μJy radio sources in the z=0.83 cluster MS1054-03

Abstract

An extremely deep 5-GHz radio observation is presented of the rich cluster MS1054−03 at redshift z=0.83. 34 radio sources are detected down to a 6σ level of 32 μJy, compared with about 25 expected from previous blank-field radio source count determinations; the sources giving rise to these excess counts lie predominantly within 2 arcmin (∼700 kpc) of the cluster centre. Existing imaging and spectroscopic observations have provided optical identifications for 21 of the radio sources and redshifts for 11, of which eight are confirmed cluster members. Four of these eight confirmed cluster sources are associated with close galaxy pairs (10–25 kpc projected offset) of similar magnitude, implying that the radio source may be triggered by an interaction. However, although MS1054−03 has a very high fraction (17 per cent) of ongoing mergers (separations ≪10 kpc), no radio emission is detected towards any of these merger events, setting a mean upper limit of 10 M⊙ yr−1 for any star-formation associated with these mergers. This supports the hypothesis that low-luminosity radio sources may be onset by initial weak interactions rather than direct mergers. The host galaxies of the other four confirmed cluster radio sources are all isolated, and show a range of morphologies from early type to Sc. A comparison between the emission-line and radio luminosities suggests that two of these four radio sources are low-luminosity active galactic nuclei (AGNs), whilst for at least one of the other two the radio emission is associated with ongoing star formation. All the radio sources associated with the galaxy pairs appear more likely to be AGN than starburst in origin. The overall proportion of radio sources associated with AGNs in this cluster (≳75 per cent) is higher than that detected at these flux-density levels in the field (40–50 per cent).