JHK spectra of the z = 2.39 radio galaxy 53W002

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 Å, and the emission lines of [O II] λ 3727, Hβ, [O III] λλ 4959, 5007, Hα, [N II] λλ 6548, 6583 and [S II] λλ 6716, 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 n_e = 1 × 10^3-4cm^-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 to be (1-1.4) × 10¹¹ M_⊙ by one-component model fitting, which is smaller than that of typical z ∼ 1 B2/6C radio galaxies. We suggest that 53W002 is currently assembling a large part of its stellar mass through merger events with the surrounding sub-galactic clumps, some of which can be identified with the Lα emitters detected in narrow-band imaging. After a few such events over the next few Gyr, 53W002 will evolve into a massive elliptical galaxy.