Stellar population analysis of MaNGA early-type galaxies: IMF dependence and systematic effects

We study systematics associated with estimating simple stellar population (SSP) parameters – age, metallicity [M/H], $\alpha$-enhancement [$\alpha$/Fe] and IMF shape – and associated $M_{\ast }\mathrm{/}L$ gradients, of elliptical slow rotators (E-SRs), fast rotators (E-FRs) and S0s from stacked spectra of galaxies in the MaNGA survey. These systematics arise from (i) how one normalizes the spectra when stacking; (ii) having to subtract emission before estimating absorption line strengths; (iii) the decision to fit the whole spectrum or just a few absorption lines; (iv) SSP model differences (e.g. isochrones, enrichment, IMF). The MILES+Padova SSP models, fit to the H${}_{\beta }$, $⟨$Fe$⟩$, TiO${}_{2\mathrm{S}\mathrm{D}\mathrm{S}\mathrm{S}}$ and [MgFe] Lick indices in the stacks, indicate that out to the half-light radius $R_e$: (a) ages are younger and [$\alpha$/Fe] values are lower in the central regions but the opposite is true of [M/H]; (b) the IMF is more bottom-heavy in the center, but is close to Kroupa beyond about $R_e\mathrm{/}2$; © this makes $M_{\ast }\mathrm{/}L$ about $2\times$ larger in the central regions than beyond $R_e\mathrm{/}2$. While the models of Conroy et al. (2018) return similar [M/H] and [$\alpha$/Fe] profiles, the age and (hence) $M_{\ast }\mathrm{/}L$ profiles can differ significantly even for solar abundances and a Kroupa IMF; different responses to non-solar abundances and IMF parametrization further compound these differences. There are clear (model independent) differences between E-SRs, E-FRs and S0s: younger ages and less enhanced [$\alpha$/Fe] values suggest that E-FRs and S0s are not SSPs, but relaxing this assumption is unlikely to change their inferred $M_{\ast }\mathrm{/}L$ gradients significantly.