Wolf-Rayet Galaxies in SDSS-IV MaNGA. II. Metallicity Dependence of the High-mass Slope of the Stellar Initial Mass Function

As hosts of living high-mass stars, Wolf-Rayet (WR) regions or WR galaxies are ideal objects for constraining the high-mass end of the stellar initial mass function (IMF). We construct a large sample of 910 WR galaxies/regions that cover a wide range of stellar metallicity (from Z similar to 0.001 to 0.03) by combining three catalogs of WR galaxies/regions previously selected from the SDSS and SDSS-IV/MaNGA surveys. We measure the equivalent widths of the WR blue bump at similar to 4650 angstrom for each spectrum. They are compared with predictions from stellar evolutionary models Starburst99 and BPASS, with different IMF assumptions (high-mass slope alpha of the IMF ranging from 1.0 to 3.3). Both singular evolution and binary evolution are considered. We also use a Bayesian inference code to perform full spectral fitting to WR spectra with stellar population spectra from BPASS as fitting templates. We then make a model selection among different alpha assumptions based on Bayesian evidence. These analyses have consistently led to a positive correlation of the IMF high-mass slope alpha with stellar metallicity Z, i.e., with a steeper IMF (more bottom-heavy) at higher metallicities. Specifically, an IMF with alpha = 1.00 is preferred at the lowest metallicity (Z similar to 0.001), and an Salpeter or even steeper IMF is preferred at the highest metallicity (Z similar to 0.03). These conclusions hold even when binary population models are adopted.

Automated summary

The research shows a positive correlation between the high-mass slope of the stellar initial mass function and stellar metallicity, with steeper initial mass functions preferred at higher metallicities. This correlation remains consistent even when binary population models are considered.