Spatially Resolved High-Ionization Nebulae in a High-Redshift Radio Galaxy: Evidence for Shock Ionization and Photoionization

Abstract

We present high-resolution, long-slit spectroscopic data obtained from the W. M. Keck I Telescope of one of the most distant radio galaxies reported to date: MG 2141+192 (B1950, z = 3.594 ± 0.003). Line emission from Lyα λ1215.7, C IV λ1549.0, and He II λ1640.5 spans an unusually large spatial extent (~120 kpc), continuing well beyond the radio lobes. We also report a large velocity field of multiple components spanning an FWZI of ~3200 km s-1. Such spectacular extended high-ionization emission of C IV and He II is rare in the published literature. The optical host galaxy is aligned with the radio axis (ΔP.A. ≈ 5°) of this Fanaroff-Riley type II radio galaxy, as is commonly observed in powerful and/or high-redshift radio galaxies. The line-emission nebulae have dissimilar velocity fields, suggesting that their origin is unlikely to be dominated by radiation from a reflected broadline region, but rather that from radiating turbulent gas. Our spatially resolved data provide information to probe the underlying energetics that power the rest-frame UV line emission. Previous modeling of radio galaxy ionization nebulae generally considered two scenarios: photoionization from a central active nucleus and shock ionization possibly resulting from radio jet interactions with the host galaxy environs. Integrated flux ratios of UV lines have frequently been used to distinguish between models. This work spatially resolves high-ionization lines to allow registered two-dimensional line ratio maps as an additional diagnostic. Comparing these maps with available shock and photoionization models reveals evidence for separate shock-ionized and photoionized emission regions. Our results imply that multiple ionization mechanisms may contribute to the line emission in active galactic nuclei, which must be carefully taken into consideration, particularly in analyses relying on total line fluxes.