Evidence of cosmic evolution of the stellar initial mass function

Theoretical arguments and indirect observational evidence suggest that the stellar IMF may evolve with time, such that it is more weighted toward high-mass stars at higher redshift. Here we test this idea by comparing the rate of luminosity evolution of massive early-type galaxies in clusters at 0: 02 <= z <= 0: 83 to the rate of their color evolution. A combined fit to the rest-frame U-V color evolution and the previously measured evolution of the M/L-B ratio gives x = -0.3(-0.7)(+0.4) for the logarithmic slope of the IMF in the region around 1M circle dot, significantly flatter than the present-day value in the Milky Way disk of x = 1.3 +/- 0.3. The best-fitting luminosity-weighted formation redshift of the stars in massive cluster galaxies is 3.7(-0.8)(+2.3) and a possible interpretation is that the characteristic mass mc had a value of similar to 2M circle dot at z similar to 4 (compared to m(c) similar to 0.1 M circle dot today), in qualitative agreement with models in which the characteristic mass is a function of the Jeans mass in molecular clouds. Such a bottom-light'' IMF for massive cluster galaxies has significant implications for the interpretation of measurements of galaxy formation and evolution. Applying a simple form of IMF evolution to literature data, we find that the volume-averaged SFR at high redshift may have been overestimated (by a factor of 3-4 at z > 4), and the cosmic star formation history may have a fairly well defined peak at z similar to 1.5. TheM/L-V ratios of galaxies are less affected than their SFRs, and future data on the stellar mass density at z > 3 will provide further constraints on IMF evolution. The formal errors likely underestimate the uncertainties, and confirmation of these results requires a larger sample of clusters and the inclusion of redder rest-frame colors in the analysis.