The [O III] Emission‐Line Nebula of the z = 3.594 Radio Galaxy 4C + 19.71

We have imaged the z = 3.594 radio galaxy 4C +19.71 in the light of the redshifted [O III] 5007 Å emission line, using a narrowband filter centered at 2.3 μm with the near-infrared camera on the Keck Telescope. The [O III] nebula of 4C +19.71 has a size of 74 × 9 kpc, and a luminosity of L_(5007) ~ 3 × 10^(37) W. The rest frame equivalent width of the 5007 Å line, averaged over the entire nebula, is 560 Å. The length of the major axis of the [O III] emission is nearly identical to the separation of the radio lobes seen at 1465 MHz (Rottgering et al.), and the position angle of the nebula is the same as that of the two radio lobes. In addition, 4C + 19.71 follows the optical emission-line versus radio-power correlation seen in other powerful radio galaxies. The [O III] and Lyα emission-line luminosities suggest that the ionized gas mass lies in the range of 2 × 10^8 - 10^9 M_☉. The O/H ratio in the nebula is at least a few tenths solar, and may be as high as a factor of 3 above solar, indicating a previous phase of star formation in 4C +19.71. Thirty-four percent of the total K-band flux is contributed by the 5007 Å emission line, and the continuum of 4C +19.71 has a K ~ 19.6 mag. This places 4C +19.71 along the K-z relation found for other radio galaxies and radio-loud quasars. If the continuum is dominated by starlight, the host galaxy has a rest frame visual luminosity of about 40 L*. There are no candidate emission-line objects at the redshift of 4C +19.71 having [O III] rest frame equivalent widths of more than about 2% that of the radio galaxy itself within a volume of 212 Mpc^3