Evolution of SMBHs in light of PTA measurements: implications for growth by mergers and accretion

Understand

We study the growth of supermassive black holes accounting for both accretion and mergers.

• The former is informed by observations of the quasar luminosity function (QLF) and the latter by the gravitational wave-background (GWB) recently detected by PTAs, while estimates of the present-day black hole mass function provide a boundary condition.
• The GWB is dominated by the most massive black holes ($\gtrsim10^{9}M_{\odot}$).
• We show that their evolution can be simplified into a two-step process: mergers dominate at $z\leq1$, while accretion peaks at $1.4\leq z\leq2$.