The nature of X-ray selected extremely red objects

J. A. Stevens, M. J. Page, R. J. Ivison, Ian Small, I. Lehmann, G. Hasinger, G. Szokoly

Research output: Contribution to journalArticle

14 Citations (Scopus)


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.

Original languageEnglish
Pages (from-to)249-258
Number of pages10
JournalMonthly Notices of the Royal Astronomical Society
Issue number1
Publication statusPublished - Jun 11 2003


  • Galaxies: Active
  • Infrared: Galaxies
  • Radio continuum: Galaxies
  • Submillimetre
  • X-rays: Galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science

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    Stevens, J. A., Page, M. J., Ivison, R. J., Small, I., Lehmann, I., Hasinger, G., & Szokoly, G. (2003). The nature of X-ray selected extremely red objects. Monthly Notices of the Royal Astronomical Society, 342(1), 249-258.