The clustering of radio galaxies atz≃ 0.55 from the 2SLAQ LRG survey

Abstract

We examine the clustering properties of low-power radio galaxies at redshift 0.4 < z < 0.8, using data from the 2SLAQ Luminous Red Galaxy (LRG) survey, and find that radio-detected LRGs (with typical optical luminosities of 3–5L* and 1.4 GHz radio powers in the range 1024–1026W Hz−1) are significantly more clustered than a matched population of radio-quiet (≲1024W Hz−1) LRGs with the same distribution in optical luminosity and colour. The measured scalelength of the two-point cross-correlation function between the full LRG sample and the radio-detected LRGs is 9.57 ± 0.50 h−1Mpc, compared to 8.47 ± 0.27 h−1Mpc for the matched sample of radio-quiet LRGs; while the implied scalelength of the auto-correlation function, r0, is 12.3 ± 1.2 h−1Mpc and 9.02 ± 0.52 h−1Mpc for the radio-detected and radio-quiet samples, respectively. We further interpret our clustering measurements in the halo model framework and demonstrate that the radio-detected LRGs have typical halo masses of 10.1 ± 1.4 × 1013h−1M⊙ and bias of 2.96 ± 0.17, compared to 6.44 ± 0.32 × 1013h−1M⊙ and 2.49 ± 0.02 for the radio-quiet sample. A model in which the radio-detected LRGs are almost all central galaxies within haloes provides the best fit to the measured clustering, and we estimate that at least 30 per cent of all 2SLAQ LRGs with the same clustering amplitude as the radio-detected LRGs are currently radio loud. Our results imply that radio-detected galaxies in the 2SLAQ LRG sample typically occupy more massive haloes than other LRGS of the same optical luminosity, so the probability of finding a radio-loud active galactic nucleus (AGN) in a massive galaxy at z∼ 0.55 is influenced by the halo mass and/or cluster environment in addition to the well-known dependence on optical luminosity. If we model the radio-detected fraction of LRGs, Frad, as a function of halo mass M, then the data are well-fitted by a power law of the form Frad∝M0.65±0.23. The observed relationship between radio emission and clustering strength could plausibly arise either through a higher fuelling rate of gas on to the central black holes of galaxies in the most massive haloes (producing more powerful radio jets) or through the presence of a denser IGM (which would provide a more efficient working surface for the jets, thus boosting their observed radio luminosity). Further work is needed to determine which of these effects is dominant.