Evidence of Cosmic Evolution of the Stellar Initial Mass Function
Theoretical arguments and indirect observational evidence suggest that the stellar initial mass function (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 for the logarithmic slope of the IMF in the region around 1 Solar mass, flatter than the present-day value in the Milky Way disk of x=1.3. The best-fitting luminosity-weighted formation redshift of the stars in massive cluster galaxies is ~3.7, and a possible interpretation is that the characteristic mass m_c had a value of ~2 Solar masses at z~4 (compared to m_c~0.1 Solar masses 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 star formation rate 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~1.5. The M/L_V ratios of galaxies are less affected than their star formation rates, 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 thes results requires a larger sample of clusters and the inclusion of redder rest-frame colors in the analysis.