Quasar spectrum classification with principal component analysis (PCA):: Emission lines in the Lyα forest

We investigate the variety in quasar UV spectra (lambda 1020-1600), with emphasis on the weak emission lines in the Ly alpha forest region, using principal component analysis (PCA). We use 50 smooth continuum-fitted quasar spectra (0.14 < z < 1.04) taken by the Hubble Space Telescope (HST) Faint Object Spectrograph. The first, second, and third principal component spectra (PCS) account for 63.4%, 14.5%, and 6.2% of the variance, respectively, and the first seven PCS account for 96.1% of the total variance. Three weak emission lines in the Ly alpha forest are identified as Fe II lambda 1070.95, Fe II + Fe III lambda 1123.17, and C III* lambda 1175.88. Using the first two standardized PCS coefficients, we introduce five classifications. By actively using PCS, we can generate artificial quasar spectra that are useful in testing the detection of quasars, DLAs, and the continuum calibration. We show that the power-law-extrapolated continuum is inadequate to perform precise measurements of the mean flux in the Ly alpha forest because of the weak emission lines and the extended tails of Ly alpha and Ly beta/O VI emission lines. We show that we miss 5.3% of the flux on average, and there are cases where we would miss 14% of the flux. These corrections are essential in the study of the intergalactic medium at high redshift in order to achieve precise measurements of physical properties, cosmological parameters, and the reionization epoch.