Galaxy properties as revealed by MaNGA II. Differences in stellar
  populations of slow and fast rotator ellipticals and dependence on
  environment
  We present estimates of stellar population (SP) gradients from stacked
spectra of slow (SR) and fast (FR) rotator elliptical galaxies from the
MaNGA-DR15 survey. We find that: 1) FRs are $\sim 5$ Gyrs younger, more metal
rich, less $\alpha$-enhanced and smaller than SRs of the same luminosity $L_r$
and central velocity dispersion $\sigma_0$. This explains why when one combines
SRs and FRs, objects which are small for their $L_r$ and $\sigma_0$ tend to be
younger. Their SP gradients are also different. 2) Ignoring the FR/SR dichotomy
leads one to conclude that compact galaxies are older than their larger
counterparts of the same mass, even though almost the opposite is true for FRs
and SRs individually. 3) SRs with $\sigma_0\le 250$ km s$^{-1}$ are remarkably
homogeneous within $\sim R_e$: they are old, $\alpha$-enhanced and only
slightly super-solar in metallicity. These SRs show no gradients in age and
$M_*/L_r$, negative gradients in metallicity, and slightly positive gradients
in [$\alpha$/Fe] (the latter are model dependent). SRs with $\sigma_0\ge 250$
km $s^{-1}$ are slightly younger and more metal rich, contradicting previous
work suggesting that age increases with $\sigma_0$. They also show larger
$M_*/L_r$ gradients. 4) Self-consistently accounting for $M_*/L$ gradients
yields $M_{\rm dyn}\approx M_*$ because gradients reduce $M_{\rm dyn}$ by $\sim
0.2$ dex while only slightly increasing the $M_*$ inferred using a Kroupa (not
Salpeter) IMF. 5) The FR population all but disappears above $M_*\ge 3\times
10^{11}M_\odot$; this is the same scale at which the size-mass correlation and
other scaling relations change. Our results support the finding that this is an
important mass scale which correlates with the environment and above which
mergers matter.
