分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： 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.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Lijie Liu
Martin Bureau
Guang-Xing Li
Timothy A. Davis
Dieu D. Nguyen
Fu-Heng Liang
Woorak Choi
Mark R. Smith
Satoru Iguchi
摘要： We present a study of molecular structures (clumps and clouds) in the dwarf galaxy NGC 404 using high-resolution (0.86x0.51 pc^2) Atacama Large Millimeter/sub-millimeter Array ^{12}CO(2-1) observations. We find two distinct regions in NGC 404: a gravitationally-stable central region (Toomre parameter Q=3-30) and a gravitationally-unstable molecular ring (Q<=1). The molecular structures in the central region have a steeper size -- linewidth relation and larger virial parameters than those in the molecular ring, suggesting gas is more turbulent in the former. In the molecular ring, clumps exhibit a shallower mass -- size relation and larger virial parameters than clouds, implying density structures and dynamics are regulated by different physical mechanisms at different spatial scales. We construct an analytical model of clump-clump collisions to explain the results in the molecular ring. We propose that clump-clump collisions are driven by gravitational instabilities coupled with galactic shear, that lead to a population of clumps whose accumulation lengths (i.e. average separations) are approximately equal to their tidal radii. Our model-predicted clump masses and sizes (and mass -- size relation) and turbulence energy injection rates (and size -- linewidth relation) match the observations in the molecular ring very well, suggesting clump-clump collisions is the main mechanism regulating clump properties and gas turbulence in that region. As expected, our collision model does not apply to the central region, where turbulence is likely driven by clump migration.
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