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

Smith, Russell J.

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
• Public URL: https://durham-repository.worktribe.com/output/1458612
• Related Public URLs: https://arxiv.org/abs/1403.6114