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

In order to construct a sample of ultra-luminous infrared galaxies (ULIRGs; with infrared luminosity LIR> 1012L${_{\odot} }$) 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 Åemission line shows a full width at half-maximum (FWHM) of 1830 km s−1and velocity shift of −770 km s−1relative to the stellar absorption lines. Furthermore, the low-ionization [O ii] 3726 Å 3729 Ådoublet also shows a large FWHM of 910 km s−1and 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${_{\odot} }$yr−1and 4 × 1044erg 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${_{\odot} }$yr−1) indicates that the feedback of the strong outflow has not severely affected the star-forming region of the galaxy.