The content of active galactic nuclei in the z = 0.83 cluster MS 1054--0321

We present a survey of X-ray point sources in a 91-ks Chandra ACIS-S observation of the z= 0.83 cluster MS 1054−0321. We detect 47 X-ray sources within the 8.3 × 8.3 arcmin2 field, of which two are immediately confirmed from pre-existing spectroscopy to be luminous active galactic nuclei (AGN) [L(0.5–8 keV)∼ 1043 erg s−1] at the redshift of the cluster. At fluxes brighter than S(0.5–8 keV)= 5 × 10−15 erg s−1 cm−2, we find a ∼2σ excess compared with predictions from field surveys, consistent with an excess of approximately six AGN. If these sources are associated with the cluster, they too are AGN with luminosities of order L(0.5–8 keV)∼ 1043 erg s−1. Combined with the recent identification of seven cluster AGN from deep radio observations by Best et al., these observations suggest significantly enhanced AGN activity in MS 1054−03 compared with local galaxy clusters. Interestingly, the excess of X-ray detected AGN is found at radial distances of between 1 and 2 Mpc, suggesting that they may be associated with infalling galaxies. The radio AGN are seen within the inner megaparsec of the cluster and are largely undetected in the X-ray, suggesting that they are either intrinsically less luminous and/or heavily obscured.