Optical and infrared properties of active galactic nuclei in the Lockman Hole

E. Rovilos, S. Fotopoulou, M. Salvato, V. Burwitz, E. Egami, G. Hasinger, G. Szokoly

Research output: Contribution to journalArticlepeer-review

18 Scopus citations


We present the observed-frame optical, near-, and mid-infrared properties of X-ray-selected active galactic nuclei (AGN) in the Lockman Hole. Using a likelihood ratio method on optical, near-infrared or mid-infrared catalogues, we assigned counterparts to 401 out of the 409 X-ray sources of the XMM-Newton catalogue. Accurate photometry was collected for all the sources from U to 24 μm. We used X-ray and optical criteria to remove any normal galaxies, galactic stars, or X-ray clusters among them and studied the multi-wavelength properties of the remaining 377 AGN. We used a mid-IR colour-colour selection to understand the AGN contribution to the optical and infrared emission. Using this selection, we identified different behaviours of AGN-dominated and host-dominated sources in X-ray-optical-infrared colour-colour diagrams. More specifically, the AGN-dominated sources show a clear trend in the f x/fRC vs. RC - K and f24 μm /fRC vs. RC - K diagrams, while the hosts follow the behaviour of non X-ray detected galaxies. In the optical-near- infrared colour-magnitude diagram we see that the known trend of redder objects to be more obscured in X-rays is stronger for AGN-dominated than for host-dominated systems. This is an indication that the trend is more related to the AGN, which contaminate the overall colours, than to any evolutionary effects. Finally, we find that a significant fraction (∼30%) of the reddest AGN are not obscured in X-rays.

Original languageEnglish (US)
Article numberA135
JournalAstronomy and astrophysics
StatePublished - 2011


  • Galaxies: Seyfert
  • Galaxies: active
  • Galaxies: statistics
  • Infrared: galaxies
  • X-rays: galaxies

ASJC Scopus subject areas

  • Astronomy and Astrophysics
  • Space and Planetary Science


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