MOND and IMF variations in early-type galaxies from $ATLA{S}^{3D}$

Abstract

MOdified Newtonian dynamics (MOND) represents a phenomenological alternative to dark matter (DM) for the missing mass problem in galaxies and clusters of galaxies. We analyze the central regions of a local sample of $\sim 220$ early-type galaxies from the $\mathrm{ATLA}{\mathrm{S}}^{3D}$ survey, to see if the data can be reproduced without recourse to DM. We estimate dynamical masses in the MOND context through Jeans analysis, and compare to $\mathrm{ATLA}{\mathrm{S}}^{3D}$ stellar masses from stellar population synthesis. We find that the observed stellar mass--velocity dispersion relation is steeper than expected assuming MOND with a fixed stellar initial mass function (IMF) and a standard value for the acceleration parameter $a_0$. Turning from the space of observables to model space, a) fixing the IMF, a universal value for $a_0$ cannot be fitted, while, b) fixing $a_0$ and leaving the IMF free to vary, we find that it is "lighter" (Chabrier-like) for low-dispersion galaxies, and "heavier" (Salpeter-like) for high dispersions. This MOND-based trend matches inferences from Newtonian dynamics with DM, and from detailed analysis of spectral absorption lines, adding to the converging lines of evidence for a systematically-varying IMF.

6 pages, 3 figures, accepted for publication on MNRAS Letters, typos corrected and further references added