The Phoenix Deep Survey: The Clustering and Environment of Extremely Red Objects

In this paper we explore the clustering properties and the environment of theExtremely Red Objects (EROs; I-K>4mag) detected in a ~180arcmin^2 deep(Ks~20mag) Ks-band survey of a region within the Phoenix Deep Survey, anon-going multiwavelength program aiming to investigate the nature and theevolution of faint radio sources. Using our complete sample of 289 EROsbrighter than Ks=20mag we estimate a statistically significant (~3.7sigma)angular correlation function signal with amplitude Aw=8.7^{+2.1}_{-1.7}x10^{-3}consistent with earlier work based on smaller samples. This amplitude suggestsa clustering length in the range ro=12-17h^{-1}Mpc, implying that EROs traceregions of enhanced density. Using a novel method we further explore theassociation of EROs with galaxy overdensities by smoothing the K-band galaxydistribution using the matched filter algorithm of Postman et al. (1996) andthen cross-correlating the resulting density maps with the ERO positions. Ouranalysis provides direct evidence that EROs are associated with overdensitiesat redshifts z>1. We also exploit the deep radio 1.4GHz data (limiting flux60microJy) available to explore the association of EROs and faint radio sourcesand whether the two populations trace similar large scale structures.Cross-correlation of the two samples (after excluding 17EROs with radiocounterparts) gives a 2sigma signal only for the sub-sample of high-z radiosources (z>0.6). Although the statistics are poor this suggests that it is thehigh-z radio sub-sample that traces similar structures with EROs.Comment: To appear in ApJ. Figure 5 is given separately in jpeg forma