The ultraviolet spectra of nearby radio galaxies

William C. Keel, Rogier Windhorst

Research output: Contribution to journalArticlepeer-review

27 Scopus citations


We report new and archival IUE SWP spectra for nine nearby radio galaxies (V < 15 mag), together with optical emission-line data for these galaxies as well as a number of candidates with weaker line emission. We examine both their UV line and continuum properties, as well as their UV and UV-optical line ratios. We find that: 1. Lyα emission is common among local radio galaxies, at modest luminosities (typically 1041-42 ergs s-1). No apparent relation is found between L(Lyα) and radio power for the nearby radio galaxies alone. Incorporating the known powerful high-redshift radio galaxies, a regression is seen suggesting that L(Lyα) ∝ (P1.4GHz)0.6±0.15, with an rms scatter ∼0.5 dex. We address the reality of this relationship, and discuss its possible consequences. 2. The Lyα/Hα ratio in low power nearby radio galaxies is 2-5 times lower than the prediction for case B recombination. Destruction of Lyα photons by grains during resonant scattering can explain the observed deficiency for reasonable metallicities. The role of internal reddening is likely to be small, since the continuum shapes do not allow enough reddening to affect Lyα so strongly. 3. The nearby radio galaxies have in general a small C IV/Lyα ratio (<0.1). Their narrow line Wλ(Lyα) values are in the range 20-200 Å. Comparison of the C IV and Lyα strengths with those in luminous active nuclei (e.g., Seyferts) suggests that most of the UV continuum comes from the stellar population, and not from the AGN. Most (4/5) of our nearby radio galaxies show an upturn in the continuum below 1450 Å, which is found in only 36% (9/25) of the nearby radio-quiet early type galaxies, and not seen in high-redshift radio galaxies. Before a detailed comparison between high- and low-redshift galaxies can be made, UV spectroscopy is needed for substantial numbers of intermediate redshift galaxies (0.1 ≲ z ≲ 1.5) of moderate radio power to address the continuity in the radio galaxy population. This can now be done with HST.

Original languageEnglish (US)
Pages (from-to)135-147
Number of pages13
JournalAstrophysical Journal
Issue number1
StatePublished - Dec 10 1991


  • Galaxies: interstellar matter
  • Galaxies: nuclei
  • Radiation mechanisms
  • Radio sources: galaxies
  • Ultraviolet: spectra

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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