“While we are excited to find so many black holes in Andromeda, we think it’s just the tip of the iceberg,” said Robin Barnard of Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Mass., and lead author of a new paper describing these results. “Most black holes won’t have close companions and will be invisible to us.”
Stellar mass black holes are what remain after sufficiently massive stars die in a supernova explosions, and are usually a few times more massive than our own Sun. These are in contrast to the supermassive black holes (SMBH) that have been found at the centre of most galaxies, being billions of times more massive than stellar mass black holes.
Previous studies of M31 had revealed a number of X-ray sources, which had been identified as possible black holes. Black holes do not emit any radiation directly, but in-falling material gets whipped into a highly energetic accretion disk, which emits high energy radiation which can be seen by X-ray telescopes like Chandra.
The astronomers studying these emissions wanted to confirm earlier predictions that they could be stellar-mass black holes, but first had to eliminate alternate explanations. Neutron stars are also capable of emitting X-rays, since they have gravitational fields almost as intense as those around black holes and can also have hot accretion disks. However, the X-rays emitted by neutron stars have different characteristics: They are less bright, and shine in different “colours”, or wavelengths of the X-ray spectrum.
The other explanation, that the observed sources could be SMBH’s of far more distant galaxies which happen to be in the same line of sight as the Chandra observations, was eliminated by noting the speed at which the X-ray emissions fluctuated. SMBH’s are vast creatures, with accretion disks bigger than our entire Solar System, which means that they are slow to change — nothing can move faster than the speed of light, which means that any change to the structure of the disk must propagate at a speed slower than light, which in turn means that there is a minimum time for changes to occur. Since the observations showed changes faster than that, the astronomers working with the data could prove that they were not looking at SMBH’s.
The black holes that were discovered are concentrated towards the central bulge of M31, which did not come as a surprise: This is the densest region of spiral galaxies, where a great deal of dust and gas formed very many stars early in the galaxy’s history, allowing plenty opportunity for 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,” 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 results of this research were published in the June 20 issue of The Astrophysical Journal (paper at arXiv:1304.7780v1 [astro-ph.HE]).