Black Holes and Stellar Observations
Reed College astrophysicists devise new way to calculate the mass of black holes in distant galaxies.
Deep in the heart of the Milky Way, at the convergence of its spiral arms, lurks a dark colossus: a black hole with a mass of more than 4 million suns.
This pattern—a massive black hole surrounded by a galactic whorl of stars—is repeated throughout the cosmos. Astronomers now believe that the mass of a black hole provides vital clues about the birth, lifespan, and fate of its surrounding galaxy, but obtaining accurate measurements for distant black holes is exceedingly difficult.
One powerful technique is to calculate stellar velocity dispersion, a statistical measure of the speed and directions in which stars are moving around their galaxy; the higher the stellar dispersion, the more massive the black hole at the center.
Unfortunately, this approach gets complicated when we look at highly elliptical galaxies, because our measurements are contaminated by the rotation of the galactic disk. (Since we are observing these galaxies from an angle, rather than straight on, we can’t always tell which stars are in the “front” and which ones are at the “back.”)
Prof. Alison Crocker and physics major Max Piper ’21 created an ingenious way to correct errors in dispersion observations, inspired by the senior thesis of Farhanul Hasan ’18 from a couple years back. They applied a mathematical formula based on the ellipticity of the observed galaxy to the calculation of stellar dispersion.
This error-correction should yield more accurate readings for stellar dispersion and more information about the black holes that are deeply entwined with the fate of the galaxies that revolve around them.