The origin of the UV excess in powerful radio galaxies: spectroscopy and polarimetry of a complete sample of intermediate-redshift radio galaxies

Abstract

A B S T R A C TWe present spectroscopic and polarimetric observations of a complete, optically unbiased sample of 2-Jy radio galaxies at intermediate redshifts ð0:15 , z , 0:7Þ. These data -which cover the nuclear regions of the target galaxies -allow us to quantify for the first time the various components that contribute to the UV excess in the population of powerful, intermediate-redshift radio galaxies. We find that, contrary to the results of previous surveys -which have tended to be biased towards the most luminous and spectacular objects in any redshift range -the contribution of scattered quasar light to the UV excess is relatively minor in most of the objects in our sample. Only seven objects (32 per cent of the complete sample) show significant polarization in the rest-frame UV, and none of the objects in our sample is polarized in the near-UV at the P . 10 per cent level. Careful measurement and modelling of our spectra have allowed us to quantify the contributions of other components to the UV excess. We show that nebular continuum (present in all objects at the 3 -40 per cent level), direct active galactic nucleus (AGN) light (significant in 40 per cent of objects) and young stellar populations (significant in 15 -50 per cent of objects) all make important contributions to the UV continuum in the population of powerful radio galaxies. These results serve to emphasize the multicomponent nature of the UV continuum in radio galaxies. The results also point to an interesting link between the optical/UV and far-IR properties of our sample objects, in the sense that the objects with the clearest evidence for optical/UV starburst activity are also the most luminous at far-IR wavelengths. This supports the idea that the cooler dust components in radio galaxies are heated by starbursts rather than by AGN.

B2 0648+27: A radio galaxy in a major merger

“…If this is the case, the merger is estimated to be relatively old (>0.1 Gyr, see e.g. Tadhunter et al 2002;Wills et al 2002), supporting the idea that al least in those objects the radio activity starts in these galaxies quite late after the merger.…”

“…A young stellar population component has now been detected in about 30% of the observed radio galaxies (Wills et al 2002;Tadhunter et al 2002) and in some of them the mass of this component is large, suggesting its origin to be related to a major merger event. If this is the case, the merger is estimated to be relatively old (>0.1 Gyr, see e.g.…”

The B3-VLA CSS sample

“…Interactions of the radio source with the galaxy interstellar medium are clear from the study of the emission lines (see, e.g., Labiano et al 2005;Holt et al 2009). The presence of young stellar populations in these objects has been revealed several times (Holt 2009;Labiano et al 2008;deVries et al 2007;Tadhunter et al 2002). Their origin is attributed to the shocks generated by the young radio source expanding into the interstellar medium (Rees 1989;Mellema et al 2002;Bicknell et al 2000), and/or as the result of a galaxy merger process which, at some later epoch, also triggered the onset of the radio source (Raimann et al 2005;Holt 2009).…”

“…There are different physical processes that can be responsible for the UV-excess (Best et al 1998;Tadhunter et al 2002;Labiano et al 2008):…”

Detection of a pair of prominent X-ray cavities in Abell 3847

“…3C 444 has been identified as a brightest galaxy at the centre of Abell 3847 cluster (Wyndham 1965) and belongs to the class of weak-line FRII radio galaxy (Tadhunter et al 2002;Dicken et al 2012) positioned at RA=22:14:25.7, DEC=-17:01:36 at a redshift of z=0.153. It is one of the famous radio sources and has been widely studied in radio bands (Kuehr et al 1979;Geldzahler & Witzel 1981;Roger et al 1986;Dicken et al 2008Dicken et al , 2009.…”