Citation:
Date Published:
October 1, 2007Abstract:
We have combined optical data from the 2dF-SDSS (Sloan Digital SkySurvey) LRG (Luminous Red Galaxy) and QSO (quasi-stellar object) (2SLAQ)redshift survey with radio measurements from the 1.4GHz VLA (Very LargeArray) FIRST (Faint Images of the Radio Sky at Twenty-cm) and NVSS (NRAOVLA Sky Survey) surveys to identify a volume-limited sample of 391 radiogalaxies at redshift 0.4 < z < 0.7. By determining an accurateradio luminosity function for luminous early-type galaxies in thisredshift range, we can investigate the cosmic evolution of theradio-galaxy population over a wide range in radio luminosity.The low-power radio galaxies in our LRG sample (those with 1.4GHz radioluminosities in the range 1024 to1025WHz-1, corresponding to Fanaroff-Riley I (FRI)radio galaxies in the local Universe) undergo significant cosmicevolution over the redshift range 0 < z < 0.7, consistent withpure luminosity evolution of the form (1 + z)k, where k = 2.0+/- 0.3. Our results appear to rule out (at the 6-7σ level) modelsin which low-power radio galaxies undergo no cosmic evolution. The mostpowerful radio galaxies in our sample (with radio luminosities above1026WHz-1) may undergo more rapid evolution overthe same redshift range.The evolution seen in the low-power radio-galaxy population implies thatthe total energy input into massive early-type galaxies from activegalactic nucleus (AGN) heating increases with redshift, and was at least50 per cent higher at z ~ 0.55 (the median redshift of the 2SLAQ LRGsample) than in the local universe.