intermediate-mass black holes, 1,000-100,000 times heavier than the Sun. Yet they would be extremely interesting to study, as Matteo Bachetti, a researcher at our Observatory and co-author of the research, explains.
“Intermediate-mass black holes are considered the ‘seeds’ from which supermassive black holes form, but they are not easy to form nowadays. It is believed that many of them are ‘primordial,’ meaning they formed during the very early stages of the universe’s life.
In fact, they would require the collapse of enormous stars, impossible to form in the current era, but present when the universe was young, or a very long process of matter accretion to increase the mass to these levels.” Unless the accretion process is extremely rapid, which, as Bachetti himself has helped to ascertain, could be another possible key to the rapid growth of these black holes.
These intermediate-mass objects, therefore, represent an important ingredient in understanding the evolution of supermassive black holes and, consequently, galaxies. However, they are traditionally very difficult to find, so much so that only a handful are known today. “The problem is certainly not that they are not bright; on the contrary, they are among the brightest sources and can be seen by X-ray satellites currently in orbit. Unfortunately, they are generally very distant, and it is difficult to distinguish them from other types of X-ray sources that may be at different distances,” says Bachetti.
But among the astronomical data archives available online lies a goldmine, and Ivan Zolotukhin knows it. Thanks to his knowledge of observatory archives worldwide and his skill in using databases known as Virtual Observatories, he managed to relate the data from the X-ray satellite XMM-Newton with those from radio, infrared, and optical telescopes, and find the key to distinguishing these intermediate black holes from other types. The final result is two new intermediate-mass black holes and 16 very likely new members of the category. Further observations at different frequencies have been requested to confirm the nature of these objects.
“This and other recent discoveries (one of which was published in the prestigious journal Science by Igor Chilingarian and Zolotukhin himself, editor’s note) unequivocally demonstrate how important it is to facilitate the sharing of astronomical data. Today, Virtual Observatory techniques allow important discoveries to be made in data that may have been obtained for entirely different studies and were lying almost unused. I hope that more and more Observatories will seize this opportunity and make their archival data available in this way,” comments Bachetti.