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Kentaro Motohara, Toru Yamada, Fumihide Iwamuro, Ryuji Hata, Tomoyuki Taguchi, Takashi Harashima, Toshinori Maihara, Masanori Iye, Chris Simpson, Michitoshi Yoshida, JHK Spectra of the z = 2.39 Radio Galaxy 53W002, Publications of the Astronomical Society of Japan, Volume 53, Issue 3, 25 June 2001, Pages 459–468, https://doi.org/10.1093/pasj/53.3.459
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Abstract
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]|$\lambda$|3727, H|$\beta$|, [O III]|$\lambda\lambda$|4959, 5007, H|$\alpha$|, [N II]|$\lambda\lambda$|6548, 6583 and [S II]|$\lambda\lambda$|6716, 6731 were detected. Using the H|$\alpha$|/H|$\beta$| line ratio, we find an extinction of |$E(B\hbox{--}V)=0.14$|. The emission-line ratios are reproduced by a cloud of electron density |$n_{\mathrm{e}}=1\times 10^{3-4}\,\mathrm{cm}^{-3}$| with solar metallicity, ionized by an |$\alpha=-0.7$| power-law continuum with ionizing parameter |$U=1\times 10^{-3}$|. 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 |$(1\hbox{--}1.4)\times10^{11}\,{{{M}_{\odot}}}$| by one-component model fitting, which is smaller than that of typical |$z\sim 1$| 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 L|$\alpha$| emitters detected in narrow-band imaging. After a few such events over the next few Gyr, 53W002 will evolve into a massive elliptical galaxy.
