Resolving the Stellar Populations in a z=4 Lensed Galaxy^*

We present deep near-infrared Keck Near-Infrared Camera (NIRC) imaging of a recently discovered z = 4.04 galaxy. This is lensed by the rich foreground cluster Abell 2390 (z ≈ 0.23) into highly magnified arcs 3''-5'' in length. Our H and K' band NIRC imaging allows us to map the Balmer+4000 Å break amplitude. In combination with high-quality archival Hubble Space Telescope (HST) WFPC2 data, we can spatially resolve stellar populations along the arcs. The WFPC2 images clearly reveal several bright knots, which correspond to sites of active star formation. However, there are considerable portions of the arcs that are significantly redder, consistent with being observed ≳100 Myr after star formation has ceased. Keck Low-Resolution Imaging Spectrograph (LRIS) long-slit spectroscopy along the arcs reveals that the Lyα emission is spatially offset by ≈1'' from the rest-UV continuum regions. We show that this line emission is most probably powered by star formation in neighboring H II regions and that the z = 4 system is unlikely to be an active galactic nucleus.