A constant limiting mass scale for flat early-type galaxies from z=1 to
  z=0: density evolves but shapes do not
  We measure the evolution in the intrinsic shape distribution of early-type
galaxies from z~1 to z~0 by analyzing their projected axis-ratio distributions.
We extract a low-redshift sample (0.04 < z < 0.08) of early-type galaxies with
very low star-formation rates from the SDSS, based on a color-color selection
scheme and verified through the absence of emission lines in the spectra. The
inferred intrinsic shape distribution of these early-type galaxies is strongly
mass dependent: the typical short-to-long intrinsic axis-ratio of high-mass
early-type galaxies (>1e11 M_sun) is 2:3, where as at masses below 1e11 M_sun
this ratio narrows to 1:3, or more flattened galaxies. In an entirely analogous
manner we select a high-redshift sample (0.6 < z < 0.8) from two deep-field
surveys: GEMS and COSMOS. We find a seemingly universal mass of ~1e11 M_sun for
highly flatted early-type systems at all redshifts. This implies that the
process that grows an early-type galaxy above this ceiling mass involves
forming round systems. Using both parametric and non-parametric tests, we find
no evolution in the projected axis-ratio distribution for galaxies with masses
>3e10 M_sun with redshift. At the same time, our samples imply an increase of
2-3x in co-moving number density for early-type galaxies at masses >3e10 M_sun,
in agreement with previous studies. Given the direct connection between the
axis-ratio distribution and the underlying bulge-to-disk ratio distribution,
our findings imply that the number density evolution of early-type galaxies is
not exclusively driven by the emergence of either bulge- or disk-dominated
galaxies, but rather by a balanced mix that depends only on the stellar mass of
the galaxy. The challenge for galaxy formation models is to reproduce this
overall non-evolving ratio of flattened to round early-type galaxies in the
context of a continually growing population.
