NEAR-INFRARED SPECTROSCOPY OF POWERFUL RADIO GALAXIES AT z\ 2.2È2.6

Near-infrared spectroscopy of eight high-redshift powerful radio galaxies (jrest D 3700È6800 AŽ ) (HzPRGs) at z\ 2.2È2.6 is presented. Strong forbidden lines and Ha emission were detected in all sources ; the data show evidence that the emission lines of the HzPRGs may contribute a substantial fraction (D25%È98%) of their total observed Hand/or K-band light. Diagnostic emission-line ratiosÈ [O III] j5007/Hb versus [S II] jj6716, 6731/HaÈfor three of the eight HzPRGs are consistent with the presence of a Seyfert 2 nucleus ; the [O III] j5007/Hb and [S II] jj6716, 6731/Ha ratios and/or limits of the remaining Ðve galaxies are inconclusive. Furthermore, all six of the galaxies for which both Hand K-band spectra were obtained have observed [O III] j5007/(Ha ] [N II] jj6548, 6583) ratios consistent with Seyfert 2 ionization. Much of the inability to detect the weaker emission lines of [S II] jj6716, 6731 in three of the galaxies and Hb in any of the galaxies may be caused by moderate amounts of dust : for the two sources with previously measured Lya Ñuxes, the observed Lya/Ha ratios are D1.5, much less than the value of 16 expected for gas in a dust-free medium photoionized by a hard, nonthermal continuum. If such a discrepancy is caused solely by dust, this ratio translates into mag A V D 0.5È1.0 (depending on which extinction curveÈMilky Way, SMC, or LMCÈis used) at the rest frame optical wavelengths of the galaxies and a corresponding factor of D1.6È2.5 reduction in optical Ñux. None of the eight HzPRGs at z\ 2.2È2.6 has a broad km s~1) emission-line core, (*vFWHM [ 1500 and it is not clear whether any have broad emission-line wings. However, the near-infrared spectrum of 3C 22, a z\ 0.937 radio galaxy with 1 km luminosity comparable to that of the radio galaxies at z\ 2.2È2.6 and a radio luminosity only 3È5 times less, shows direct evidence for broad Ha emission wings. Such a feature is indicative of the presence of a partially obscured Seyfert 1 nucleus. Given that 3C 22 is at the luminosity distance of the sample of HzPRGs at z\ 2.2È2.6, a thorough search for such a D13 faint feature in the more distant galaxies may require 8È10 m class telescopes. These new data, along with recent UV-to-optical polarimetry showing evidence of high polarization in many HzPRGs, provide evidence that many HzPRGs are predominantly ionized by an active nucleus, and that a signiÐcant fraction of their spectral energy distribution may be caused by nonthermal emission from an active galactic nucleus (AGN). Subject headings : galaxies : active È galaxies : ISM È galaxies : Seyfert È infrared : galaxies È radio continuum: galaxies