  A constant stellar-mass to light ratio $M_{\star}/L$ has been widely-used in
studies of galaxy dynamics and strong lensing, which aim at disentangling the
mass density distributions of dark matter and baryons. In this work, we take
early-type galaxies from the cosmological hydrodynamic IllustrisTNG-100
simulation to investigate possible systematic bias in the inferences due to a
constant $M_{\star}/L$ assumption. To do so, we construct two-component matter
density models, where one component describes the dark matter distribution, the
other one for the stellar mass, which is made to follow the light profile by
assuming a constant factor of $M_{\star}/L$. Specifically, we adopt multiple
commonly used dark matter models and light distributions. We fit the
two-component models directly to the {\it total} matter density distributions
of simulated galaxies to eliminate systematics from other modelling procedures.
We find that galaxies in general have more centrally-concentrated stellar mass
profile than their light distribution. This is more significant among more
massive galaxies, for which the $M_{\star}/L$ profile rises up markedly towards
the centre and may often exhibit a dented feature due to on-going star
formation at about one effective radius, encompassing a quenched bulge region.
As a consequence, a constant $M_{\star}/L$ causes a model degeneracy to be
artificially broken under specific model assumptions, resulting in strong and
model-dependent biases on estimated properties, such as the central dark matter
fraction and the initial mass function. Either a steeper dark matter profile
with an over-predicted density fraction, or an over-predicted stellar mass
normalization ($M_{\star}/L$) is often obtained through model fitting. The
exact biased behaviour depends on the slope difference between mass and light,
as well as on the adopted models for dark matter and light.

Astrophysics of Galaxies