Polycyclic aromatic hydrocarbon (PAH) luminous galaxies at z textttchar126 1
ABSTRACT ensuremath Aimsensuremath<?iensuremath>. The NEP-deep survey, an extragalactic AKARI survey towards the north ecliptic pole (NEP), provides a comprehensive wavelength coverage from 2 to 24 ensuremath ensuremathmuensuremath<?iensuremath>m using all 9 photometric bands of the infrared camera (IRC). It allows us to photometrically identify galaxies whose mid-IR emission is clearly dominated by PAHs. ensuremath ensuremath<?brensuremath>ensuremath ensuremath<?brensuremath> ensuremath Methodsensuremath<?iensuremath>. We propose a single-colour selection method to identify such galaxies, using two mid-IR flux ratios at 11-to-7 ensuremath ensuremathmuensuremath<?iensuremath>m and 15-to-9 ensuremath ensuremathmuensuremath<?iensuremath>m (PAH-to-continuum flux ratio in the rest frame), which are useful for identifying starburst galaxies at z textttchar126 0.5 and 1,respectively. We perform a fitting of the spectral energy distributions (SEDs) from optical to mid-IR wavelengths, using an evolutionary starburst model with a proper treatment of radiative transfer (SBURT), in order to investigate their nature. ensuremath ensuremath<?brensuremath>ensuremath ensuremath<?brensuremath> ensuremath Resultsensuremath<?iensuremath>. The SBURT model reproduces observed optical-to-mid-IR SEDs of more than a half of the PAH-selected galaxies. Based on the 8 ensuremath ensuremathmuensuremath<?iensuremath>m luminosity, we find ultra luminous infrared galaxies (ULIRGs) among PAH-selected galaxies. Their PAH luminosity is higher than local ULIRGs with a similar luminosity, and the PAH-to-total IR luminosity ratio is consistent with that of less luminous starburst galaxies. They are a unique galaxy population at high redshifts, and we call these PAH-selected ULIRGs "PAH-luminous" galaxies. Although they are not as massive as submillimetre galaxies at z textttchar126 2, they have the stellar mass of ensuremath> 3 x 10ensuremath 10ensuremath<?supensuremath>ensuremath Mensuremath<?iensuremath>ensuremath ?ensuremath<?subensuremath> and therefore are moderately massive.