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Second Black hole at the Heart of the Milky Way.


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Milky Way galaxy.
Milky Way galaxy.

A French/US team of Astronomers have discovered a second black hole is lurking at the heart of our Milky Way galaxy, completely separate from the supermassive black hole that we've known about for years. This new object, IRS 13E, contains only 1,400 stellar masses, which is much less that the 4 million stellar masses of our supermassive black hole. IRS 13E probably used to be located far away from the galactic centre, where a cluster of stars could safely form. All that's left now are a few massive stars whipping around the black hole as it spirals towards the centre of our galaxy.

Using archived science verification data from the Hokupa’a/QUIRC Adaptive Optics system on Gemini North, a French/US team of Astronomers led by Jean-Pierre Maillard of the Institut d’Astrophysique de Paris has confirmed the physical association of a cluster of massive stars in the Infrared source IRS 13 near the center of the Milky Way galaxy.

The team also used data from Hubble Space Telescope, the Chandra X-Ray Observatory, the Canada-France-Hawai’i telescope (CFHT), and the Very Large Array to provide broad spectral coverage to complement the Gemini data. The Gemini observations consisted of deconvolved H and Kp band images that identified the existence of two formerly undetected sources within IRS 13E. In all, seven individual massive stars appear to be associated with what the team believes was once a larger cluster of massive stars held together by a central intermediate-mass black hole of about 1,300 solar masses. (This black hole is distinct from the black hole at the galactic center which has a mass of about four million solar masses.) The seven individual stars of IRS 13E seen within a diameter of about 0.5" (or projected 0.6 light-year across) are co-moving westward with a similar velocity of about 280 kilometers per second in the plane of the sky.

The compactness of the cluster and the common proper motion of the components suggest that they are kept together by a massive source, a stellar black hole at the center of IRS 13E. The size of the cluster allow to infer a mean orbit radius. The radial velocities (+/- 30 kilometers per second) of the individual stars derived from the BEAR Fourier Transform Spectrometer (CFHT) measurements can be used to estimate the average orbital velocity. The authors then explored a range of orbital assumptions and were able to constraint the mass of the holding black hole to about 1,300 solar masses rather robustly.

The team also speculates that this cluster was once located farther from the galactic center, where the stars could form away from the extreme gravitational influence of the central supermassive black hole. IRS 13E seems to be the wreckage or remnant core of a once larger cluster of stars that is now spiraling towards Sgr A* at the galactic center.

This theory also explains the existence of other massive stars around the galactic center, which are thought to be stars stripped from the cluster due to the gravitational environment around the galaxy’s central black hole.

The Gemini data for this work were obtained by a team led by Francois Rigaut (Gemini Observatory) as part of an adaptive optics demonstration run in July 2000. The results are published in Astronomy and Astrophysics, Volume 423, pgs 155-167 (2004)




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