Skip to main content

Research Repository

Advanced Search

The IMF-sensitive 1.14-μm Na i doublet in early-type galaxies

Smith, Russell J.; Alton, Padraig D.; Lucey, John R.; Conroy, C.; Carter, D.

The IMF-sensitive 1.14-μm Na i doublet in early-type galaxies Thumbnail


Padraig D. Alton

C. Conroy

D. Carter


We present J-band spectroscopy of passive galaxies focusing on the Na i doublet at 1.14 μm. Like the Na i 0.82 μm doublet, this feature is strong in low-mass stars and hence may provide a useful probe of the initial mass function (IMF). From high signal-to-noise composite spectra, we find that Na i 1.14 μm increases steeply with increasing velocity dispersion, σ, and for the most massive galaxies (σ ≳ 300 km s−1) is much stronger than predicted from synthetic spectra with Milky Way-like IMFs and solar abundances. Reproducing Na i 1.14 μm at high σ likely requires either a very high [Na/H], or a bottom-heavy IMF, or a combination of both. Using the Na D line to break the degeneracy between IMF and abundance, we infer [Na/H] ≈ +0.5 and a steep IMF (single-slope-equivalent x ≈ 3.2, where x = 2.35 for Salpeter), for the high-σ galaxies. At lower mass (σ = 50–100 km s−1), the line strengths are compatible with Milky Way (MW)-like IMFs and near-solar [Na/H]. We highlight two galaxies in our sample where strong gravitational lensing masses favour MW-like IMFs. Like the high-σ sample on average, these galaxies have strong Na i 1.14 μm; taken in isolation their sodium indices imply bottom-heavy IMFs which are hard to reconcile with the lensing masses. An alternative full-spectrum-fitting approach, applied to the high-σ sample, recovers an IMF less heavy than Salpeter, but under-predicts the Na i 1.14 μm line at the 5σ level. We conclude that current models struggle to reproduce this feature in the most massive galaxies without breaking other constraints, and caution against over-reliance on the sodium lines in spectroscopic IMF studies.


Smith, R. J., Alton, P. D., Lucey, J. R., Conroy, C., & Carter, D. (2015). The IMF-sensitive 1.14-μm Na i doublet in early-type galaxies. Monthly Notices of the Royal Astronomical Society: Letters, 454(1), L71-L75.

Journal Article Type Article
Acceptance Date Sep 3, 2015
Online Publication Date Sep 30, 2015
Publication Date Nov 21, 2015
Deposit Date Dec 16, 2015
Publicly Available Date Jan 6, 2016
Journal Monthly Notices of the Royal Astronomical Society: Letters
Electronic ISSN 1745-3933
Publisher Oxford University Press
Peer Reviewed Peer Reviewed
Volume 454
Issue 1
Pages L71-L75
Keywords Galaxies: elliptical and lenticular, cD, Galaxies: stellar content.
Related Public URLs


Published Journal Article (807 Kb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society: Letters ©: 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

You might also like

Downloadable Citations