AKARI mid-infrared slitless spectroscopic survey of star-forming
  galaxies at $z\lesssim 0.5$
  Deep MIR surveys have revealed numerous strongly star-forming galaxies at
redshift z~<2. Their MIR fluxes are produced by a combination of continuum and
PAH emission features. The PAH features can dominate the total MIR flux, but
are difficult to measure without spectroscopy. We aim to study star-forming
galaxies by using a blind spectroscopic survey at MIR wavelengths to understand
evolution of their star formation rate (SFR) and specific SFR up to z~=0.5, by
paying particular attention to their PAH properties. We conducted a
low-resolution (R~=50) slitless spectroscopic survey at 5-13um of 9um
flux-selected sources (>0.3 mJy) around the North Ecliptic Pole with the
Infrared Camera (IRC) onboard AKARI. After removing 11 AGN candidates by using
the IRC photometry, we identified 48 PAH galaxies with PAH 6.2, 7.7, and 8.6um
features at z<0.5. The rest-frame optical-MIR spectral energy distributions
(SEDs) based on CFHT and AKARI/IRC imaging covering 0.37-18um were produced,
and analysed in conjunction with the PAH spectroscopy. We defined the PAH
enhancement by using the luminosity ratio of the 7.7um PAH feature over the
3.5um stellar component of the SEDs. The rest-frame SEDs of all PAH galaxies
have a universal shape with stellar and 7.7um bumps, except that the PAH
enhancement significantly varies as a function of the PAH luminosities. We
identified a PAH-enhanced population at z~>0.35, whose SEDs and luminosities
are typical of luminous infrared galaxies. They show particularly larger PAH
enhancement at high luminosity, implying that they are vigorous star-forming
galaxies with elevated specific SFR. Our composite starburst model that
combines a very young and optically very thick starburst with a very old
population can successfully reproduce most of their SED characteristics,
although we could not confirm this optically thick component from our spectral
analysis.
