Multi-Wavelength Analysis of 18$\mu $m-Selected Galaxies in the AKARI/Infrared-Camera monitor field towards the North Ecliptic Pole

We present an initial analysis of AKARI 18$\mu $m-selected galaxies using all 9 photometric bands at 2–24$\mu $m available in the Infrared Camera (IRC), in order to demonstrate new capabilities of AKARI cosmological surveys with this unprecedented wavelength coverage at mid-infrared (MIR) bands. We detected 72 sources at 18$\mu $m in an area of 50.2 arcmin$^2$ in the AKARI/IRC monitor field towards the North Ecliptic Pole (NEP). From this sample, 25 galaxies with probable redshifts of $z \gtrsim$ 0.5 were selected with a single colour cut (N2$-$N3$\gt$ 0.1) for a detailed spectral energy distribution (SED) analysis with ground-based $BVRi'z'JK$ data. Using an SED radiative transfer model of starbursts covering the wavelength range UV–submm, we derived photometric redshifts from the optical-MIR SEDs of 18$\mu $m-selected galaxies. From the best-fit SED models, we showed that the IRC all-band photometry is capable of tracing a steep rise in the flux at the blue side of the polycyclic aromatic hydrocarbon (PAH) 6.2$\mu $m emission feature. This indicates that the IRC all-band photometry is useful to constrain the redshift of infrared galaxies, specifically for dusty galaxies with a less prominent 4000Å break. Also, we found that the flux dip between the PAH 7.7 and 11.2$\mu $m emission feature is recognizable in the observed SEDs of galaxies at $z \sim 1$. This pilot study suggests the possibility of detecting many interesting galaxy properties in the NEP-Deep and Wide surveys, such as a systematic difference in SEDs between high–and low-$z$ Ultra-Luminous InfraRed Galaxies, and a large variation of the PAH inter-band strength ratio in galaxies at high redshifts.