Spectroscopic Identifications of Spitzer Sources in the SWIRE/XMM-Newton/ELAIS-S1 Field A Large Fraction of Active Galactic Nucleus with High F(24 mum)/F(R) Ratio

We present a catalog of optical spectroscopic identifications of sources detected by Spitzer at 3.6 or 24 mum down to ~10 and ~280 muJy, respectively, in the SWIRE/XMM-Newton/ELAIS-S1 field and classified via line width analysis and diagnostic diagrams. A total of 1376 sources down to R ~ 24.2 mag have been identified (1362 detected at 3.6 mum, 419 at 24 mum, and 405 at both) by low-resolution optical spectroscopy carried out with FORS2, VIMOS, and EFOSC2 at the Very Large Telescope and 3.6 m ESO telescope. The spectroscopic campaigns have been carried out over the central 0.6 deg<SUP>2</SUP> area of ELAIS-S1 which, in particular, has also been observed by XMM-Newton and Chandra. We find the first direct optical spectroscopic evidence that the fraction of active galactic nuclei (AGNs; mostly AGN2) increases with increasing F(24 mum)/F(R) ratio, reaching values of 70(20)% in the range 316 < F(24 mum)/F(R) < 1000. We present an Infrared Array Camera-Multiband Imaging Photometer color-color diagram able to separate AGN1 from obscured AGN2 candidates. After having corrected for the spectroscopic incompleteness of our sample, the result is that the AGN fraction at F(24 mum) ~0.8 mJy is ~22(7)% and decreases slowly to ~19(5)% down to F(24 mum) ~ 0.3 mJy.