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~0.001 up to Z~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 ~4650 A 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 up 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 model
selection among different {\alpha} assumptions based on Bayesian evidence.
These analyses have consistently led to a positive correlation of IMF high-mass
slope {\alpha} with stellar metallicity Z, i.e. with steeper IMF (more
bottom-heavy) at higher metallicities. Specifically, an IMF with {\alpha}=1.00
is preferred at the lowest metallicity (Z~0.001), and a Salpeter or even
steeper IMF is preferred at the highest metallicity (Z~0.03). These conclusions
hold even when binary population models are adopted.
