  The optical-IR images of the Northern and Southern Hubble Deep Fields are
used to measure the spectral and density evolution of early-type galaxies. The
mean optical SED is found to evolve passively towards a mid F-star dominated
spectrum by z ~ 2. We demonstrate with realistic simulations that hotter
ellipticals would be readily visible if evolution progressed blueward and
brightward at z > 2, following a standard IMF. The colour distributions are
best fitted by a `red' IMF, deficient above ~2 M_solar and with a spread of
formation in the range 1.5 < z_f < 2.5. Traditional age dating is spurious in
this context, a distant elliptical can be young but appear red, with an
apparent age >3 Gyrs independent of its formation redshift. Regarding density
evolution, we demonstrate that the sharp decline in numbers claimed at z > 1
results from a selection bias against distant red galaxies in the optical,
where the flux is too weak for morphological classification, but is remedied
with relatively modest IR exposures revealing a roughly constant space density
to z ~ 2. We point out that the lack of high mass star-formation inferred here
and the requirement of metals implicates cooling-flows of pre-enriched gas in
the creation of the stellar content of spheroidal galaxies. Deep-field X-ray
images will be very helpful to examine this possibility.

Astrophysics