Understanding the AGN-Host Connection in Broad Mg II Emission-Selected AGN-Host Hybrid Quasars

We study the issue of active galactic nucleus (AGN)-host connection in intermediate-z (1.2>z > 0.4) galaxies with hybrid spectra (hybrid QSOs for short). The observed spectra redward of the Balmer limit are dominated by starlight, and the spectra at the blue end by both an AGN continuum and an Mg II broad emission line. This unique property allows us to examine both an AGN and its host galaxy in an individual galaxy simultaneously. First, 15 hybrid QSOs are selected from the Sloan Digital Sky Survey (SDSS) Data Release 6. The spectra are then analyzed in detail for three objects: SDSS J162446.49+461946.7, SDSS J102633.32+103443.8, and SDSS J090036.44+381353.0. Our spectral analysis shows that the current star formation activities are strongly suppressed, and that the latest burst ages range from ~400 Myr to 1 Gyr. Based on the Mg II black hole masses, the three hybrid QSOs are consistent with the D_n (4000) - L/L _Edd sequence that was previously established in local AGNs. The three hybrid QSOs are located in the middle range of the sequence, which implies that the hybrid QSOs are at the transition stage not only from young to old AGNs, but also from a host-dominated phase to an AGN-dominated phase.