The phrase "galaxy masses" refers to the amount of matter contained within a galaxy. It represents how much stuff, like stars, gas, and dust, exists in a particular galaxy. Full definition
The mass of each black hole is roughly proportional to the mass of the galaxy itself, and makes up approximately half a percent of the total galaxy mass. (stsci.edu)
So the observatories can't spend all of their time estimating galaxy mass. (space.com)
This provides an independent test for astronomers» usual methods of estimating distant galaxy masses — which rely on extrapolation from their nearby cousins. (sciencedaily.com)