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

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, ${{\rm Y}}_{\star }\sim R^{-\mu }$, 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.

19 pages, accepted for publication in MNRAS

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• Combining Stellar Populations with Orbit-Superposition Dynamical Modelling - the Formation History of the Lenticular Galaxy NGC 3115
• The Fornax 3D project: a two-dimensional view of the stellar initial mass function in the massive lenticular galaxy FCC 167
• Star formation quenching imprinted on the internal structure of naked red nuggets
• Modeling Nearly Spherical Pure-Bulge Galaxies with a Stellar Mass-to-Light Ratio Gradient under the $\Lambda$CDM and MOND Paradigms: II. The Orbital Anisotropy of Slow Rotators within the Effective Radius
• Initial Mass Function Variation in two Elliptical Galaxies using Near-Infrared Tracers