An overdensity of extremely red objects around faint mid-IR galaxies

We have searched for Extremely Red Objects (EROs) around faint mid-IR selected galaxies in ELAIS fields. We find a significant overdensity, by factors of 2 to 5, of these EROs compared to field EROs in the same region and literature random field ERO counts. The excess is similar to that found previously in the fields of known high redshift quasars and AGN. While with the present data it cannot be definitely shown whether the overdensity is physically connected to the mid-IR source, a derived radial distribution does suggest this. The fraction of EROs among K-selected galaxies is high in the mid-IR fields in agreement with the picture that the EROs responsible for the overdensity are members of high redshift clusters of galaxies. We find R - K > 5 selected EROs to be more clustered around the mid-IR galaxies than I - K > 4 EROs, though statistics are weak because of small numbers. However, this would be consistent with a cluster/galaxy group scenario if, as we argue, the former colour selection finds preferentially more strongly clustered early type galaxies, whereas the latter selection includes a larger fraction of dusty EROs. Finally, using the mid-IR data, we are able to limit the fraction of ULIRG type very dusty EROs at K < 18 mag to less than ∼10% of the total ERO population.