Evolutionary implications from SDSS J085338.27+033246.1: A spectacular narrow-line seyfert 1 galaxy with young poststarburst

We analyze the physical properties of the poststarburst active galactic nucleus (AGN) SDSS J085338.27+033246.1 according to its optical spectrum and discuss its implications on AGNs' evolution. The spectra principal component analysis (PCA) method is developed to extract emission lines and absorption features from the total light spectrum. The emission-line analysis indicates that the object can be classified as a narrow-line Seyfert 1 galaxy with Fe II/H beta(B) 2:4 +/- 0:2, large Eddington ratio (similar to 0.34), small black hole mass (similar to 1:1; 107 M-circle dot), and intermediately strong radio emission. A simple single stellar population model indicates that the absorption features are rather well reproduced by a similar to 100 Myr old starburst with amass of similar to 7;109 M-circle dot. However, the current star formation rate, similar to 3:0 M-circle dot yr(-1), inferred from the [O II] emission is much smaller than the past average star formation rate, similar to 70 M-circle dot yr(-1). The line ratio diagnosis using Baldwin-Phillips-Terlevich (BPT) diagrams indicates that the narrow emission lines are almost entirely emitted from H II regions. We further discuss a possible evolutionary path that links AGN and starburst phenomena.