SDSS-IV MaNGA: environmental dependence of the Mgb/⟨Fe⟩-σ_* relation for nearby galaxies

We use a sample of ~3000 galaxies from the MaNGA MPL-7 internal data release to study the α abundance distribution within low-redshift galaxies. We use the Lick index ratio Mgb/⟨Fe⟩ as an αabundance indicator to study relationships between the α abundance distribution and galaxy properties such as effective stellar velocity dispersion within 0.3 effective radii (σ_*), galaxy environment, and dark matter halo formation time (z_f ). We find that (1) all galaxies show a tight correlation between Mgb/⟨Fe⟩ and σ_*;(2) "old" (Hβ < 3) low-σ_* galaxies in high local density environment and inner regions within galaxy groups are enhanced in Mgb/⟨Fe⟩, while "young" (Hβ > 3) galaxies and high-mass galaxies show no or less environmental dependence; (3) "old" galaxies with high-z_f  show enhanced Mgb⟨Fe⟩/ over low- and medium-z_f ; and (4) Mgb/⟨Fe⟩ gradients are close to zero and show dependence on σ_* but no obvious dependence on the environment or z_f . Our study indicates that stellar velocity dispersion or galaxy mass is the main parameter driving the Mgb/⟨Fe⟩ enhancement, although environments appear to have modest effects, particularly for low- and medium-mass galaxies.