Star watchers count black holes in globular clusters

Recently. black hole candidates have been seen in four globular clusters

The universe is strange and fascinating because it holds many a glittering clue to its secrets — bright and dark. To follow the clues and unravel these secrets is the astrophysicist’s privilege. In a recent paper published in The Astrophysical Journal, Sourav Chatterjee of Tata Institute of Fundamental Research (TIFR), Mumbai, and a team of international collaborators, trace such clues to estimate the number of black holes in globular clusters.

Globular clusters

Globular clusters are massive bunches of stars that are typically about 9-12 billion years old. While the Milky Way has about 150 globular clusters, their number could go up to a few thousands in other galaxies. They are densely packed with stars. Within the distance equal to that between the Sun and its nearest star neighbour, a globular cluster would have about a million stars. “Due to the high stellar densities, the stars interact with other stars frequently. These dynamical interactions are key to understanding how black holes form binaries that may be detected via gravitational waves, X-ray or radio emission,” says Sourav Chatterjee in an email to The Hindu.

Recently, astronomers have detected black hole candidates in four globular clusters in the Milky Way. “They are either discovered via X-ray or radio emissions or by monitoring the radial velocity of a companion star,” Dr Chatterjee explains. Among these, the team studied three globular clusters within the Milky Way: 47 Tuc, M 10, M22. These three globular clusters have candidates identified to be black holes through radio and X-ray observations. “The observations were done using archival data from the ACS survey done using the Hubble space telescope. All theoretical models and data reduction software were built by us,” he adds.

This has sparked interest in understanding how these black holes evolve and estimating how many black holes are there within a globular cluster. The team shows that the clue to knowing the number of black holes in the cluster lies in the mass segregation observed between the giant stars and the less massive main sequence stars. They find that the measure of mass segregation is related inversely to the number of black holes in the cluster. According to Dr. Chatterjee, this can be an effective probe to indirectly estimate the number of black holes in a globular cluster aided by calibrations from numerical models.

Milky Way

The team has estimated that 47 Tuc, M10, and M22 globular clusters within the Milky Way contain 150, 50 and 200 black holes, respectively.

Not all black holes that form in a cluster are retained. Some of them are ejected at birth and some are forced out later. Once this has stabilised, the galaxy or cluster acquires a stable retention fraction.

“Newlin [N. Weatherford, a collaborator from North Western University, USA] and I will use the same method to constrain the number of retained black holes in all globular clusters for which the necessary data is available,” Says Dr Chatterjee.

“These constraints will help us understand the retention fraction of black holes in globular clusters. This has wide implications. For example, the rate of black hole mergers observable by LIGO is dependent strongly on this [constraint],” he adds.

Source: Read Full Article