two separate black hole-neutron star mergers are confirmed for the
first time. Have LIGO and VIRGO yet enabled any statistical estimates
on the general population numbers of black holes per galaxy, say, in
the universe, and maybe their mass distributions?
[[Mod. note -- Yes.
Another excellent article on this past week's announcement
(of the detection of two BH-NS mergers by Ligo/Virgo) is
This includes a link to the full research paper (which is open-access).
It also has a cute subtitle. :)
To return to the author's question, there has been a lot of work done
on trying to estimate the population distribution of black holes. It's
tricky both because of the small number of observed coalescences, and
because of the strong selection biases in the observations (lower-mass
black holes produce weaker gravitational waves, and so are less likely
to be observed).
For example, GW 190814 was the coalescence of a 23 Msun black hole and
a 2.6 Msun "compact object" (maybe a small black hole, or maybe a neutron
star near the upper limit of possible NS masses). In Astrophysical Journal
896, L44 (https://doi.org/10.3847/2041-8213/ab960f
), the authors estimated
the density of "GW190814-like" systems as 7 +16/-6 (i.e., between 1 and 23)
mergers/year per cubic gigaparsec. For systems with more comparable masses,
arXiv:1811.12940 estimated a density of 53 +56/-27 (i.e., between 26 and 109)
mergers/year per cubic gigaparsec. (There might be a more up-to-date
estimate published that I haven't seen.)
We don't yet have any direct gravitational-wave observations of
supermassive black holes (e.g., in galaxy or quasar centers), although
there are strong astronomical reasons to suspect that most galaxies
contain supermassive black holes). The European Space Agency's planned
LISA mission (currently scheduled for 2034) should directly observe
gravitational waves from these systems (among others), which will give
us direct information about their properties and how common they are.