Deep imaging of the field of the high-redshift radio source 4C 23.56

We present deep infrared and optical imaging of a 1.25 arcmin(2) field centered on the giant (similar to 0.5 Mpc) high-redshift (z = 2.479) ultra-steep-spectrum radio source 4C 23.56. The optical/infrared continuum morphology of the host galaxy is asymmetric, with strong color gradients and an enormous biconical emission-line nebula. The morphology of 4C 23.56 is best described by a dusty galaxy illuminated by a beam from an active galactic nucleus that is scattered into our line of sight. We determine the K' band number counts in the field to a limit of K' = 22.5 mag and find a marginally significant excess of objects with K' = 19-21 mag, compared to other K' band surveys. Spectral energy distributions (SEDs) of 117 objects in the field are constructed from aperture photometry in five bands (U', NB, V, I, and K'). From the SEDs we distinguish three subsets of objects as possible candidates for z similar to 2.48 companions: emission-line galaxies, very red galaxies, and flat-spectrum star-forming galaxies. The field has nine candidate objects with line emission in a narrow band centered on the redshifted Ly alpha line, compared to an expected number of at most three foreground CO rrl emitters that could mimic Ly alpha sources. We find a high density of faint, very red objects with I-K' > 4 mag in the field, including two with I-K' > 6 mag.