We use cookies to ensure that we give you the best experience on our website. You can change your cookie settings at any time. Otherwise, we'll assume you're OK to continue.

Durham University

Department of Physics

Staff profile

Publication details for Dr Russell Smith

Smith, Russell J., Alton, Padraig D., Lucey, John 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.

Author(s) from Durham


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.