2008ApJ...674...29V

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^+0.4_–0.7for the logarithmic slope of the IMF in the region around 1 M_☉, 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^+2.3_–0.8, and a possible interpretation is that the characteristic mass m_c had a value of ∼2 M_☉at z∼4 (compared to m_c∼0.1 M_☉ 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∼1.5. The M/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.