The High‐Redshift Radio Galaxy MRC 0406−244

Abstract

We present HST and ground-based multiwavelength observations of the distant radio galaxy MRC 0406-244. Ground-based visible and near-IR images reveal two distinct components that are aligned with the radio source axis. One of these has the red optical-to-IR and blue UV colors characteristic of radio galaxies, while the other is red in all colors. A Ly? image reveals a nebula that is 3'' ? 5'' in extent and is aligned with the radio source. The HST images reveal a spatially resolved continuum associated with the southeastern component that is aligned with the radio axis and which has a complex morphology, including a double nucleus and taillike features suggestive of a tidal origin. By comparing the luminosities of discrete knots in the extended continuum to the luminosity of radio quasars from the MRC/1 Jy survey, we make a quantitative assessment of the optical depth required for scattering models for the alignment effect. The required optical depth is difficult to produce from Thomson scattering by electrons associated with the emission-line nebulae, although our inferred ?T is within an order of magnitude of the required value. A putative intercloud medium with densities ne < 1 cm-3 fails to produce the required Thomson depth by an order of magnitude or more. The required optical depth for dust necessitates a dust-to-gas ratio comparable to the Galactic value. The spectral energy distribution of the extended continuum to the northwest of the nucleus is nearly flat in f? units and is only marginally consistent with a Thomson scattering origin. We detect an extension to the J, H, and Ksh images to the southeast along the radio axis. We believe that this is primarily line emission, the implied Ly?/H? ratio being near unity. The spectral energy distribution of the galaxy near the northwest radio lobe is better fitted by an intermediate-age population at z = 1.7 than by any plausibly aged model at z = 2.4. Thus we believe that this galaxy lies in the foreground and that there is no evidence for a large-scale infrared alignment effect in this object. The morphology of the extended continuum and the emission-line kinematics in MRC 0406-244 are highly suggestive of a merger. The spectral energy distributions of the extended continuum and the difficulty in producing the required Thomson depth lead us to favor scattering by dust in tidal features as the origin of the aligned continuum in this source.