The Phoenix Deep Survey: The clustering and environment of extremely red objects

In this paper we explore the clustering properties and environment of the extremely red objects (EROs; I - K > 4 mag) detected in a ≈ 180 arcmin² deep (K_s ≈ 20 mag) K_s-band survey of a region within the Phoenix Deep Survey, an ongoing multiwavelength program aiming to investigate the nature and evolution of faint radio sources. Using our complete sample of 289 EROs brighter than K_s = 20 mag, we estimate a statistically significant (≈3.7 σ) angular correlation function signal with amplitude A_w = 8.7_-1.7^+2.1 × 10^-3 (assuming w(θ) = A_wθ^-0.8 with θ in degrees), consistent with earlier work based on smaller samples. This amplitude suggests a clustering length in the range r_o = 12-17 h^-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 available deep radio 1.4 GHz data (limiting flux 60 μJy) 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 17 EROs with radio counterparts) gives a 2 σ signal only for the subsample of high-z radio sources (z > 0.6). Although the statistics are poor, this suggests that it is the high-z radio subsample that traces similar structures with EROs.