M*/L gradients driven by IMF variation: large impact on dynamical stellar mass estimates

Abstract

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 ( Mdyn∗ ), though stellar population (MSP∗) based estimates of early-type galaxy stellar masses are also affected. This is because Mdyn∗ is usually calibrated using the velocity dispersion measured in the central regions (e.g. Re/8) where stars are expected to dominate the mass (i.e. the dark matter fraction is small). On the other hand, MSP∗ 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 Mdyn∗ by as much as a factor of two for the most massive galaxies. Large ϒ*-gradients have four main consequences: First, Mdyn∗ cannot be estimated independently of stellar population synthesis models. Secondly, if there is a lower limit to ϒ* and gradients are unknown, then requiring Mdyn∗=MSP∗ constrains them. Thirdly, if gradients are stronger in more massive galaxies, then accounting for this reduces the slope of the correlation between Mdyn∗/MSP∗ of a galaxy with its velocity dispersion. In particular, IMF-driven gradients bring Mdyn∗ and MSP∗ into agreement, not by shifting MSP∗ upwards by invoking constant bottom-heavy IMFs, as advocated by a number of recent studies, but by revising Mdyn∗ estimates in the literature downwards. Fourthly, accounting for ϒ* gradients changes the high-mass slope of the stellar mass function ϕ(Mdyn∗) 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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