[O II] λ3727 emission from the companion to the quasar BR 1202-0725 at z = 4.7

Abstract

Results of a narrow-band imaging for the redshifted [O II] λ 3727 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(1)] filter at a location ∼ 2″.4 northwest from the quasar, where the presence of a companion has been reported in Lyman α emission and the rest-frame UV continuum. We identify this line as [O II] λ 3727 emission and confirm that the source really is a companion at z = 4.7. The [O II] λ 3727 flux from the companion is estimated to be 2.5 × 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(fisheye sign) yr-1, even without assuming an extinction correction. This value is higher than those derived from the Lyman α 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(fisheye sign), yr-1, depending on the assumed extinction curves.