Radio galaxies at $z\sim$ 2.5: Results from Keck spectropolarimetry

Abstract

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 Nv emission lines and the degree of ultraviolet continuum polarization which may represent the dispersal of dust associated with the chemical enrichment of the spheroid.

Far-infrared/millimetre emission in 3C sources

“…The dust reprocessed starburst light dominates the spectrum at long wavelengths (λ > 10 µm), while the reprocessing of AGN energy is neglible. This is consistent with the model in Vernet et al (2001) for the z ∼ 2.5 radio galaxy, 4C+48.48.…”

“…4, together with results from independent observations, seems to support this picture: (a) the dusty torus emits between 10 and 50 µm and heated by the AGN, (b) an additional cool component emitting at 100-140 µm and representing the reprocessed stellar energy in the host environment is clearly seen in radio galaxies and it may be outshone by the stronger non-thermal beamed component in quasars, (c) the difference at UV-optical wavelengths between the two classes is due to the obscuration of the AGN in the radio galaxies by the optically thick torus. The optical/UV flux which remains visible in these objects can be a combination of scattered AGN light and a direct view of a young stellar population (Vernet et al 2001).…”

“…For instance, the detection of significant linear polarization in the UV/blue aligned light is identified with scattered light from a hidden AGN due to externally illuminated dust which acts as a very efficient reflector of UV light (di Serego Alighieri et al 1989;Scarrott et al 1990;Cimatti et al 1993;di Serego Alighieri et al 1996;Cimatti & di Serego Alighieri 1995;Manzini & di Serego Alighieri 1996;Best & Longair 1999;Vernet et al 2001).…”

Witnessing the transformation of a quasar host galaxy at z = 1.6

“…With a WISE magnitude (Vega) of [3.4]=17.4±0.2, we estimate a stellar mass of ∼10 11.8 -10 12.5 M⊙, under the assumption that the rest-frame 1.3 µm emission is dominated by stars, with our range of mass to light ratio be- [NeIV] and MgII. We show some of our MAPPINGS 1e photoionization model calculations, together with shock models from Allen et al (2008), the measured line flux ratios of the gband knot of 5C 7.245 (this work), and the flux ratios of other HzRGs as reported by Vernet et al (2001) and Solórzano-Iñarrea et al (2004). We have labeled one end of each model locus to indicate shock velocity or ionization parameter U as appropriate.…”

GMASS ultradeep spectroscopy of galaxies at z ~ 2

“…As for other objects at intermediate/high redshifts, the presence of the carriers of the 2175 Å feature is controversial in quasars/active galactic nuclei (AGN) (cf. Pitman et al 2000;Maiolino et al 2001;Hopkins et al 2004;Vernet et al 2001) and intervening Mg II absorbers (Malhotra 1997;Wang et al 2004;Vanden Berk et al 2008), whereas it is likely in Ca II absorbers at intermediate redshifts (Wild & Hewett 2005).…”