A group of scientists, led by Robin Barnard of the Harvard-Smithsonian Center for Astrophysics in Cambridge Mass., identified 26 new potential black holes in the globular clusters near the center of Andromeda.
"While we are excited to find so many black holes in Andromeda, we think it's just the tip of the iceberg," said
Barnard in a NASA
press release. "Most black holes won't have close companions and will be invisible to us."
Researchers conducted an extensive analysis of 13 years of data using over 150 Chandra observations. The 26 new candidates identified are in addition to nine previously identified black holes, bringing the total to 35.
"We are particularly excited to see so many black hole candidates this close to the center, because we expected to see them and have been searching for years," said Barnard.
The scientists needed to use such a large quantity of data to tell the difference between the smaller black holes at the center of Andromeda and larger supermassive black holes in distant galaxies.
These particular black holes, known as stellar-mass black holes, are the smallest known to exist. Amounting to little more than few dozen solar masses, which is a unit of mass in astronomy equal to the mass of the Sun, these black holes are the remnants of dead stars that collapsed under their own weight millions of years ago. In contrast, supermassive black holes may range from hundreds of thousands to millions of solar masses.
The black hole candidates are identified in the images below.
The first image shows an optical image of the Andromeda galaxy with an X-ray close-up of the center.
The second image notes the location of the black holes in that X-ray close-up.
Seven of the potential black holes can be found within 1,000 light-years of the center of the galaxy, more than the number of similar black holes thought to exist near the center of the Milky Way.
One indicator that led scientists to suspect that Andromeda contained so many black holes was the size of the bulge of stars in its middle. The more stars, the greater the likelihood of black hole formation.
"When it comes to finding black holes in the central region of a galaxy, it is indeed the case where bigger is better," said
co-author Stephen Murray of Johns Hopkins University. "In the case of Andromeda we have a bigger bulge and a bigger supermassive black hole than in the Milky Way, so we expect more smaller black holes are made there as well."
These results of Barnard’s and Murray’s study will be published in the June 20 issue of The Astrophysical Journal.
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