Evidence for Reduced Specific Star Formation Rates in the Centers of
  Massive Galaxies at z = 4
  We perform the first spatially-resolved stellar population study of galaxies
in the early universe (z = 3.5 - 6.5), utilizing the Hubble Space Telescope
Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS)
imaging dataset over the GOODS-S field. We select a sample of 418 bright and
extended galaxies at z = 3.5 - 6.5 from a parent sample of ~ 8000
photometric-redshift selected galaxies from Finkelstein et al. (2015). We first
examine galaxies at 3.5< z < 4.0 using additional deep K-band survey data from
the HAWK-I UDS and GOODS Survey (HUGS) which covers the 4000A break at these
redshifts. We measure the stellar mass, star formation rate, and dust
extinction for galaxy inner and outer regions via spatially-resolved spectral
energy distribution fitting based on a Markov Chain Monte Carlo algorithm. By
comparing specific star formation rates (sSFRs) between inner and outer parts
of the galaxies we find that the majority of galaxies with the high central
mass densities show evidence for a preferentially lower sSFR in their centers
than in their outer regions, indicative of reduced sSFRs in their central
regions. We also study galaxies at z ~ 5 and 6 (here limited to high spatial
resolution in the rest-frame ultraviolet only), finding that they show sSFRs
which are generally independent of radial distance from the center of the
galaxies. This indicates that stars are formed uniformly at all radii in
massive galaxies at z ~ 5 - 6, contrary to massive galaxies at z < 4.
