Triggered Star Formation in a Massive Galaxy at z = 3.8: 4C 41.17^*

We present deep spectropolarimetric observations obtained with the W. M. Keck Telescope of the very high redshift (z = 3.79786 ± 0.0024) radio galaxy 4C 41.17. We find that the bright, spatially extended rest-frame UV continuum emission from this galaxy, which is aligned with the radio axis, is unpolarized (P_2σ < 2.4%). This implies that scattered AGN light, which is generally the dominant contributor to the rest-frame UV emission in z ~ 1 radio galaxies, is unlikely to be a major component of the UV flux from 4C 41.17. The resulting total light spectrum shows absorption lines and P Cygni-like features that are similar to those detected in the spectra of the recently discovered population of star forming galaxies at slightly lower (z ~ 2-3) redshifts. It may be possible for a galactic outflow to contribute partially to the absorption line profiles of the low ionization species; however, it is unlikely that the high-velocity wings of the high ionization lines are dominated by a galactic wind since the outflow mass implied by the absorption line strengths is very large. The detection of the S V λ1502 stellar photospheric absorption line, the shape of the blue wing of the Si IV profile, the unpolarized continuum emission, the inability of any AGN-related processes to account for the UV continuum flux, and the overall similarity of the UV continuum spectra of 4C 41.17 and the nearby star-forming region NGC 1741B1 strongly suggest that the UV light from 4C 41.17 is dominated by young, hot stars. If all of the UV emission is due to starlight from a young population, the implied star formation rate is roughly 140-1100 h⁻²₅₀ M_☉ yr^-1. The deep spectroscopy presented here combined with the morphology of the system at radio and optical wavelengths and the possibly comparable ages for the radio source structure and the UV stellar population suggest that star formation in 4C 41.17 was triggered by the expansion of the radio source into the ambient medium. Our current observations are consistent with the hypothesis that 4C 41.17 is undergoing its major epoch of star formation at z ~ 4, and that by z ~ 1 it will have evolved to have spectral and morphological properties similar to those observed in known z ~ 1 powerful radio galaxies.