Year: 2018
Type: article
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 (|$M_*^{\rm dyn}$|), 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. Re/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.
DOI: 10.1093/mnras/sty781
Language: English
Subfield: Astronomy and Astrophysics
Field: Physics and Astronomy
Domain: Physical Sciences
Open Access status: green
Award: G6898514348
OpenAlex ID: W2772311955