Extremely red galaxy counterparts to 7C radio sources

We present RIJHK imaging of seven radio galaxies from the 7C Redshift Surve y (7CRS) which lack strong emission lines and we use these data to investig ate their spectral energy distributions (SEDs) with models that constrain t heir redshifts. Six of these seven galaxies have extremely red colours (R - K > 5.5) and we find that almost all of them lie in the redshift range 1 < z < 2. We also present near-infrared spectroscopy of these galaxies which demonstrate that their SEDs are not dominated by emission lines, although t entative lines, consistent with H alpha at z = 1.45 and z = 1.61, are found in two objects. Although the red colours of the 7CRS galaxies can formally be explained by stellar populations that are either very old or young and heavily reddened, independent evidence favours the former hypothesis. At z similar to 1.5 at least 1/4 of powerful radio jets are triggered in massive (>L*) galaxies, which formed the bulk of their stars several Gyr earlier, that is at epochs corresponding to redshifts z greater than or similar to 5 . if a similar fraction of all z similar to 1.5 radio galaxies are old, the n extrapolation of the radio luminosity function shows that, depending on t he radio source lifetimes, between 10 and 100 per cent of the near-IR selec ted extremely red object (ERO) population undergo a radio outburst at epoch s corresponding to 1 < z < 2. An ERO found serendipitously in the field of one of the 7CRS radio sources appears to be a radio-quiet analogue of the 7 CRS EROs with an emission line likely to be [O II] at z = 1.20. The implica tion is that some of the most massive elliptical galaxies formed the bulk o f their stars at z greater than or similar to 5 and these objects probably undergo at least two periods of active galactic nucleus activity: one at hi gh redshift during which the black hole forms and another one at an epoch c orresponding to z similar to 1.5.