Supermassive Black Holes and Their Relationships with Their Host Galaxies

We review how the masses of black holes in active galactic nuclei are measured and outline the current limitations and uncertainties. Masses have been measured directly by emission-line reverberation for nearly 50 relatively nearby AGNs, but uncertainties due to the unknown geometry and projection effects limit the accuracy of these masses to similar to 0.3 dex. Reverberation studies show that there is a very tight relationship between the broad-line region radius and the AGN luminosity, with an intrinsic scatter of similar to 0.1 dex, which shows (1) that the largest source of systematic uncertainty in the black hole mass determinations is how the velocity field of the broad-line region is characterized, not the size of the broad-line region, and (2) that the size of the broad-line region can be estimated to fairly high accuracy from the AGN luminosity alone, thus providing a powerful indirect method of estimating black hole masses in even distant AGNs.