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Variations in the initial mass function in early-type galaxies: a critical comparison between dynamical and spectroscopic results

Smith, Russell J.

Variations in the initial mass function in early-type galaxies: a critical comparison between dynamical and spectroscopic results Thumbnail


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Abstract

I present a comparison between published dynamical (ATLAS3D) and spectroscopic (Conroy & van Dokkum) constraints on the stellar initial mass function (IMF) in early-type galaxies, using the 34 galaxies in common between the two works. Both studies infer an average IMF mass factor α (the stellar mass relative to a Kroupa-IMF population of similar age and metallicity) greater than unity, i.e. both methods favour an IMF which is heavier than that of the Milky Way, on average over the sample. However, on a galaxy-by-galaxy basis, there is no correlation between α inferred from the two approaches. I investigate how the two estimates of α are correlated systematically with the galaxy velocity dispersion, σ, and with the Mg/Fe abundance ratio. The spectroscopic method, based on the strengths of metal absorption lines, yields a correlation only with metal abundance ratios: at fixed Mg/Fe, there is no residual correlation with σ. The dynamical method, applied to exactly the same galaxy sample, yields the opposite result: the IMF variation correlates only with dynamics, with no residual correlation with Mg/Fe after controlling for σ. Hence although both methods indicate a heavy IMF on average in ellipticals, they lead to incompatible results for the systematic trends, when applied to the same set of galaxies. The sense of the disagreement could suggest that one (or both) of the methods has not accounted fully for the main confounding factors, i.e. element abundance ratios or dark matter contributions. Alternatively, the poor agreement might indicate additional variation in the detailed shape of the IMF, beyond what can currently be inferred from the spectroscopic features.

Citation

Smith, R. J. (2014). Variations in the initial mass function in early-type galaxies: a critical comparison between dynamical and spectroscopic results. Monthly Notices of the Royal Astronomical Society, 443(1), L69-L73. https://doi.org/10.1093/mnrasl/slu082

Journal Article Type Article
Acceptance Date Jun 2, 2014
Online Publication Date Jul 2, 2014
Publication Date Sep 1, 2014
Deposit Date Jun 17, 2014
Publicly Available Date Sep 25, 2014
Journal Monthly Notices of the Royal Astronomical Society: Letters
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 443
Issue 1
Pages L69-L73
DOI https://doi.org/10.1093/mnrasl/slu082
Related Public URLs https://arxiv.org/abs/1403.6114

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Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society. © 2014 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.







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