Galaxy structure from multiple tracers: III. Radial variations in M87's IMF

TL;DR

This study uses multiple dynamical tracers to constrain the radial variation of the stellar mass-to-light ratio in M87, revealing a declining ratio consistent with an IMF that varies from Salpeter-like in the core to Chabrier-like outward.

Contribution

First to constrain stellar mass-to-light ratio gradients in an ETG using multiple tracers, indicating a non-universal, radially varying IMF in M87.

Findings

Radially declining stellar mass-to-light ratio in M87.

IMF likely varies from Salpeter-like in the center to Chabrier-like outward.

Steep gradient cannot be explained by metallicity, age, dust, or star formation history alone.

Abstract

We present the first constraints on stellar mass-to-light ratio gradients in an early-type galaxy (ETG) using multiple dynamical tracer populations to model the dark and luminous mass structure simultaneously. We combine the kinematics of the central starlight, two globular cluster populations and satellite galaxies in a Jeans analysis to obtain new constraints on M87's mass structure, employing a flexible mass model which allows for radial gradients in the stellar mass-to-light ratio. We find that, in the context of our model, a radially declining stellar-mass-to-light ratio is strongly favoured. Modelling the stellar mass-to-light ratio as following a power law, $\Upsilon_{\star} \sim R^{-\mu}$, we infer a power-law slope $\mu = -0.54 \pm 0.05$; equally, parameterising the stellar-mass-to-light ratio via a central mismatch parameter relative to a Salpeter IMF, $\alpha$, and scale radius $R_M$, we find $\alpha > 1.48$ at $95\%$ confidence and $R_M = 0.35 \pm 0.04$ kpc. We use stellar population modelling of high-resolution 11-band HST photometry to show that such a steep gradient cannot be achieved by variations in only the metallicity, age, dust extinction and star formation history if the stellar initial mass function (IMF) remains spatially constant. On the other hand, the stellar mass-to-light ratio gradient that we find is consistent with an IMF whose inner slope changes such that it is Salpeter-like in the central $\sim 0.5$ kpc and becomes Chabrier-like within the stellar effective radius. This adds to recent evidence that the non-universality of the IMF in ETGs may be confined to their core regions, and points towards a picture in which the stars in these central regions may have formed in fundamentally different physical conditions.