SDSS-IV MaNGA: The [$\alpha$/Fe] of Early-Type Galaxies
  The mean stellar alpha-to-iron abundance ratio ([$\alpha$/Fe]) of a galaxy is
an indicator of galactic star formation timescale. It is important for
understanding the star formation history of early-type galaxies (ETGs) as their
star formation processes have basically stopped. Using the model templates
which are made by Vazdekis et al., we apply the pPXF based spectral fitting
method to estimate the [$\alpha$/Fe] of 196 high signal-to-noise ratio ETGs
from the MaNGA survey. The velocity dispersions within 1R$_e$ ($\sigma_{e}$)
range from 27 to 270 km/s. We find a flat relation between the mean
[$\alpha$/Fe] within the 1R$_e^{maj}$ ellipses and log($\sigma_{e}$), even if
limiting to the massive sample with log($\sigma_{e}$/km s$^{-1}$)$>$1.9.
However, the relation becomes positive after we exclude the Mg$_1$ feature in
our fits, which agrees with the results from the previous work with other
stellar population models, albeit with relatively large scatter. It indicates
that the spectral fits with Vazdekis models could give basically the consistent
predictions of [$\alpha$/Fe] with previous studies when the Mg$_b$ index is
used, but do not work well at the Mg$_1$ band when their $\alpha$-enhanced
version is employed in the metal-rich regime. We suggest avoiding this rather
wide index, which covers 471\AA, as it might suffer from other effects such as
flux-calibration issues. For reference, we also measure the stellar population
radial gradients within 1R$_e^{maj}$ ellipses. Due to the low resolution of age
estimations for old objects and the Mg$_1$ issue, the uncertainties of these
gradients cannot be neglected.
