We present the results of optical identifications for 257 mid-infrared sources detected with a deep 15 μm survey over approximately 80 arcmin 2 area in the AKARI performance verification field near the North Ecliptic Pole. The 15 μm fluxes of the sources range from 1 mJy down to 40 μJy, approximately one half of which are below 100 μJy. Optical counterparts were searched for within a 2″-3″ radius in both the BVRi′z′ catalog generated by using the deep Subaru/Suprime-cam field, which covers one-third of the performance verification field, and the g′r′i′z′ catalog based on observations made with MegaCam at CFHT. We found that the B -R and R-z′ colors of sources with successful optical identifications are systematically redder than that of the entire optical sample in the same field. Moreover, approximately 40% of the 15 μm sources show colors with R-L15 > 5, which cannot be explained by the spectral energy distribution (SED) of normal quiescent spiral galaxies, but are consistent with SEDs of redshifted (z > 1) starburst or ultraluminous infrared galaxies. This result indicates that the fraction of the ultraluminous infrared galaxies in our faint 15 μm sample is much larger than that in our brighter 15 μm sources, which is consistent with the evolving mid-infrared luminosity function derived by recent studies based on Spitzer 24 μm deep surveys. Based on an SED fitting technique, the nature of the faint 15 μm sources is further discussed for a selected number of sources with available K8-band data.
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