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

Abstract

In this paper we explore the clustering properties and environment of the extremely red objects (EROs; I À K > 4 mag) detected in a %180 arcmin 2 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 ¼ 20 mag, we estimate a statistically significant (%3.7 ) angular correlation function signal with amplitude A w ¼ 8:7 þ2:1 À1:7 ; 10 À3 (assuming w() ¼ A w À0:8 , with in degrees), consistent with earlier work based on smaller samples. This amplitude suggests a clustering length in the range r o ¼ 12 17 h À1 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 k 1. We also exploit the available deep radio 1.4 GHz data (limiting flux 60 Jy) 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 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.

The FIRST radio survey: Panchromatic properties of FIRST radio sources identified in the Boötes and Cetus fields

“…This result may indicate that star-forming galaxies actually dominate the sub-mJy population. This is also reported by Gruppioni et al (1999b), in the study of the Marano field, Georgakakis et al (2005) in the Phoenix survey and in the FIRST-APM sample.…”

Optical colours of AGN in the extended Chandra deep field South: obscured black holes in early type galaxies

“…Brusa et al (2005) find that almost all ERO X-ray sources with redshift information from the literature are obscured with N H > 10 22 cm −2 (see also Severgnini et al 2005). While our sources are not typically EROs 1 , they share some observational characteristics (colours redder than the respective median, X-ray absorption) and could be their analogous sources in more moderate redshifts (EROs usually have z 1; Georgakakis et al 2005), at least for those not being dust reddened. Red evolved galaxies are therefore rather common, and theoretical models of galaxy and AGN evolution should be fine-tuned to predict such a behaviour in evolved systems.…”

The Clustering of Extremely Red Objects

“…Previous studies have all found that EROs are very strongly clustered (e.g., Daddi et al 2000Daddi et al , 2002Firth et al 2002;Roche et al 2002;Brown et al 2005;Georgakakis et al 2005;Kong et al 2006Kong et al , 2009Kim et al 2011). However, the existing estimates of the clustering strengths, as measured by the scale r 0 where the two-point correlation strength is unity, show a broad range (r 0 = 5.5-17 h −1 Mpc) that makes it difficult to link the higher redshift EROs with local galaxy populations.…”

“…An additional problem is that the angular correlation function (w(θ )) is usually fit as a fixed power law, which may not correctly represent the data even if it is the only option given the sample sizes (e.g., Firth et al 2002;Georgakakis et al 2005). Furthermore, the uncertainties in the correlation function are frequently assumed to be Poisson with no correlations between data points (e.g., Kong et al 2009;Kim et al 2011).…”