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.

Abstract

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.

Citation

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. https://doi.org/10.1093/mnrasl/slv132

• 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
• DOI: https://doi.org/10.1093/mnrasl/slv132
• Keywords: Galaxies: elliptical and lenticular, cD, Galaxies: stellar content.
• Public URL: https://durham-repository.worktribe.com/output/1396171
• Related Public URLs: http://arxiv.org/abs/1509.01253