A DEEP HUBBLE SPACE TELESCOPE H-BAND IMAGING SURVEY OF MASSIVE GAS-RICH MERGERS. II. THE QUEST QSOs

We report the results from a deep Hubble Space Telescope (HST) NICMOS H-band imaging survey of 28 z < 0.3 QSOs from the Palomar-Green (PG) sample. This program is part of QUEST (Quasar/ULIRG Evolution Study) and complements a similar set of data on 26 highly nucleated ULIRGs presented in Paper I. Our analysis indicates that the fraction of QSOs with elliptical hosts is higher among QSOs with undetected far-infrared (FIR) emission, small infrared excess (LIR/LB < 10), and luminous hosts. The hosts of FIR-faint QSOs show a tendency to have less pronounced merger-induced morphological anomalies and larger QSO-to-host luminosity ratios on average than the hosts of FIR-bright QSOs, consistent with late-merger evolution from FIR-bright to FIR-faint QSOs. The spheroid sizes (∼ 0.3–5.5 kpc) and total host luminosities (∼ 0.6–7.2 L*H) of the radio-quiet PG QSOs in our sample are statistically indistinguishable from the ULIRG hosts presented in Paper I, while those of radio-loud PG QSOs are systematically larger and more luminous. ULIRGs and PG QSOs with elliptical hosts fall near, but not exactly on, the fundamental plane of inactive spheroids. We confirm the systematic trend noted in Paper I for objects with small (≲ 2 kpc) spheroids to be up to ∼ 1 mag brighter than inactive spheroids. The host colors and wavelength dependence of their sizes support the idea that these deviations are at least in part due to non-nuclear star formation. However, the amplitudes of these deviations depend mainly on host sizes, and possibly on infrared excess, but not on merger phase, QSO-to-host luminosity ratio, optical spectral type, active galactic nucleus fractional contribution to the bolometric luminosity, or host R − H color. Taken at face value (i.e., no correction for extinction or the presence of a young stellar population), the H-band spheroid-host luminosities imply black hole masses ∼ (5–200) × 107 M☉ and sub-Eddington mass accretion rates for both QSOs and ULIRGs. These results are compared with published black hole mass estimates derived from other methods.