  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.
