Radio galaxies at z ~ 2.5: Results from Keck spectropolarimetry

Abstract

In classifying the ensemble of powerful extragalactic radio sources, considerable evidence has accumulated that radio galaxies and quasars are orientation-dependent manifestations of the same parent population: massive spheroidal galaxies containing correspondingly massive black holes. One of the key factors in establishing this unification has been the signature of a hidden quasar detected in some radio galaxies in polarized light. The obscuration of our direct view of the active nucleus usually, but not necessarily exclusively, by a thick nuclear disk or torus can act conveniently as a "natural coronograph" that allows a much clearer view of the host of a radio galaxy than of a quasar. In this study, we exploit the opportunity to eliminate the quasar glare by performing sensitive spectropolarimetry with the Keck II telescope of a sample of radio galaxies with redshifts around 2.5. This represents the epoch when quasars were many times more common that they are now and is likely to be the period during which their host galaxies were being assembled into what become the most massive galaxies in the Universe today. We show that dust-reflected quasar light generally dominates the restframe ultraviolet continuum of these sources and that a highly clumped scattering medium results in almost grey scattering of the active galactic nucleus photons. The observations, however, do not exclude a substantial star formation rate averaged over a Gyr of evolution. The sub-mm reradiation from the scattering dust is likely to represent only a small fraction (∼10% ) of the total far infrared luminosity. An analysis of the emission lines excited in the interstellar medium of the host galaxy by the hard quasar radiation field reveals evidence of a dramatic chemical evolution within the spheroid during this epoch. Secondary nitrogen production in intermediate mass stars produces a characteristic signature in the Nv/Civ and Nv/Heii line ratios which has been seen previously in the broad line region of quasars at similar redshifts. We find intriguing correlations between the strengths of the Lyα and N v emission lines and the degree of ultraviolet continuum polarization which may represent the dispersal of dust associated with the chemical enrichment of the spheroid.

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