FeII emission in 14 low-redshift quasars.: I.: Observations

We present the spectra of 14 quasars with a wide coverage of rest wavelengths from 1000 to 7300 angstrom. The redshift ranges from z = 0.061 to 0.555 and the luminosity from M-B = - 22.69 to - 26.32. These spectra of high quality result from combining Hubble Space Telescope spectra with those taken from ground-based telescopes. We describe the procedure of generating the template spectrum of Fe II line emission from the spectrum of a narrow-line Seyfert 1 galaxy, I Zw 1, that covers two wavelength regions of 2200 - 3500 and 4200 - 5600 angstrom. Our template Fe II spectrum is semiempirical in the sense that the synthetic spectrum calculated with the CLOUDY photoionization code is used to separate the Fe II emission from the Mg II lambda 2798 line. The procedure of measuring the strengths of Fe II emission lines is twofold: ( 1) subtracting the continuum components by fitting models of the power-law and Balmer continua in the continuum windows, which are relatively free from line emissions, and ( 2) fitting models of the Fe II emission based on the Fe II template to the continuum-subtracted spectra. From 14 quasars including I Zw 1, we obtained the Fe II fluxes in five wavelength bands ( U1 [ 2200 - 2660 angstrom], U2 [ 2660 - 3000 angstrom], U3 [ 3000 - 3500 angstrom], O1 [ 4400 - 4700 angstrom], and O2 [ 5100 - 5600 8]), the total flux of Balmer continuum, and the fluxes of Mg II lambda 2798, H alpha, and other emission lines, together with the full widths at half-maximum ( FWHMs) of these lines. Regression analysis was performed by assuming a linear relation between any two of these quantities. Eight correlations were found with a confidence level higher than 99%: ( 1) larger Mg II FWHM for larger H alpha FWHM, ( 2) larger Gamma for fainter M-B, ( 3) smaller Mg II FWHM for larger Gamma, ( 4) larger Mg II FWHM for smaller Fe II( O1)/ Mg II, ( 5) larger M-BH for smaller Gamma, ( 6) larger MBH for smaller Fe II( O1)/ Mg II, ( 7) larger [ O III] / H beta for larger Mg II FWHM, and ( 8) larger Fe II( O1)/ Mg II for larger Fe II( O1)/ Fe II( U1). The fact that six of these eight are related to FWHM or MBH (proportional to FWHM2) may imply that MBH is a fundamental quantity that controls Gamma or the spectral energy distribution ( SED) of the incident continuum, which in turn controls the Fe II emission. Furthermore, it is worthy of noting that Fe II( O1)/ Fe II( U1) is found to tightly correlate with Fe II( O1)/ Mg II, but not with Fe II( U1)/ Mg II.