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 (WR) galaxies drawn from the SloanDigital Sky Survey to study whether and how the slope of the stellar initialmass function depends on metallicity. We calculate for each object its oxygenabundance according to which we divide our sample into four metallicitysubsamples. For each subsample, we then measure three quantities: theequivalent width of \hb emission line, the equivalent width of WR bump around4650\AA, and the WR bump-to-\hb intensity ratio, and compare to the predictionsof the same quantities by evolutionary synthesis models of Schaerer & Vacca.Such comparisons lead to a clear dependence of the slope of initial massfunction ($\alpha$) on metallicity in that galaxies at higher metallicitiestend to have steeper initial mass functions, with the slope index ranging from$\alpha\sim$1.00 for the lowest metallicity of $Z=0.001$ to $\alpha\sim$3.30for the highest metallicity $Z=0.02$. We have carefully examined the possiblesources of systematic error either in models or in our observationalmeasurements and shown that these sources do not change this result.Comment: 12 pages, 6 figures, ApJ accepte