A Sample of IRAS Infrared-selected Seyfert 1.5 Galaxies: Infrared Color α(60, 25)-dominated Eigenvector 1

The well-documented E1 relationships are first extended to infrared color α(60, 25) and flux ratio [O III]/Hβn by comparing emission-line properties to continuum properties in infrared wavelengths. Both direct correlations and a principal component analysis are used in a sample of 50 IRAS IR-selected Seyfert 1.5 galaxies. In addition, to confirm the correlations of E1 in Boroson & Green, our eigenvector 1 turns out to be dominated by the mid-infrared color α(60, 25) and most strongly affected by RFe, [O III]/Hβn, and EW(Hβb). Our analysis indicates that the objects with large E1 tend to coexist with relatively young nuclear stellar populations, which implies that E1 is related to the nuclear star formation history. The IR-dominated eigenvector 1 can therefore be inferred to be interpreted as the "age" of an AGN. In confirmation of the work of Xu and coworkers, it is clear that the extreme Seyfert galaxies with both large RFe and large [O III]/Hβn are rare in our universe.