Separating Accretion and Mergers in the Cosmic Growth of Black Holes
  with X-ray and Gravitational Wave Observations
  Black holes across a broad range of masses play a key role in the evolution
of galaxies. The initial seeds of black holes formed at $z \sim 30$ and grew
over cosmic time by gas accretion and mergers. Using observational data for
quasars and theoretical models for the hierarchical assembly of dark matter
halos, we study the relative importance of gas accretion and mergers for black
hole growth, as a function of redshift ($0<z<10$) and black hole mass ($ 10^3
\, \mathrm{M_{\odot}} < M_{\bullet} < 10^{10} \, \mathrm{M_{\odot}}$). We find
that (i) growth by accretion is dominant in a large fraction of the parameter
space, especially at $M_{\bullet} > 10^8 \, \mathrm{M_{\odot}}$ and $z>6$; and
(ii) growth by mergers is dominant at $M_{\bullet} < 10^5 \,
\mathrm{M_{\odot}}$ and $z>5.5$, and at $M_{\bullet} > 10^8 \,
\mathrm{M_{\odot}}$ and $z<2$. As the growth channel has direct implications
for the black hole spin (with gas accretion leading to higher spin values), we
test our model against $\sim 20$ robust spin measurements available thus far.
As expected, the spin tends to decline toward the merger-dominated regime,
thereby supporting our model. The next generation of X-ray and
gravitational-wave observatories (e.g. Lynx, AXIS, Athena and LISA) will map
out populations of black holes up to very high redshift ($z\sim 20)$, covering
the parameter space investigated here in almost its entirety. Their data will
be instrumental to providing a clear picture of how black holes grew across
cosmic time.
