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We measure the evolution in the intrinsic shape distribution of early-typegalaxies 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 withvery low star-formation rates from the SDSS, based on a color-color selectionscheme and verified through the absence of emission lines in the spectra. Theinferred intrinsic shape distribution of these early-type galaxies is stronglymass dependent: the typical short-to-long intrinsic axis-ratio of high-massearly-type galaxies (>1e11 M_sun) is 2:3, where as at masses below 1e11 M_sunthis ratio narrows to 1:3, or more flattened galaxies. In an entirely analogousmanner we select a high-redshift sample (0.6 < z < 0.8) from two deep-fieldsurveys: GEMS and COSMOS. We find a seemingly universal mass of ~1e11 M_sun forhighly flatted early-type systems at all redshifts. This implies that theprocess that grows an early-type galaxy above this ceiling mass involvesforming round systems. Using both parametric and non-parametric tests, we findno 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 of2-3x in co-moving number density for early-type galaxies at masses >3e10 M_sun,in agreement with previous studies. Given the direct connection between theaxis-ratio distribution and the underlying bulge-to-disk ratio distribution,our findings imply that the number density evolution of early-type galaxies isnot exclusively driven by the emergence of either bulge- or disk-dominatedgalaxies, but rather by a balanced mix that depends only on the stellar mass ofthe galaxy. The challenge for galaxy formation models is to reproduce thisoverall non-evolving ratio of flattened to round early-type galaxies in thecontext of a continually growing population.

A constant limiting mass scale for flat early-type galaxies from z=1 to z=0: density evolves but shapes do not