JHK Spectra of the z = 2.39 Radio Galaxy 53W002^1

We present low-resolution, near-IR JHK spectra of the weak z =2 .39 radio galaxy 53W002, obtained with the OH-airglow Suppressor spectrograph (OHS) and Cooled Infrared Spectrograph and Camera for OHS (CISCO) on the Subaru Telescope. They cover rest-frame wavelengths of 3400–7200 u A, and the emission lines of [O II] λ3727, Hβ ,[ OIII] λλ4959, 5007, Hα ,[ NII] λλ6548, 6583 and [S II] λλ6716, 6731 were detected. Using the Hα/Hβ line ratio, we find an extinction of E(B − V )=0 .14. The emission-line ratios are reproduced by a cloud of electron density ne =1 × 10 3−4 cm −3 with solar metallicity, ionized by an α = −0.7 power-law continuum with ionizing parameter U =1 ×10 −3 . In addition to these emission lines, we make the first spectroscopic confirmation of the Balmer discontinuity in a high-z radio galaxy. Together with rest-frame UV photometry from the literature, we show that at least 1/3 of the present stellar mass was formed in the current starburst. The stellar mass was estimated