MUSE observations of M87: radial gradients for the stellar initial-mass function and the abundance of sodium

Abstract

Based on Multi-Unit Spectroscopic Explorer (MUSE) integral-field data we present evidence for a radial variation at the low-mass end of the stellar initial-mass function (IMF) in the central regions of the giant early-type galaxy NGC 4486 (M87). We used state-of-the-art stellar-population models and the observed strength of various IMF-sensitive absorption-line features to solve for the best low-mass tapered `bimodal' form of the IMF, while accounting also for variations in stellar metallicity, the overall α-elements abundance and the abundance of individual elements such as Ti, O, Na, and Ca. Our analysis reveals a strong negative IMF gradient corresponding to an exceeding fraction of low-mass stars compare(Cappellari & d to the case of the Milky Way towards the centre of M87, which drops to nearly Milky Way levels by 0.4 R<SUB>e</SUB>. Such IMF variations correspond to over a factor 2 increase in stellar mass-to-light M/L ratio compared to the case of a Milky Way IMF, consistent with independent constraints on M/L radial variations in M87 from dynamical models. We also looked into the abundance of sodium in M87, which turned up to be super-Solar over the entire radial range of our MUSE observations and to exhibit a considerable negative gradient. These findings suggest an additional role of metallicity in boosting the Na-yields in the central, metal-rich regions of M87 during its early and brief star formation history. Our work adds M87 to the few objects that presently have radial constraints on their IMF or [Na/Fe] abundance, while also illustrating the accuracy that MUSE could bring to this kind of investigations.