Young Stars and Nonstellar Emission in the Aligned Radio Galaxy 3C 256

Abstract

We present ground-based images of the z=1.824 radio galaxy 3C 256 in the standard BVRIJHK filters and an interference filter centered at 8800 Å, a Hubble Space Telescope image in a filter dominated by Lyα emission (F336W), and spectra covering rest-frame wavelengths from Lyα to [O III] λ5007. Together with published polarimetry observations, we use these to decompose the overall spectral energy distribution into nebular continuum emission, scattered quasar light, and stellar emission. The nebular continuum and scattered light together comprise half (one-third) of the V-band (K-band) light within a 4" aperture and are responsible for the strong alignment between the optical/near-infrared light and the radio emission. The stellar emission is dominated by a population estimated to be 100-200 Myr old (assuming a Salpeter IMF) and formed in a short burst with a peak star formation rate of 1-4x103 Msolar yr-1. The total stellar mass is estimated to be no more than 2x1011 Msolar, which is far less than other luminous radio galaxies at similar redshifts, and suggests that 3C 256 will undergo further star formation or mergers.