Spectral principal component analysis of the Hβ region of low-redshift SDSS quasars

Studying the relationships among quasar spectral features is essential to unveil the origins of the emission lines and the quasars’ physical processes. Principal component analysis (PCA) is a powerful tool to investigate correlations between variables. Here, we present the results of PCA on the spectra of low-redshift SDSS quasars. The rest-frame wavelength range studied is 4000 – 5500 Å, involving some typical features of quasar spectra, such as Hβ, [O iii] and Fe ii emission lines. The first principal component is the anti-correlation between [O iii] and Fe ii, the well-known eigenvector one (EV1). The next six principal components also show clear (anti-)correlations between line strengths and/or velocity widths of various features, which agree well with measured spectral properties. By comparing the weights of these principal components with other quasar properties, we can identify their underlying drivers. We find that the second principal component represents spectral slope, and can quantify quasar host fraction, intrinsic slope and reddening well. The third component exhibits the velocity width variation of Hβ, and may be a proxy for orientation. In addition, we calculate the fractional-contribution spectra to investigate which components dominate the variance at individual wavelength ranges. Our results also indicate that the optical Fe ii emission may have distinct origins.