Extremely Red Objects

Deep multiwavelength surveys have revealed a new population of sources that are relatively bright at near-infrared wavelengths (K-band) although extremely faint at optical wavebands (R-band). These objects have very red optical-infrared colours (hence their name Extremely Red Objects; EROs) indicating either a dust-enshrouded starburst or an evolved stellar population at high redshifts, z~1.

We quantify the large-scale structure of this population by cross-correlating the positions of EROs with galaxy overdensities identified by smoothing the (spatial) distribution of near-IR selected galaxies. The top-left figure shows the positions of EROs (black dots) on the smoothed galaxy distribution image. Dark areas correspond underdense regions, while, white peaks are galaxy overdensities. Visual inspection as well as more rigorous statistical analysis shows that EROs are more frequently associated with high density peaks, suggesting rich environments for this population, i.e. groups or clusters.

We also explore the cosmological significance of EROs by estimating the contribution of this dusty population to the star-formation history of the Universe at z~1. Using ultra-deep radio data obtained as part of the Phoenix Deep Survey we apply stacking analysis methods to estimate the mean radio luminosity of EROs, which provides a dust-independent estimate of their mean star-formation rate (SFR). The results are shown in the bottom-left figure, plotting the SFR density of EROs in comparison with the total SFR density of the Universe as a function of redshift estimated from a number of independent studies. EROs in this figure contribute at least 10% of the total SFR density at z~1 and are therefore a non negligible component of the SFR at this redshift. The importance of this finding is that the dusty starburst EROs are largely underrepresented at UV/optical studies (due to dust obscuration) resulting in incompleteness in samples compiled at these wavelengths.