The secret of the growth of black holes

A study carried out on more than a hundred galaxies by a team of researchers from NASA’s Chandra X-ray Observatory has revealed something that was already suspected. Some growing black holes are carrying out a ‘mass killing of stars’, devouring them by the thousands in what looks like a true large-scale stellar demolition process. Of the hundred galaxies investigated, 29 showed evidence of rapidly growing black holes, all of them in the central regions of their galaxies, where the density of stars is higher.

The work, published in ‘The Astrophysical Journal’, could help explain how a small, stellar-mass black hole can grow to a hundred million times the mass of the Sun. These are the elusive ‘intermediate-mass black holes’, much larger than those formed by the collapse of a single star but smaller than the supermassive black holes that reign at the center of galaxies. The study shows that destroying stars on a large scale is an efficient, albeit extremely violent, way for at least some stellar-mass black holes to grow exponentially in size.

black holes

Until now, astronomers have been able to thoroughly study two different classes of black holes: the smallest, or stellar-mass, which rarely exceed 30 solar masses; and the supermassive ones that reside in the galactic centers, which can reach tens of billions of times the mass of the Sun. But in recent years there has been increasing evidence that there is also a third class, of intermediate size (IMBH , Intermediate Mass Black Holes), although none have been unequivocally detected so far.

According to the study, the key for a small black hole to grow to become another of intermediate mass is in the environment that surrounds it. For this reason, the researchers decided to analyze clusters with a high density of stars just in the areas where there is a greater accumulation, in the central regions of the galaxies.

Models built by the team show that if the density of stars in a cluster is above a certain value, a stellar-mass black hole in the center of the cluster will undergo rapid growth by shredding and then ingesting the abundant ones at thousands of stars that are in the vicinity.

In fact, among the clusters studied with Chandra, those that showed a density above that critical value had about twice as many growing black holes as those that were below the density threshold. The process suggested in the study, moreover, can occur at any time in the history of the Universe, which implies that intermediate-mass black holes can form billions of years after the Big Bang, even in the present.

Font: Jose Manuel Nieves / ABC

Reference article: https://www.abc.es/ciencia/abci-revelan-secreto-crecimiento-agujeros-negros-masiva-masiva-estrellas-202205030911_noticia.html