$M_{\ast }/L$ gradients driven by IMF variation: Large impact on dynamical stellar mass estimates

Abstract

Within a galaxy the stellar mass-to-light ratio ${{\rm Y}}_{\ast }$ is not constant. Spatially resolved kinematics of nearby early-type galaxies suggest that allowing for a variable initial mass function (IMF) returns significantly larger ${{\rm Y}}_{\ast }$ gradients than if the IMF is held fixed. If ${{\rm Y}}_{\ast }$ is greater in the central regions, then ignoring the IMF-driven gradient can overestimate $M_{\ast }^{\mathrm{dyn}}$ by as much as a factor of two for the most massive galaxies, though stellar population estimates $M_{\ast }^{\mathrm{SP}}$ are also affected. Large ${{\rm Y}}_{\ast }$-gradients have four main consequences: First, $M_{\ast }^{\mathrm{dyn}}$ cannot be estimated independently of stellar population synthesis models. Second, if there is a lower limit to ${{\rm Y}}_{\ast }$ and gradients are unknown, then requiring $M_{\ast }^{\mathrm{dyn}}=M_{\ast }^{\mathrm{SP}}$ constrains them. Third, if gradients are stronger in more massive galaxies, then $M_{\ast }^{\mathrm{dyn}}$ and $M_{\ast }^{\mathrm{SP}}$ can be brought into agreement, not by shifting $M_{\ast }^{\mathrm{SP}}$ upwards by invoking constant bottom-heavy IMFs, as advocated by a number of recent studies, but by revising $M_{\ast }^{\mathrm{dyn}}$ estimates in the literature downwards. Fourth, accounting for ${{\rm Y}}_{\ast }$ gradients changes the high-mass slope of the stellar mass function $\varphi (M_{\ast }^{\mathrm{dyn}})$, 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 ${{\rm Y}}_{\ast }$ gradients in larger samples.

13 pages, 7 figures, MNRAS in press

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