Modelling biases from constant stellar mass-to-light ratio assumption in galaxy dynamics and strong lensing

ABSTRACT 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 distributions of dark matter and baryons. However, systematic biases arising from constant $M_\star /L$ assumption have not been fully quantified. In this work, we take massive early-type galaxies from the TNG100 simulation to investigate possible systematic biases in the inferences due to a constant $M_\star /L$ assumption. We construct two-component matter density models, where one component describes the dark matter, the other for the stars, which is made to follow the light profile by assuming a constant $M_\star /L$. We fit the two-component model directly to the total matter density distributions of simulated galaxies to eliminate systematics coming from other model assumptions. We find that galaxies generally have more centrally concentrated stellar mass profile than their light distribution. Given the light profiles adopted (i.e. single- and double-Sérsic profiles), the assumption of a constant $M_\star /L$ would artificially break the model degeneracy between baryons and dark matter for non-constant$M_\star /L$ systems. For such systems, without knowing the true $M_{\star }/L$ but assuming a constant ratio, the two-component modelling procedure tend to generally overestimate $M_{\star }/L$ by $30{\small --}50~{{\ \rm per\ cent}}$, and underestimate the central dark matter fraction $f_{\rm DM}$ by $\sim 20~{{\ \rm per\ cent}}$ on average.