  We re-examine the assembly history of field spheroidals as a potentially
powerful discriminant of galaxy formation models. Whereas monolithic collapse
and hierarchical, merger-driven, models suggest radically different histories
for these galaxies, neither the theoretical predictions nor the observational
data for field galaxies have been sufficiently reliable for precise conclusions
to be drawn. A major difficulty in interpreting the observations, reviewed
here, concerns the taxonomic definition of spheroidals in merger-based models.
Using quantitative measures of recent star formation activity drawn from the
internal properties of a sample of distant field galaxies in the Hubble Deep
Fields, we undertake a new analysis to assess the continuous formation of
spheroidal galaxies. Whereas abundances and redshift distributions of modelled
spheroidals are fairly insensitive to their formation path, we demonstrate that
the distribution and amount of blue light arising from recent mergers provides
a more sensitive approach. With the limited resolved data currently available,
the rate of mass assembly implied by the observed colour inhomogeneities is
compared to that expected in popular Lambda-dominated cold dark matter models
of structure formation. These models produce as many highly inhomogeneous
spheroidals as observed, but underpredict the proportion of homogeneous,
passive objects. We conclude that colour inhomogeneities, particularly when
combined with spectroscopic diagnostics for large, representative samples of
field spheroidals, will be a more valuable test of their physical assembly
history than basic source counts and redshift distributions. Securing such data
should be a high priority for the Advanced Camera for Surveys on Hubble Space
Telescope.
