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

M*/L gradients driven by IMF variation: large impact on dynamical stellar mass estimates
M*/L gradients driven by IMF variation: large impact on dynamical stellar mass estimates
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.
0035-8711
2560–2571
Bernardi, M.
056c9e26-95ca-450e-89c2-e728ae416950
Sheth, R.K.
a98226d3-8c7b-4bf1-b15e-c9a7da652ac5
Dominguez-Sanchez, H.
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Fischer, J.-L.
c5e0ab8b-0b9e-4421-aec5-46ff9a7bd3f9
Chae, K.-H.
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Huertas-Company, M.
cabe382d-b8c8-4871-ac0e-d7a5b57fd5ee
Shankar, F.
b10c91e4-85cd-4394-a18a-d4f049fd9cdb
Bernardi, M.
056c9e26-95ca-450e-89c2-e728ae416950
Sheth, R.K.
a98226d3-8c7b-4bf1-b15e-c9a7da652ac5
Dominguez-Sanchez, H.
4d0f4aa7-7547-4348-aa78-ab189206a102
Fischer, J.-L.
c5e0ab8b-0b9e-4421-aec5-46ff9a7bd3f9
Chae, K.-H.
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Huertas-Company, M.
cabe382d-b8c8-4871-ac0e-d7a5b57fd5ee
Shankar, F.
b10c91e4-85cd-4394-a18a-d4f049fd9cdb

Bernardi, M., Sheth, R.K., Dominguez-Sanchez, H., Fischer, J.-L., Chae, K.-H., Huertas-Company, M. and Shankar, F. (2018) M*/L gradients driven by IMF variation: large impact on dynamical stellar mass estimates. Monthly Notices of the Royal Astronomical Society, 477 (2), 2560–2571. (doi:10.1093/mnras/sty781).

Record 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 ( 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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Accepted/In Press date: 20 March 2018
e-pub ahead of print date: 26 March 2018
Published date: 21 June 2018

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Local EPrints ID: 419128
URI: http://eprints.soton.ac.uk/id/eprint/419128
ISSN: 0035-8711
PURE UUID: 8b71abd3-4e0a-4f30-b8da-e0c52d34ca31

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Date deposited: 06 Apr 2018 16:30
Last modified: 16 Dec 2019 18:19

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Contributors

Author: M. Bernardi
Author: R.K. Sheth
Author: H. Dominguez-Sanchez
Author: J.-L. Fischer
Author: K.-H. Chae
Author: M. Huertas-Company
Author: F. Shankar

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