Discovery of a strong ionized-gas outflow in an AKARI-selected ultra-luminous infrared galaxy at z = 0.5

Abstract

In order to construct a sample of ultra-luminous infrared galaxies (ULIRGs; with infrared luminosity L_IR > 10¹² L_{⊙}) at 0.5 < z < 1, we are conducting an optical follow-up program for bright 90 μm far-infrared sources with a faint optical (i < 20 mag) counterpart selected in the AKARI Far-Infrared Surveyor (FIS) Bright Source catalog (Ver.2). AKARI-FIS-V2 J0916248+073034, identified as a ULIRG at z = 0.49 in the spectroscopic follow-up observation, indicates signatures of an extremely strong outflow in its emission line profiles. Its [O III] 5007 Å emission line shows a full width at half-maximum (FWHM) of 1830 km s^-1 and velocity shift of -770 km s^-1 relative to the stellar absorption lines. Furthermore, the low-ionization [O II] 3726 Å 3729 Å doublet also shows a large FWHM of 910 km s^-1 and velocity shift of -380 km s^-1. After the removal of an unresolved nuclear component, the long-slit spectroscopy 2D image possibly shows that the outflow extends to a radius of 4 kpc. The mass outflow and energy ejection rates are estimated to be 500 M_{⊙} yr^-1 and 4 × 10⁴⁴ erg s^-1, respectively, which imply that the outflow is among the most powerful ones observed in ULIRGs and quasi-stellar objects at 0.3 < z < 1.6. The co-existence of the strong outflow and intense star formation (star formation rate of 990 M{{⊙} yr^-1) indicates that the feedback of the strong outflow has not severely affected the star-forming region of the galaxy.