The blue companion of 3C 65: A star-forming galaxy with a probable redshift of 2.8

We present BVRIK images and photometry of the z = 1.175 radio galaxy 3C 65 and its extended blue companion, along with a 6 cm radio map. These data are used to test the hypotheses that the companion is scattering the continuum of a hidden active nucleus within the radio galaxy and/or is undergoing a starburst triggered by interaction with its radio plasma. However, we find that the object's spectral energy distribution (SED) is best fitted by a model of constant star formation with moderate dust extinction for a redshift of approximately 2.8, while no fit to the SED can be obtained for redshifts equal to or below that of 3C 65. The object thus appears to be part of the recently discovered population of star-forming galaxies at z similar to 3, which is also consistent with its morphology and size as revealed by Hubble Space Telescope WFPC2 images. The age of the object is not well-constrained by the model fits and could range from 0.2 to 3.5 Gyr. Our best-fitting model has an age of 1.4 Gyr, which at z congruent to 2.8 does not violate standard cosmologies.