An international group of astronomers using NASA’s Chandra X-ray Observatory has reported the detection of 26 black hole candidates in Messier 31, also known as the Andromeda Galaxy.
“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,” said Dr Robin Barnard of Harvard-Smithsonian Center for Astrophysics, lead author of a paper accepted for publication in the Astrophysical Journal (arXiv.org version).
The black hole candidates belong to the stellar mass category, meaning they formed in the death throes of very massive stars and typically have masses five to 10 times that of our Sun. Astronomers can detect these otherwise invisible objects as material is pulled from a companion star and heated up to produce radiation before it disappears into the black hole.
To classify those objects as black holes, the team observed that these X-ray sources had special characteristics: that is, they were brighter than a certain high level of X-rays and also had a particular X-ray color. Sources containing neutron stars, the dense cores of dead stars that would be the alternate explanation for these observations, do not show both of these features simultaneously. But sources containing black holes do.
“By observing in snapshots covering more than a dozen years, we are able to build up a uniquely useful view of M31. The resulting very long exposure allows us to test if individual sources are black holes or neutron stars,” said study co-author Dr Michael Garcia, also from Harvard-Smithsonian Center for Astrophysics.
The research group previously identified nine black hole candidates within the region covered by the Chandra data, and the present results increase the total to 35. Seven of them are within 1,000 light-years of the Andromeda Galaxy’s center. That is more than the number of black hole candidates with similar properties located near the center of our own galaxy. This is not a surprise to astronomers because the bulge of stars in the middle of Andromeda is bigger, allowing more black holes to form.
“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 supermassive black hole than in the Milky Way, so we expect more smaller black holes are made there as well,” said study senior author Dr Stephen Murray of Johns Hopkins University.
Bibliographic information: R. Barnard et al. 2013. Chandra identification of 26 new black hole candidates in the central region of M31. ApJ, accepted for publication; arXiv: 1304.7780black holes, andromeda galaxy, black hole <BR/>