Black holes galore in Andromeda galaxy

Black holes have been seen before in the Andromeda Galaxy, but astronomers have just announced the discovery of 26 more in our sister galaxy. Once further confirmations are complete, this will bring the total number of known black holes in Andromeda up to 35. This is the largest find of such bodies ever outside the Milky Way.

Black holes are tough to find, since they give off (almost) no light of their own. It is only radiation given off by objects falling into them which gives away their presence. The new black hole candidates were found in binary systems, where they orbit neutron stars or other black holes. The newest such objects found each measure between five and 10 times the mass of our Sun, which formed from the collapse of once-giant stars.

"While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg. Most black holes won't have close companions and will be invisible to us," Robin Barnard, an astronomer at the Harvard-Smithsonian Center for Astrophysics, said.

Seven of the 26 possible new black holes lie within 1,000 light years of the galactic center of Andromeda, otherwise known as M31. This matches predictions, which said that the massive central bulge of the galaxy would be surrounded by such objects.

"When it comes to finding black holes in the central region of a galaxy, it is indeed the case where bigger is better. In the case of Andromeda, we have a bigger bulge and a bigger super-massive black hole than in the Milky Way, so we expect more smaller black holes are made there as well," Stephen Murray, of Johns Hopkins University and the Center for Astrophysics, said.

Using more than 150 images taken in x-rays by the Chandra observatory over the course of 13 years, the researchers were able to determine that the objects being studied were not neutron stars. The team could tell this from the wavelengths and intensity of the x-ray signal.

Nor were the objects distant super-massive black holes lying at the center of other galaxies. This was confirmed from the brightness and variability of the x-rays coming form the sources, which vary at a much faster rate than super-massive black holes.

Eight of the new possible black holes were discovered in globular clusters, densely-packed, ancient collections of stars that orbit large galaxies.

Observations of the new black hole candidates was confirmed by the XMM-Newton X-ray Observatory, operated by the European Space Agency (ESA).

Results of the findings will be published in The Astrophysical Journal June 20.

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