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Understanding the AGN-Host Connection in Broad Mgii Emission Selected AGN-Host Hybrid Qausars

We study the issue of 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 blue end by both an AGN continuum and a MgII broad emission line. Such unique property allows us to examine both AGN and its host galaxy in individual galaxy simultaneously. At first, 15 hybrid QSOs are selected from the Sloan Digital Sky Survey Data Release 6. The spectra are then analyzed in detail in three objects: SDSS J162446.49+461946.7, SDSS J102633.32+103443.8 and SDSS J090036.44+381353.0. Our spectral analyzing shows that the current star formation activities are strongly suppressed, and that the latest burst ages range from ~400Myr to 1Gyr. Basing upon the MgII-based 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 AGN, but also from host-dominated phase to AGN-dominated phase.