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

Results of a narrow-band imaging for the redshifted [Oil] A3727 emission around a quasar at z = 4.7 obtained with the Subaru telescope and CISCO (a Cassegrain near infrared camera) are presented. A significant emission line is detected in the narrow-band H2 [v = 1-0 S(l)] filter at a location ~ 2." 4 northwest from the quasar, where the presence of a companion has been reported in Lyman a emission and the rest-frame UV continuum. We identify this line as [Oil] A3727 emission and confirm that the source really is a companion at z = 4.7. The [Oil] A3727 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-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 a emission or from the UV continuum. Thus, provided that the difference is caused by dust extinction, the extinction-corrected star-formation rate is calculated to be 45 to 2300 M® yr -1 , depending on the assumed extinction curves.