Supermassive Black Holes at the Centers of Galaxies

For over three decades, the giant elliptical galaxy Messier 87 in the Virgo Cluster has hosted the most massive known black hole in the local universe.  New observational data and improved stellar orbit models in the past several years have substantially expanded and revised dynamical measurements of black hole masses at the centers of nearby galaxies.  I will describe recent progress in discovering black holes up to twenty billion solar masses in ongoing surveys of massive elliptical galaxies.  I will discuss the implications of this new population of ultra-massive black holes, including its impact on our understanding of the symbiotic relationships between black holes and galaxies, and on the gravitational waves signals from merging supermassive black hole binaries targeted by ongoing pulsar timing array experiments.