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.
- Publication:
-
Monthly Notices of the Royal Astronomical Society
- Pub Date:
- June 2003
- DOI:
- 10.1046/j.1365-8711.2003.06534.x
- arXiv:
- arXiv:astro-ph/0302289
- Bibcode:
- 2003MNRAS.342..249S
- Keywords:
-
- galaxies: active;
- infrared: galaxies;
- radio continuum: galaxies;
- submillimetre;
- X-rays: galaxies;
- Astrophysics
- E-Print:
- MNRAS, in press, 10 pages, 7 figures