[O II] lambda 3727 emission from the companion to the quasar BR 1202-0725 at z=4.7

Results of a narrow-band imaging for the redshifted [O II] lambda 3727 emis sion around a quasar at z = 4.7 obtained with the Subaru telescope and CISC O (a Cassegrain near infrared camera) are presented. A significant emission line is detected in the narrow-band H-2 [nu = 1-0 S(1)] filter at a locati on similar to 2." 4 northwest from the quasar, where the presence of a comp anion has been reported in Lyman cu emission and the rest-frame UV continuu m. We identify this line as [OII] lambda 3727 emission and confirm that the source really is a companion at z = 4.7. The [OII] lambda 3727 flux from t he companion is estimated to be 2.5 x 10(-17) erg s(-1) cm(-2). If the comp anion is a star-forming object, the inferred star-formation rate is as high as 45-230 M. yr(-1), even without assuming an extinction correction. This value is higher than those derived from the Lyman alpha emission or from th e UV continuum. Thus, provided that the difference is caused by dust extinc tion, the extinction-corrected star-formation rate is calculated to be 45 t o 2300 M. yr(-1), depending on the assumed extinction curves.