Observations of Paschen alpha in a Complete Sample of Radio Galaxies

Abstract

We present infrared spectrophotometry of the Paalpha (n = 4-3)emission line of hydrogen, together with optical spectrophotometry ofHalpha and Hbeta, of a flux-limited sample of 11 radio sources. Thesample consists of all FR II 3CR radio sources with 0.1 < R.A. < 16^h^ which contains eight narrow-line radiogalaxies (3CR 135, 184.1, 219, 223, 236, 319, 321, and 327), twobroad-line radio galaxies (3CR 234 and 3CR 303), and one quasar (3CR273). The observations were aimed at the detection of obscured broadlines, hidden from our view by dust absorption, as a means of testingtheories of the unification of radio galaxies and quasars. All theobjects except 3CR 236 and 3CR 273 show significant reddening of thenarrow and broad lines, typically of order A_V_ ~ 1.5 for narrow linesand ~3 for broad lines. We detect highly obscured broad-line regions in3CR 184.1, 219, and 223, which appear to be narrow-line objects in theoptical, so these should be reclassified as broad-line radio galaxies.In all cases except 3CR 273 and 3CR 303, the broad lines are reddenedmore than the narrow lines, locating much of the dust responsible forabsorbing the broad-line emission between the broad- and narrow-lineregions. The dereddened line luminosities range up to those oflow-luminosity quasars. The results are broadly consistent with modelswhich seek to unify radio galaxies and quasars through orientation,where an axisymmetric equatorial obscuring region hides the quasarnucleus from view unless the radio axis is pointing close to our line ofsight. These data provide the first opportunity to model thedistribution of broad-line region extinctions in a complete sample,rather than model just the fraction of quasars and radio galaxies. Wedevelop a simple unification model that matches the observeddistribution of extinctions, explains our observations, and makespredictions about the fraction of obscured quasars that will be presentin samples of higher radio luminosity.