Understanding AGN-Host Connection in Partially Obscured Active Galactic Nuclei. III. Properties of ROSAT-Selected SDSS Active Galactic Nuclei

As the third paper of our series of studies that aim at examining the AGN-host co-evolution by using partially obscured active galactic nuclei (AGNs), we extend the broad-line composite galaxies (composite AGNs) into ROSAT-selected Seyfert 1.8/1.9 galaxies based upon the ROSAT All Sky Survey/Sloan Digital Sky Survey Data Release 5 (SDSS-DR5) catalog given by Anderson et al. The SDSS spectra of a total of 92 objects are analyzed by the same method used in our previous studies, after requiring the signal-to-noise ratio in the SDSS r' band to be larger than 20. Combining the ROSAT-selected Seyfert galaxies with the composite AGNs reinforces the tight correlation between the line ratio [O I]/Hα versus D_n (4000), and establishes a new tight correlation between [S II]/Hα versus D_n (4000). Both correlations suggest that the two line ratios are plausible age indicators of the circumnuclear stellar population for typical Type I AGNs in which the stellar populations are difficult to derive from their optical spectra. The ROSAT-selected Seyfert galaxies show that the two correlations depend on the soft X-ray spectral slope α _X , which is roughly estimated from the hardness ratios by requiring that the X-ray count rates within 0.1-2.4 keV be larger than 0.02 counts s^-1. However, we fail to establish a relationship between α _X and D_n (4000), which is likely caused by the relatively large uncertainties of both the parameters (especially for α _X because of the AGN intrinsic obscuration). The previously established L/L _Edd-D_n (4000) evolutionary sequence is reinforced again by the extension to the ROSAT-selected Seyfert galaxies. These X-ray-selected Seyfert galaxies are, however, biased against the two ends of the sequence, which implies that the X-ray Seyfert galaxies present a population at a middle evolutionary stage.