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 can not 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 .