The nature of X-ray selected extremely red objects

Abstract

We report on the X-ray, optical, near-infrared, submillimetre and radio properties of five extremely red objects (EROs) selected at X-ray wavelengths by XMM-Newton in the Lockman Hole field. They all have enough counts in the X-ray band to allow spectral fitting: four are most probably obscured, Compton-thin active galactic nuclei (AGNs) with redshift-dependent absorbing column densities of 10²²-10²⁴ cm^-2, whilst the fifth is best fitted by a thermal spectrum and is likely to be a massive elliptical galaxy in a deep gravitational potential. Their optical/near-infrared colours and sizes suggest that X-ray selected EROs comprise a mixture of dusty `starburst' galaxies and non-dusty galaxies that are dominated by either starlight or light from an active nucleus. The colour diagnostics are supported by the submillimetre and radio data; the two AGNs with `starburst' colours have submillimetre or radio flux densities that imply large star formation rates, whilst those with `elliptical' colours do not. The one source detected in the submillimetre waveband has narrow emission lines at a redshift of 1.45. Although the bulk of its radio emission originates from processes other than star formation, it is most probably a radio-quiet ultraluminous infrared galaxy.