The Fine‐Structure Constant as a Probe of Chemical Evolution and Asymptotic Giant Branch Nucleosynthesis in Damped Lyα Systems

Abstract

Evidence from a large sample of quasar absorption-line spectra in damped Lyα systems has suggested a possible time variation of the fine-structure constant, α. The most statistically significant portion of this sample involves the comparison of Mg and Fe wavelength shifts using the many-multiplet (MM) method. However, the sensitivity of this method to the abundance of heavy isotopes, especially Mg, is enough to imitate an apparent variation in α in the redshift range 0.5 < z < 1.8. We implement recent yields of intermediate mass (IM) stars into a chemical evolution model and show that the ensuing isotope distribution of Mg can account for the observed variation in α provided the early initial mass function was particularly rich in IM stars or that the heavy Mg isotope yields from asymptotic giant branch stars are even higher than in present-day models. As such, these observations of quasar absorption spectra can be used to probe the nucleosynthetic history of low-metallicity damped Lyα systems in the redshift range 0.5 < z < 1.8. This analysis, in conjunction with other abundance measurements of low-metallicity systems, reinforces the mounting evidence that star formation at low metallicities may have been strongly influenced by a population of IM stars. Such IM stars have a significant influence on other abundances, particularly nitrogen. We constrain our models with independent measurements of N, Si, and Fe in damped Lyα systems as well as C/O in low-metallicity stars. In this way, we obtain consistent model parameters for this chemical evolution interpretation of the MM method results.