Optical Identification of 15$\mu $m Sources in the AKARI Performance Verification Field toward the North Ecliptic Pole

Abstract

Abstract We present the results of optical identifications for 257 mid-infrared sources detected with a deep 15$\;\mu$m survey over approximately 80 arcmin$^2$ area in the AKARI performance verification field near the North Ecliptic Pole. The 15$\;\mu$m fluxes of the sources range from 1 mJy down to 40 $\mu $Jy, approximately one half of which are below 100 $\mu $Jy. Optical counterparts were searched for within a 2$^{\prime\prime}$-3$^{\prime\prime}$ 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$\;\mu$m sources show colors with $R-$L15$\gt$ 5, which cannot be explained by the spectral energy distribution (SED) of normal quiescent spiral galaxies, but are consistent with SEDs of redshifted ($z\gt1$) starburst or ultraluminous infrared galaxies. This result indicates that the fraction of the ultraluminous infrared galaxies in our faint 15$\;\mu$m sample is much larger than that in our brighter 15$\;\mu$m sources, which is consistent with the evolving mid-infrared luminosity function derived by recent studies based on Spitzer 24$\;\mu$m deep surveys. Based on an SED fitting technique, the nature of the faint 15$\;\mu$m sources is further discussed for a selected number of sources with available $K_{\rm s}$-band data.