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