[OII] λ 3727 Emission from the Companion to the Quasar BR 1202–0725 atz= 4.7

Results of a narrow-band imaging for the redshifted [OII]3727 emission arounda quasar at z=4.7 obtained with the Subaru telescope and CISCO (a Cassegrainnear infrared camera) are presented. A significant emission line is detected inthe narrow-band H_2 (v=1-0 S(1)) filter at a location 2.4" northwest from thequasar, where the presence of a companion has been reported in Lyman alphaemission and the rest-frame UV continuum. We identify this line as [OII]3727emission and confirm that the source really is a companion at z=4.7. The[OII]3727 flux from the companion is estimated to be 2.5 x 10^{-17} erg s^{-1}cm^{-2}. If the companion is a star forming object, the inferred star formationrate is as high as 45-230 M_{solar} yr^{-1} even without assuming theextinction correction. This value is higher than those derived from the Lymanalpha emission or from the UV continuum. Thus, provided that the difference iscaused by dust extinction, the extinction corrected star formation rate iscalculated to be 45 to 2300 M_{solar} yr^{-1} depending on the assumingextinction curves.Comment: 15 pages including 3 figures. Accepted for publication in PAS