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 α abundance 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.