How Do Star-Forming Galaxies at Z>3 Assemble Their Masses?
  We investigate how star-forming galaxies typically assemble their masses at
high redshift. Using the deep multi-wavelength coverage of the GOODS dataset,
we measure stellar mass of a large sample of star-forming galaxies at z~4 and
5, and make a robust determination of stellar mass function (SMF). We report a
broad correlation between stellar mass and UV luminosity, such that more
UV-luminous galaxies are, on average, more massive. However, the correlation
has a substantial intrinsic scatter evidenced by a non-negligible number of
UV-faint but massive galaxies. Furthermore, the low-mass end of the SMF does
not rise as steeply as the UV luminosity function (alpha_UVLF} -(1.7-1.8),
alpha_SMF -(1.3-1.4)) of the same galaxies. In a smooth formation scenario
where star formation (SF) is sustained at the observed rates for a long time,
these galaxies would have accumulated more mass (by a factor of ~3) than
observed and therefore the SMF would mirror more closely that of the UVLF. The
relatively shallow slope of the SMF is due to the fact that many of the
UV-selected galaxies are not massive enough, and therefore are too faint in
their rest-frame optical bands, to be detected in the current observations. Our
results favor a episodic formation history in which SF in low-mass galaxies
vary significantly over time, a scenario favored by galaxy clustering. Our
findings for the UV-faint galaxies are in contrast with those found for more
UV-luminous galaxies, which exhibit tighter SFR-M_star correlations. The
discrepancy may suggest that galaxies at different luminosities may have
different evolutionary paths.
