Spectral energy distributions of the brightest palomar-green quasars at intermediate redshifts

Todd M. Tripp, Jill Bechtold, Richard F. Green

Research output: Contribution to journalArticle

27 Scopus citations


We have combined low-dispersion International Ultraviolet Explorer (IUE) spectra with the optical/near-IR spectrophotometry of Neugebauer et al. (1987) in order to study the spectral energy distributions of seven of the brightest Palomar-Green quasars at intermediate redshifts (0.9 ≤ zem ≤ 1.5). Some of these PG quasars are barely detectable in long IUE exposures, so we have used the Gaussian Extraction (GEX) technique to maximize the signal-to-noise of the IUE data, and we have co-added all spectra available from the IUE archive for each QSO unless the ultraviolet spectra varied significantly from one exposure to the next. We have corrected the spectral energy distributions for Milky Way reddening using the observed neutral hydrogen column densities on each sight line and the gas-to-dust relation recently derived by Diplas & Savage. Six of the seven quasars are detected down to λ ≪ 700 Å in the rest frame, and consequently continuum reddening due to dust in the immediate vicinity of the quasar can have a dramatic effect on the spectral energy distributions, even if the intrinsic E(B-V) is very small by Galactic standards. In order to explore the possible importance of intrinsic continuum reddening, we have assembled a heuristic extinction curve which extends to λ ≪ 912 Å. Since the 2200 Å bump has never been detected in quasar spectra, longward of the Lyman limit our heuristic extinction curve is based on the Small Magellanic Cloud extinction curve (which contains very little 2200 Å bump). Shortward of the Lyman limit, our extinction curve smoothly increases down to λ ≈700 Å (hv ≈ 18 eV) and then turns over in crude accordance with theoretical work based on the Kramers-Kronig relations and laboratory studies. Using this heuristic extinction curve, we derive reasonable upper limits on the intrinsic E(B -V) for each quasar based on the maximum amount of continuum dereddening which can be applied without giving the big blue bump a complex shape. For six out of the seven quasars, we find that the intrinsic E(B-V) < 0.03, and the seventh QSO requires intrinsic E(B-V) < 0.045. We briefly discuss some of the implications of the derived intrinsic continuum reddening limits. We use geometrically thin accretion disk models to derive the black hole masses and accretion rates implied by the spectral energy distributions. Even if we neglect intrinsic reddening, we find that a large fraction of the quasars require super-Eddington accretion rates (which is not consistent with the thin disk assumption). Comparison of the data in this paper to a large body of data from the literature on the accretion disk MBH -Ṁ grid calculated by Wandel & Petrosian reveals that our quasars are among the brightest in the sky at 1450 Å, and ostensibly suggests that the fraction of quasars which require super-Eddington accretion rates is much smaller than the fraction that we derive from our data alone. However, intrinsic continuum reddening has been ignored in this comparison, and a small amount of intrinsic reddening will push more of the quasars into the super-Eddington regime. We also plot the recent reverberation monitoring results on NGC 5548 and NGC 3783 on the Wandel & Petrosian grid, and we find that these Seyfert galaxies appear to vary along lines of constant MBH. Continuum flux from two of the quasars in our main sample, PG 1338+416 and PG 1630+377, is detected at λrest < 584 Å. These quasars can in principle be used for the He éGunn-Peterson test, but the S/N of IUE spectra of individual objects is usually too low to place interesting limits on the Gunn-Peterson optical depth. In order to improve the S/N, we have formed a composite spectrum from the spectra of five quasars detected with IUE at λrest < 584 Å, and we have used this composite spectrum to place a tighter limit on τGP,He I. From the composite spectrum we find that at <zem> = 1.58, τGP,He I ≤ 0.09 at the 3 σ level. We briefly discuss intermediate-redshift Lyman limit systems (0.5 ≤ zLL ≤ 1.6) detected in the IUE spectra of five quasars, including lower limits on N(H I) in each Lyman limit system.

Original languageEnglish (US)
Pages (from-to)533-552
Number of pages20
JournalAstrophysical Journal
Issue number2
StatePublished - Oct 1 1994


  • Accretion, accretion disks
  • Dust, extinction
  • Infrared: galaxies
  • Intergalactic medium
  • Quasars: general
  • Ultraviolet: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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