Host Dynamics and Origin of Palomar‐Green QSOs

Abstract

We present host galaxy velocity dispersions of 12 local (mainly Palomar-Green) QSOs measured directly from the stellar CO absorption features in the H band. The mean bulge dispersion of the QSOs in our sample is 186 km s-1 with a standard deviation of 24 km s-1. The measurement of the stellar dispersion in QSOs enables us to place them on observational diagrams such as the local black hole mass-bulge velocity dispersion relation and the fundamental plane of early-type galaxies. Concerning the former relation, these QSOs have higher black hole masses than most Seyfert 1 AGNs with similar velocity dispersions. On the fundamental plane, PG QSOs are located between the regions occupied by moderate-mass and giant ellipticals. The QSO bulge and black hole masses, computed from the stellar velocity dispersions, are of order 1011 and 108 M☉, respectively. The Eddington efficiency of their black holes is on average 0.25, assuming that all of the bolometric luminosity originates from the active nucleus. Our data are consistent with other lines of evidence that Palomar-Green QSOs are related to galaxy mergers with gas-rich components and that they are formed in a manner similar to the most massive ultraluminous infrared galaxies, regardless of their far-infrared emission. However, PG QSOs seem to have smaller host dispersions and different formation mechanisms than QSOs with supermassive black holes of 5 × 108-109 M☉ that accrete at low rates and reside in massive spheroids.