The black holes we’ve observed in the universe typically fall into two categories: small star-sized black holes, and gargantuan black holes lurking at the centers of galaxies. Now, a new black-hole discovery sheds some light on the gray area between these extremes.
Illustrations of two types of accreting black holes: a stellar-mass black hole accreting from a binary companion (top) and a supermassive black hole accreting gas in a galaxy’s center (bottom). [Top: ESA/NASA/Felix Mirabel; Bottom: ESO/M. Kornmesser]Stellar-mass black holes of up to 100 solar masses are scattered by the millions throughout galaxies. At the opposite end of the spectrum, most galaxies are thought to contain just one massive black hole: a black hole of millions to tens of billions of solar masses that lies in the galaxy’s core.
Intriguingly, the mass of these central black holes seems to be inherently tied to that of their host. An empirical relationship known as the M-σ relation shows a correlation between a central black hole’s mass and the spread of star velocities in its host galaxy’s bulge, which acts as a proxy for the bulge mass. The M-σ relation and other, similar relationships show that black holes seem to grow ...