From the July 2013 issue

Is there a correlation between the mass of a supermassive black hole and the amount of stars within its galaxy? If so, what is it?

David Irvin, Calabasas, California
By | Published: July 22, 2013 | Last updated on May 18, 2023
Mass relationships
The mass of a galaxy’s central supermassive black hole appears to relate to the amount of stars within that galaxy’s bulge component. This plot compares a black hole’s mass (Y-axis) to the random motions of stars due to the gravitational pull of the matter in the host galaxy’s bulge (X-axis). // Astronomy: Roen Kelly, after Kayhan Gultekin, et al.
Yes, there is a correlation between a supermassive black hole’s mass and the stellar content of its host galaxy’s bulge. In some galaxies (like spirals), the bulge is smaller, round, and confined to the center, while giant elliptical galaxies are essentially all bulge.

We have several ways to measure the amount of stars in the bulges of galaxies. One is simply by measuring the light from those stars. Another is by determining the mass of the bulge (because it is mostly stars). In a third method, we measure the speed of the random motions of stars due to the bulge’s gravitational attraction — we call this “velocity dispersion.”

No matter which method we use, we find that the more stars in a galaxy, the larger the mass of its black hole. However, the third measurement, velocity dispersion, appears to have the tightest correlation.
We’re not sure why this correlation between black hole mass and host galaxy properties exists. One of the most compelling explanations — but by no means certain — is “quasar feedback.” In this model, when galaxies merge, their bulges also merge, and the new, larger central black hole swallows a lot of gas; as the material funnels toward the black hole, friction and radiative processes cause it to glow as a bright “quasar.” The quasar shines until its powerful winds and jets heat up and/or expel all the gas in the galaxy so that it can’t make any new stars or feed the black hole. Then, the black hole stops growing.

The correlation between black hole mass and amount of stars is not perfect. The smallest galaxies, in particular, appear to have much smaller black holes than we expect based on the amount of stars in their bulges. This could be because they never went through a quasar phase.

Kayhan Gültekin
University of Michigan, Ann Arbor