Two-Component Structure of the Hbeta Broad-Line Region in Quasars. I.
  Evidence from Spectral Principal Component Analysis
  We report on a spectral principal component analysis (SPCA) of a sample of
816 quasars, selected to have small Fe II velocity shifts with spectral
coverage in the rest wavelength range 3500--5500 \AA. The sample is explicitly
designed to mitigate spurious effects on SPCA induced by Fe II velocity shifts.
We improve the algorithm of SPCA in the literature and introduce a new
quantity, \emph{the fractional-contribution spectrum}, that effectively
identifies the emission features encoded in each eigenspectrum. The first
eigenspectrum clearly records the power-law continuum and very broad Balmer
emission lines. Narrow emission lines dominate the second eigenspectrum. The
third eigenspectrum represents the Fe II emission and a component of the Balmer
lines with kinematically similar intermediate velocity widths. Correlations
between the weights of the eigenspectra and parametric measurements of line
strength and continuum slope confirm the above interpretation for the
eigenspectra. Monte Carlo simulations demonstrate the validity of our method to
recognize cross talk in SPCA and firmly rule out a single-component model for
broad Hbeta. We also present the results of SPCA for four other samples that
contain quasars in bins of larger Fe II velocity shift; similar eigenspectra
are obtained. We propose that the Hbeta-emitting region has two kinematically
distinct components: one with very large velocities whose strength correlates
with the continuum shape, and another with more modest, intermediate velocities
that is closely coupled to the gas that gives rise to Fe II emission.
