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We present a catalog of optical spectroscopic identifications of sourcesdetected by Spitzer at 3.6 or 24 micron down to 10 and 280 microJy,respectively, in the SWIRE/XMM-Newton/ELAIS-S1 field and classified via linewidth analysis and diagnostic diagrams. A total of 1376 sources down to R~24.2mag have been identified (1362 detected at 3.6 micron, 419 at 24 micron, and405 at both) by low-resolution optical spectroscopy carried out with FORS2,VIMOS, and EFOSC2 at the Very Large Telescope and 3.6m ESO telescopes. Thespectroscopic campaigns have been carried out over the central 0.6 squaredegrees area of ELAIS-S1 which, in particular, has also been observed byXMM-Newton and Chandra. We find the first direct optical spectroscopic evidencethat the fraction of active galactic nuclei (AGN; mostly AGN2) increases withincreasing F(24 micron)/F(R) ratio, reaching values of 70(+/-20)% in the range316<F(24 micron)F(R)<1000. We present an IRAC-MIPS color-color diagram able toseparate AGN1 from obscured AGN2 candidates. After having corrected for thespectroscopic incompleteness of our sample, it results that the AGN fraction atF(24 micron)=0.8 mJy is ~22(+/-7)% and decreases slowly to ~19(+/-5)% down toF(24 micron)=0.3 mJy.

astrophysical journal/the astrophysical journal

Spectroscopic identifications of Spitzer sources in the  SWIRE/XMM-NEWTON/ELAIS-S1 field: a large fraction of Active Galactic Nuclei  with high F(24micron)/F(R) ratio