Photometric studies of PAH emission from distant infrared galaxies

Using extensive mid-IR datasets from AKARI, i.e. 9 band photometry covering the wavelength range from 2 μm to 24 μm and the unbiased spectroscopic survey for sources with S _ν (9μm)>0.3 mJy, we investigated the PAHs emission features in distant starburst galaxies. PAH-selected galaxies, selected with an extremely red mid-IR colour due to PAHs, are found to have a peculiar rest-frame 11-to-8 μm flux ratio, which is systematically smaller than nearby starburst/AGN spectral templates. This may indicate a systematic difference in the physical condition of the ISM between nearby and distant starburst galaxies.