Dynamical stellar mass-to-light ratio gradients: Evidence for very centrally concentrated IMF variations in ETGs?

Abstract

Evidence from different probes of the stellar initial mass function (IMF) of massive early-type galaxies (ETGs) has repeatedly converged on IMFs more bottom-heavy than in the Milky Way (MW). This consensus has come under scrutiny due to often contradictory results from different methods on the level of individual galaxies. In particular, a number of strong lensing probes are ostensibly incompatible with a non-MW IMF. Radial gradients of the IMF -- related to gradients of the stellar mass-to-light ratio $\hat{I}\yen$ -- can potentially resolve this issue. We construct Schwarzschild models allowing for $\hat{I}\yen$-gradients in seven massive ETGs with MUSE and SINFONI observations. We find dynamical evidence that $\hat{I}\yen$ increases towards the center for all ETGs. The gradients are confined to sub-kpc scales. Our results suggest that constant-$\hat{I}\yen$ models may overestimate the stellar mass of galaxies by up to a factor 1.5. For all except one galaxy, we find a radius where the total dynamical mass has a minimum. This minimum places the strongest constraints on the IMF outside the center and appears at roughly 1 kpc. We consider the IMF at this radius characteristic for the main body of each ETG. In terms of the IMF mass-normalization $\hat{I}\pm$ relative to a Kroupa IMF, we find on average a MW-like IMF $<\hat{I}{\pm }_{main}>=1.03\pm 0.19$. In the centers, we find concentrated regions with increased mass normalizations that are less extreme than previous studies suggested, but still point to a Salpeter-like IMF, $<\hat{I}{\pm }_{cen}>=1.54\pm 0.15$

38 pages, 16 figures, accepted for publication in ApJ