* Astronomy

The sticky star cluster that's mostly black hole

At the centre of the Milky Way, a cluster of three stars perches precariously on the edge of oblivion, a mere stone's throw in cosmic terms from the supermassive black hole at the galaxy's core. Like a delicate spiderweb that somehow survives a hurricane, the group's continued existence seems impossible in the face of the black hole's extreme gravity, which should rip the cluster apart.
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Title: A Black Hole in the Galactic Centre Complex IRS 13E?
Authors: R. Schödel, A. Eckart, C. Iserlohe, R. Genzel and T. Ott

The IRS 13E complex is an unusual concentration of massive, early-type stars at a projected distance of ~0.13 pc from the Milky Way's central supermassive black hole Sagittarius A* (Sgr A*). Because of their similar proper motion and their common nature as massive, young stars, it has recently been suggested that IRS 13E may be the remnant of a massive stellar cluster containing an intermediate-mass black hole (IMBH) that binds its members gravitationally in the tidal field of Sgr A*. Here, we present an analysis of the proper motions in the IRS 13E environment that combines the currently best available data with a time line of 10 years. We find that an IMBH in IRS 13E must have a minimum mass of ~104 Msun in order to bind the source complex gravitationally. This high-mass limit in combination with the absence so far of compelling evidence for a nonthermal radio and X-ray source in IRS 13E make it appear unlikely that an IMBH exists in IRS 13E that is sufficiently massive to bind the system gravitationally.

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