The fine-structure constant as a probe of chemical evolution and asymptotic giant branch nucleosynthesis in damped Lyα systems

Evidence from a large sample of quasar absorption-line spectra in damped Lyalpha systems has suggested a possible time variation of the fine-structure constant, alpha. 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 alpha 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 alpha 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 Lyalpha 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 Lyalpha 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.