Understanding the AGN-Host Connection in Partially Obscured Active Galactic Nuclei. I. The Nature of AGN+H II Composites

The goal of our serial papers is to examine the evolutionary connection between an active galactic nucleus (AGN) and star formation in the AGN's host galaxy for partially obscured AGNs (i.e., Seyfert 1.8 and 1.9 galaxies). Taking advantage of these galaxies, the properties of both components can be studied together by direct measurements. In this paper, we focus on broad-line composite galaxies (composite AGNs), which are located between the theoretical and empirical separation lines in the [N II]/Hα versus [O III]/Hβ diagram. These galaxies are selected from the composite galaxies provided by the SDSS DR4 MPA/JHU catalogs. After we reanalyze the spectra, we perform a fine classification for the 85 composite AGNs in terms of Baldwin-Phillips-Terlevich (BPT) diagrams. All objects located below the three theoretical separation lines are associated with a young stellar population (<1 Gyr), while either a young or old stellar population is identified in the individual multiply classified objects. The multiply classified objects with very old stellar populations are located in the LINER region in the [O I]/Hα versus [O III]/Hβ diagram. We then consider the connection between AGNs and star formation to derive the key results. The Eddington ratio inferred from the broad Hα emission, the age of the stellar population of the AGN's host as assessed by D_n(4000) , and the line ratio [O I]/Hα are found to be related to each other. These relations strongly support an evolutionary scenario in which AGNs evolve from a high L/L_Edd state with a soft spectrum to a low L/L_Edd state with a hard spectrum as the young stellar population ages and fades. The significant correlation between the line ratio [O I]/Hα and D_n(4000) leads us to suggest that the line ratio could be used to trace the age of the stellar population in type I AGNs.