$M_*/L$ gradients driven by IMF variation: Large impact on dynamical stellar mass estimates
Résumé
Within a galaxy the stellar mass-to-light ratio ϒ_* is not constant. Recent studies of spatially resolved kinematics of nearby early-type galaxies suggest that allowing for a variable initial mass function (IMF) returns significantly larger ϒ_* gradients than if the IMF is held fixed. We show that ignoring such IMF-driven ϒ_* gradients can have dramatic effect on dynamical ($$M_*^{\rm dyn}$$), though stellar population ($$M_*^{\rm SP}$$) based estimates of early-type galaxy stellar masses are also affected. This is because $$M_*^{\rm dyn}$$ is usually calibrated using the velocity dispersion measured in the central regions (e.g. R_e/8) where stars are expected to dominate the mass (i.e. the dark matter fraction is small). On the other hand, $$M_*^{\rm SP}$$ is often computed from larger apertures (e.g. using a mean ϒ_* estimated from colours). If ϒ_* is greater in the central regions, then ignoring the gradient can overestimate $$M_*^{\rm dyn}$$ by as much as a factor of two for the most massive galaxies. Large ϒ_*-gradients have four main consequences: First, $$M_*^{\rm dyn}$$ cannot be estimated independently of stellar population synthesis models. Secondly, if there is a lower limit to ϒ_* and gradients are unknown, then requiring $$M_*^{\rm dyn}=M_*^{\rm SP}$$ constrains them. Thirdly, if gradients are stronger in more massive galaxies, then accounting for this reduces the slope of the correlation between $$M_*^{\rm dyn}/M_*^{\rm SP}$$ of a galaxy with its velocity dispersion. In particular, IMF-driven gradients bring $$M_*^{\rm dyn}$$ and $$M_*^{\rm SP}$$ into agreement, not by shifting $$M_*^{\rm SP}$$ upwards by invoking constant bottom-heavy IMFs, as advocated by a number of recent studies, but by revising $$M_*^{\rm dyn}$$ estimates in the literature downwards. Fourthly, accounting for ϒ_* gradients changes the high-mass slope of the stellar mass function $$\phi (M_*^{\rm dyn})$$, and reduces the associated stellar mass density. These conclusions potentially impact estimates of the need for feedback and adiabatic contraction, so our results highlight the importance of measuring ϒ_* gradients in larger samples.
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