We analyze the ultraviolet (UV) emission properties of a sample of 13 quasars studied with the Hubble Space Telescope (HST). The quasars redshifts are in the range 0.165 ≤ z ≤ 2.06, and their absolute magnitudes are in the range -30.1 ≤ Mv ≤ -24.4. The data were obtained with the Faint Object Spectrograph and have a spectral resolution of ∼ 1300 and a signal-to-noise ratio S/N ∼ 20 per resolution element. We include in the analysis optical spectra covering Hβ and the narrow [O III] λλ4959, 5007 doublet for 7 of the 13 quasars. We find that the profile of the O VI + Lyβ blend can be reproduced well using a "symmetric Lyα" template. The remarkable similarity of the Lyα and O VI profiles rules out significant line emission from a high-velocity and high ionization level component inner to the broad line region (the very broad line region, VBLR), suggested in a number of earlier studies of active galactic nuclei. The Hβ line profile is in most cases different from the Lyα and C IV profiles, which indicates that Hβ and the bulk of the UV lines are not produced by the same distribution of clouds in the broad line region (BLR). The following previously unobserved emission features were found in our spectra: (1) λ ∼ 832 Å, most likely O II λ833 + O III λλ833, 835 emission. (2) λ ∼ 1070 Å, most likely an Fe II multiplet. (3) λ ∼ 1175 Å, probably due to C III*. Possible implications of these features are briefly mentioned. The narrow [O III]-like components in both Lyα and C III] are generally significantly weaker than expected based on the observed [O III] λ5007 flux. This result is consistent with the 1993 finding of Wills et al. and implies that the narrow line region (NLR) in quasars produces weaker UV/optical emission line ratios compared with the NLR in Seyfert 2 galaxies. A study of a combined sample of 18 quasars observed with HST (including Paper I) suggests that the equivalent widths of all lines, excluding N V, decreases with increasing continuum luminosity. The slopes of the EW versus luminosity relations for the different lines do not clearly favor explanations which invoke either a decrease in ionization parameter, or softening of the ionizing continuum, with increasing luminosity, as the single physical mechanism producing this effect. We verify with our larger sample the conclusions of Paper I concerning the presence of N III λ991, C III λ977, and Lyβ, the similarity of the C IV λ1549 and Lyα profiles, and the generally small velocity shifts of the broad line peaks relative to the peak of [O III] λ5007. In particular, we find a very small average shift for O VI (120 ± 270 km s-1 relative to Lyα), which does not support the suggestion made in earlier studies that the line peak velocity shift increases systematically with ionization level. The software developed for the line profile fitting and template deblending is available upon request.
All Science Journal Classification (ASJC) codes
- Astronomy and Astrophysics
- Space and Planetary Science