  In this paper we explore the clustering properties and the environment of the
Extremely 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, an
on-going multiwavelength program aiming to investigate the nature and the
evolution of faint radio sources. Using our complete sample of 289 EROs
brighter 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 suggests
a clustering length in the range ro=12-17h^{-1}Mpc, implying that EROs trace
regions of enhanced density. Using a novel method we further explore the
association of EROs with galaxy overdensities by smoothing the K-band galaxy
distribution using the matched filter algorithm of Postman et al. (1996) and
then cross-correlating the resulting density maps with the ERO positions. Our
analysis provides direct evidence that EROs are associated with overdensities
at redshifts z>1. We also exploit the deep radio 1.4GHz data (limiting flux
60microJy) available to explore the association of EROs and faint radio sources
and whether the two populations trace similar large scale structures.
Cross-correlation of the two samples (after excluding 17EROs with radio
counterparts) gives a 2sigma signal only for the sub-sample of high-z radio
sources (z>0.6). Although the statistics are poor this suggests that it is the
high-z radio sub-sample that traces similar structures with EROs.
