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

• Published in: The Astrophysical journal Vol. 923; no. 1; pp. 120 - 135
• Main Authors: Liang, Fu-Heng, Li, Cheng, Li, Niu, Zhou, Shuang, Yan, Renbin, Mo, Houjun, Zhang, Wei
• Format: Journal Article
• Language: English
• Published: Philadelphia The American Astronomical Society 01.12.2021; IOP Publishing
• Subjects: Astronomical models; Astronomy & Astrophysics; ASTRONOMY AND ASTROPHYSICS; Astrophysics; Bayesian analysis; Binary stars; Galaxies; Galaxy evolution; Initial mass function; Massive stars; Metallicity; Slopes; Spectra; Star formation; Starburst galaxies; Stars & galaxies; Statistical inference; Stellar evolution; Stellar evolutionary models; Stellar models; Wolf-Rayet stars
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/ac2bff

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 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 ∼4650 Å for each spectrum. They are compared with predictions from stellar evolutionary models Starburst99 and BPASS , with different IMF assumptions (high-mass slope α 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 α assumptions based on Bayesian evidence. These analyses have consistently led to a positive correlation of the IMF high-mass slope α with stellar metallicity Z , i.e., with a steeper IMF (more bottom-heavy) at higher metallicities. Specifically, an IMF with α = 1.00 is preferred at the lowest metallicity ( Z ∼ 0.001), and an Salpeter or even steeper IMF is preferred at the highest metallicity ( Z ∼ 0.03). These conclusions hold even when binary population models are adopted.