Extremely red objects in the UKIDSS Uultra Deep Survey Early Data Release

We construct a sample of extremely red objects (EROs) within the UKIDSS Ultra Deep Survey by combining the Early Data Release with optical data from the Subaru/XMM–Newton Deep Field. We find a total of 3715 objects over 2013 arcmin2 with R−K > 5.3 and K≤ 20.3, which is a higher surface density than found by previous studies. This is partly due to our ability to use a small aperture in which to measure colours, but is also the result of a genuine overdensity of objects compared to other fields. We separate our sample into passively evolving and dusty star-forming galaxies using their RJK colours and investigate their radio properties using a deep radio map. The dusty population has a higher fraction of individually detected radio sources and a higher mean radio flux density among the undetected objects, but the passive population has a higher fraction of bright radio sources, suggesting that active galactic nuclei are more prevalent among the passive ERO population.