Optical (R-band) and near-infrared (J, K-bands) images of the faint radio source PDFJ011423 discovered in the course of the Phoenix Deep Survey. This object is faint at optical (R=21.1mag) although relatively bright at near-infrared (K=15.3mag). Thus, with an optical-infrared colour of R-K=5.8mag, this source belongs to the class of Extremely Red Objects (EROs). Optical spectroscopy has revealed a galaxy with strong narrow emission lines at z=0.653. Detailed study using multiwavelength data (optical, near and far-infrared) suggests a dusty starburst with a small contribution from an active galactic nucleus (Afonso et al. 2001).

Surveys at near-infrared wavelengths (NIR; 2.2µm) have revealed a class of galaxies that are very faint at optical although relatively bright at NIR, resulting in distinctively red optical-infrared colours (R-K>5.5). These systems, dubbed Extremely Red Objects (EROs), are believed to be almost equally split between dusty active galaxies (starbursts or AGN) and ellipticals at moderate to high redshifts (z>1). The former owe their red colours to heavy dust obscuration of the underlying stellar or AGN emission, while the latter have an intrinsically red spectral energy distribution that appears even redder due to their high redshift (k-correction). The identification of either type of galaxies (dusty or old) at high-z has significant cosmological implications, providing significant impetus in ERO studies. Open questions include:

• What is the relative fraction of starbursts and AGN among the dusty ERO subpopulation.
• What are the low redshift descendants of these high-z objects. To address this issue one has to determine properties such as stellar mass and large scale environment to compare against local populations.
• Is this population a significant component of the Universe at high redshift. For example what is the contribution of these objects to the total mass density and star-formation rate density of the Universe at z~1.

We combine optical (UBVRI), near-IR (KJ) and radio continuum (1.4GHz) observations obtained as part of the Phoenix Deep Survey to explore the properties of EROs. Firstly we quantify the environment of these systems and show that they reside, on average, in overdense regions, groups or clusters (Georgakakis et al. 2005). Using the 1.4GHz radio emission as dust-free SFR diagnostic, we estimate the mean SFR of dusty EROs, independent of dust induced biases. Comparison with SFR estimates based on optical emission lines (i.e. affected by dust), suggest a mean optical extinction A_V>0.5. We also estimate a lower limit of about 10% for the contribution of EROs to the global SFR density (Georgakakis et al. 2006). We further estimate the stellar masses of these systems a find that they are massive galaxies. However, not all massive galaxies at z~1 are EROs. These systems represent about 50% of the mass density of the Universe at these redshifts (Georgakakis et al. 2006).