Galaxy properties as revealed by MaNGA – II. Differences in stellar populations of slow and fast rotator ellipticals and dependence on environment

Abstract

ABSTRACT We present estimates of stellar population (SP) gradients from stacked spectra of slow rotator (SR) and fast rotator (SR) elliptical galaxies from the MaNGA-DR15 survey. We find that (1) FRs are ∼5 Gyr younger, more metal rich, less α-enhanced and smaller than SRs of the same luminosity Lr and central velocity dispersion σ0. This explains why when one combines SRs and FRs, objects which are small for their Lr and σ0 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 σ0 ≤ 250 km s−1 are remarkably homogeneous within ∼Re: they are old, α-enhanced, and only slightly supersolar in metallicity. These SRs show no gradients in age and M*/Lr, negative gradients in metallicity, and slightly positive gradients in [α/Fe] (the latter are model dependent). SRs with σ0 ≥ 250 km s−1 are slightly younger and more metal rich, contradicting previous work suggesting that age increases with σ0. They also show larger M*/Lr gradients. (4) Self-consistently accounting for M*/L gradients yields Mdyn ≈ M* because gradients reduce Mdyn by ∼0.2 dex while only slightly increasing the M* inferred using a Kroupa (not Salpeter) initial mass function. (5) The SR population starts to dominate the counts above $M_*\ge 3\times 10^{11}\, \mathrm{M}_\odot$; 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 that correlates with the environment and above which mergers matter.