The host galaxies of radio-loud AGN: colour structure
  We construct a sample of 3,516 radio-loud host galaxies of active galactic
nuclei (AGN) from the optical Sloan Digital Sky Survey (SDSS) and Faint Images
of the Radio Sky at Twenty cm (FIRST). These have 1.4 GHz luminosities in the
range 10E23-1025 WHz^{-1}, span redshifts 0.02<z<0.18, are brighter than
r*_{petro}<17.77 mag and are constrained to `early-type' morphology in colour
space (u*-r*>2.22 mag). Optical emission line ratios (at >3 sigma) are used to
remove type 1 AGN and star-forming galaxies from the radio sample using BPT
diagnostics. For comparison, we select a sample of 35,160 radio-quiet galaxies
with the same r*-band magnitude-redshift distribution as the radio sample. We
also create comparison radio and control samples derived by adding the NRAO VLA
Sky Survey (NVSS) to quantify the effect of completeness on our results.
  We investigate the effective radii of the surface brightness profiles in the
SDSS r and u bands in order to quantify any excess of blue colour in the inner
region of radio galaxies. We define a ratio R=r_{e}(r)/r_{e}(u) and use maximum
likelihood analysis to compare the average value of R and its intrinsic
dispersion between both samples. R is larger for the radio-loud AGN sample as
compared to its control counterpart, and we conclude that the two samples are
not drawn from the same population at >99% significance. Given that star
formation proceeds over a longer time than radio activity, the difference
suggests that a subset of galaxies has the predisposition to become radio loud.
We discuss host galaxy features that cause the presence of a radio-loud AGN to
increase the scale size of a galaxy in red relative to blue light, including
excess central blue emission, point-like blue emission from the AGN itself,
and/or diffuse red emission. We favour an explanation that arises from the
stellar rather than the AGN light.
