"By observing in snapshots covering more than a dozen years, we are able to build up a uniquely useful view of M31," said co-author Michael Garcia, also of CfA. "The resulting very long exposure allows us to test if individual sources are black holes or neutron stars."
While neutron stars would have exhibited a certain level of X-ray emission and wavelength, these sources were more energetic and brighter, indicating that they are very likely stellar-mass black holes - the collapsed remains of stars five to ten times the mass of our sun.
Because the Andromeda galaxy's central bulge is bigger than the Milky Way's, more black holes can form there - a scenario that was expected by astronomers and now supported by these Chandra results.
ANALYSIS: Andromeda's Bling: Tiny Greedy Quasar Found
"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 and CfA. "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."
The team's results will be published in the June 20 issue of The Astrophysical Journal.
Read more on the Chandra press release here.
Image credit: X-ray (NASA/CXC/SAO/R.Barnard, Z.Lee et al.), Optical (NOAO/AURA/NSF/REU Prog./B.Schoening, V.Harvey; Descubre Fndn./CAHA/OAUV/DSA/V.Peris)