Stellar populations of massive early-type galaxies observed by MUSE

Abstract

ABSTRACT Stellar population studies of massive early-type galaxies (ETGs) suggest that the stellar initial mass function may not be universal. In particular, the centres of ETGs seem to contain an excess of low-mass dwarf stars compared to our own Galaxy. Through high-resolution data from MUSE (Multi Unit Spectroscopic Explorer), we carry out a detailed study of the stellar populations of eight massive ETGs. We use full spectrum fitting to determine ages, element abundances, and initial mass function (IMF) slopes for spatially binned spectra. We measure flat gradients in age and [Mg/Fe] ratio, as well as negative gradients in metallicity and [Na/Fe]. We detect IMF gradients in some galaxies, with the centres hosting bottom-heavy IMFs and mass excess factors between 1.5 and 2.5 compared to a Kroupa IMF. The IMF slope below 0.5 M$_\odot$varies for our galaxy sample between 1 and 2.8, with negative radial gradients, while the IMF slope between 0.5 and 1M$_\odot$ has a steep value of $\sim$3 with mildly positive gradients for most galaxies. For M87, we find excellent agreement with the dynamical mass-to-light ratio (M/L) as a function of radius. For the other galaxies, we find systematically higher M/L from stellar populations compared to orbit-based dynamical analysis of the same data. This discrepancy increases with NaI strength, suggesting a combination of calibration issues of this line and correlated uncertainties.