Element Abundance Ratios in Spiral and Irregular Galaxies

Element Abundance Ratios in Spiral and Irregular Galaxies 

The abundance ratios of heavy elements are sensitive to the initial mass function (IMF), the star formation history, and variations in stellar nucleosynthesis with, e.g., metallicity. In particular, comparison of abundances of elements produced in stars with relatively long lifetimes (such as C, N, Fe, and the s-process elements) with those produced in short-lived stars (such as O) probe the star formation history. Below, I review the accumulated data on C, N, S, and Ar abundances (relative to O) in spiral and irregular galaxies, covering two orders of magnitude in metallicity (as measured by O/H). The data are taken from a variety of sources on abundances for H II regions in the literature. 

Helium, the second most abundant element, has significance for cosmology and stellar structure. Most 4He was produced in the Big Bang, and the primordial mass fraction Yp is a constraint on the photon/baryon ratio and thus on the cosmological model. The He mass fraction also affects stellar structure, but He is difficult to measure in stars and so must be inferred from other measurements. On the other hand, He I recombination lines are relatively easy to measure in H II regions, and so a large amount of data is available on He/H in ionized nebulae. 

Donald R. Garnett Element Abundances In Nearby Galaxies 

Depending on which estimate is considered most reliable, Yp either agrees with the best current estimate of D/H under standard Big Bang nucleosynthesis (for Yp = 0.244, or it does not). 

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