re: Milky Way Houses Up To 100 Million Black Holes

How they determined:


We present an empirical approach for interpreting gravitational wave signals of binary black hole mergers under the assumption that the underlying black hole population is sourced by remnants of stellar evolution. Using the observed relationship between galaxy mass and stellar metallicity, we predict the black hole count as a function of galaxy stellar mass. We show, for example, that a galaxy like the Milky Way should host millions of ~30~M? black holes and dwarf satellite galaxies like Draco should host ~100 such remnants


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From article:

It stands to reason, then, if we can figure out how galaxies formed, grew, and created stars over their history, we can run simulations that can tell us approximately how many black holes should exist in a galaxy of any size and merger history. That's precisely what the work of Oliver D. Elbert, James S. Bullock, and Manoj Kaplinghat has recently attempted to do. What they found is that there are three questions you need to know the answer to in order to come up with an estimate for black holes:

What is the total mass of the galaxy?

What is the total mass-in-stars of the galaxy?

And what is the metallicity of the galaxy? (i.e., what percent of the galaxy's mass is elements heavier than hydrogen and helium?)

If you can observe and/or reconstruct these three properties, you can pick out not only how many black holes are inside, but what the typical masses of those black holes are.