SDSS-IV MaNGA: Environmental dependence of the Mgb/<Fe>-sigma_* relation
  for nearby galaxies
  We use a sample of ~3000 galaxies from the MaNGA MPL-7 internal data release
to study the alpha abundance distribution within low-redshift galaxies. We use
the Lick index ratio Mgb/<Fe> as an alpha abundance indicator to study
relationships between the alpha abundance distribution and galaxy properties
such as effective stellar velocity dispersion within 0.3 effective radii
(sigma_*), galaxy environment, and dark matter halo formation time (z_f). We
find that (1) all galaxies show a tight correlation between Mgb/<Fe> and
sigma_*; (2) `old' (H_beta < 3) low-sigma_* galaxies in high local density
environment and inner regions within galaxy groups are enhanced in Mgb/<Fe>,
while `young' (H_beta>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; (4) Mgb/<Fe> gradients are close to zero and
show dependence on sigma_* 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.
