Big Galaxies and Big Black Holes and Nanohertz Gravitational Waves and Even Bigger Black Holes

In recent years pulsar timing arrays (PTAs) have produced compelling evidence of a nanohertz scale gravitational wave background (GWB). The most plausible origin of this background is a population of mergers of supermassive black holes (SMBHs).  The GWB amplitude measured by these PTA experiments is strongly related to the SMBH population. Here we use precision measurements of the luminosities, stellar populations, and dynamical masses of a collection of the most massive local galaxies to refine the local galaxy stellar mass function (GSMF) and the local black hole mass function. We find that prior GSMF measurements systematically under-estimate the mass of the largest galaxies and that when using our new GSMF we find a higher characteristic amplitude for the GWB which is consistent with PTA measurements. We will then turn to discussion precision stellar dynamical measurements and modelling of nearby massive galaxies using triaxial Schwarzschild modeling, including substantially refined measurements of two of the largest local ultramassive black holes.