Two stellar-mass black holes found in M22

Globular clusters are extremely dense spheroidal collections of stars, containing between 100,000 to a million stars.   Because of the extremely high densities near the centre of globular clusters (100-1000 stars per pc3 in the core compared to 0.14 stars per pc3 in the solar neighbourhood), dynamical interactions and even collisions between stars can occur. The densest objects in globular clusters – neutron stars and black holes – are expected to migrate towards the centre of the cluster as a result of mass segregation.  It was thought that interactions between these dense stellar remnants in the cores of globular clusters would lead to either their ejection for the cluster and/or collisions and mergers.  It has been suggested that globular cluster cores are the place to search for intermediate-mass black holes, objects a few 100s to a few 1000s of solar masses, though none have been confirmed in globular cluster to date.  These are thought to be like scaled-down supermassive black holes found in the centres of massive galaxies.

In this week’s edition of the journal Nature astronomers using the Very Large Array (VLA) have found two radio sources in the Milky Way globular cluster M22, which they argue are stellar-mass black holes (about 10-20 solar masses each) that are accreting matter. Strader et al. argue that finding two stellar-mass black holes in one globular cluster indicates that the dynamical ejection of black holes from cluster cores is not as efficient as previously thought. They suggest that there might be 5-100 stellar-mass black holes in M22.

(a) Ground-based image of globular cluster M22 (Credit: D. Matthews/A. Block/NOAO/AURA/NSF.) (b) and (c) Radio sources M22-VLA1 and M22-VLA2 from archival HST/Advanced Camera for Surveys F814W image. (Image from Strader et al. 2012.)

These are the first stellar-mass black holes to be found in any globular clusters in our Milky Way galaxy, and also are the first found by radio observations (rather than X-ray observations).  Future radio observations from arrays like the VLA and Australia Telescope Compact Array will shred light on the ultimate fate of stelar-mass black holes in globular clusters.

Swinburne PhD student Anna Sippel is delighted by the news. Anna is running the largest, most realistic N-body simulations of globular clusters and finds that of order 10 stellar-mass black holes can survive in a Hubble time. “These observations support my model predictions.”

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