Wolf-Rayet galaxies in the Sloan Digital Sky Survey: The metallicity dependence of the initial mass function

We use a large sample of 174 Wolf-Rayet (W-R) galaxies drawn from the Sloan Digital Sky Survey to study whether and how the slope of the stellar initial mass function depends on metallicity. We calculate for each object its oxygen abundance, according to which we divide our sample into four metallicity subsamples. For each subsample, we then measure three quantities: the equivalent width of the H beta emission line, the equivalent width of the W-R bump around 4650 8, and the W-R bump-to-H beta intensity ratio, and compare to the predictions of the same quantities by evolutionary synthesis models of Schaerer & Vacca. Such comparisons lead to a clear dependence of the slope of initial mass function (alpha) on metallicity in that galaxies at higher metallicities tend to have steeper initial mass functions, with the slope index ranging from alpha similar to 1.00 for the lowest metallicity, Z = 0.001, to alpha similar to 3.30 for the highest metallicity, Z = 0.02. We have carefully examined the possible sources of systematic error, either in models or in our observational measurements, and have shown that these sources do not change this result.