Near‐Infrared Spectroscopy of Powerful Radio Galaxies atz= 2.2–2.6

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Abstract
Near-infrared spectroscopy (λrest ~ 3700-6800 Å) of eight high-redshift powerful radio galaxies (HzPRGs) at z = 2.2-2.6 is presented. Strong forbidden lines and Hα emission were detected in all sources; the data show evidence that the emission lines of the HzPRGs may contribute a substantial fraction (~25%-98%) of their total observed H- and/or K-band light. Diagnostic emission-line ratios— [O III] λ5007/Hβ versus [S II] λλ6716, 6731/Hα—for three of the eight HzPRGs are consistent with the presence of a Seyfert 2 nucleus; the [O III] λ5007/Hβ and [S II] λλ6716, 6731/Hα ratios and/or limits of the remaining five galaxies are inconclusive. Furthermore, all six of the galaxies for which both H- and K-band spectra were obtained have observed [O III] λ5007/(Hα + [N II] λλ6548, 6583) ratios consistent with Seyfert 2 ionization. Much of the inability to detect the weaker emission lines of [S II] λλ6716, 6731 in three of the galaxies and Hβ in any of the galaxies may be caused by moderate amounts of dust: for the two sources with previously measured Lyα fluxes, the observed Lyα/Hα ratios are ~1.5, 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 AV ~ 0.5-1.0 mag (depending on which extinction curve—Milky Way, SMC, or LMC—is used) at the rest frame optical wavelengths of the galaxies and a corresponding factor of ~1.6-2.5 reduction in optical flux. None of the eight HzPRGs at z = 2.2-2.6 has a broad (ΔvFWHM > 1500 km s-1) emission-line core, 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 μm luminosity comparable to that of the radio galaxies at z = 2.2-2.6 and a radio luminosity only 3-5 times less, shows direct evidence for broad Hα emission wings. Such a feature is indicative of the presence of a partially obscured Seyfert 1 nucleus. Given that 3C 22 is at ~1/3 the luminosity distance of the sample of HzPRGs at z = 2.2-2.6, a thorough search for such a faint feature in the more distant galaxies may require 8-10 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 significant fraction of their spectral energy distribution may be caused by nonthermal emission from an active galactic nucleus (AGN).
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