Chemically peculiar stars in integrated light stellar population models and local group galaxies

Abstract

ABSTRACT Integrated light models that incorporate common types of chemically peculiar (CP) stars are assembled using synthetic spectra. Selected spectral features encode significant age information for populations with ages ∼50 Myr < age < ∼2 Gyr. Due to the alleviation of template mismatch, the inclusion of CP star features in model spectra improves the accuracy of recovered stellar population parameters, but we are not able to show that new or unique age information can be extracted from the weak CP features compared to continuum fitting and strong-feature strengths, at least at the present state of the art. An age-extraction routine that recovers two- and three-burst age structures is employed to analyse the spectra of local group galaxies. NGC 224 (M 31) has a stellar population too old for the types of CP stars we examine. NGC 221 (M 32) also shows no CP spectral features. It appears to contain a component at age ∼1 Gyr at 1 per cent by mass in addition to its dominant 4.7 Gyr population. Unlike Sloan Digital Sky Survey galaxy spectrum averages, NGC 205 (M 110) contains no features due to mercury–manganese (HgMn) stars. This excludes the age range associated with HgMn production, and its near-nuclear spectrum is best fitted by a 68 ± 2 Myr population superimposed on an older population with a 1.85 ± 0.1 Gyr component. Both NGC 205 and NGC 221 have an ancient component whose mass is not easy to constrain given the overwhelming light dominance of the younger populations.