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

We present J-band spectroscopy of passive galaxies focusing on the Na I doublet at 1.14 mu m. Like the Na I 0.82 mu m 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 mu m increases steeply with increasing velocity dispersion, s, and for the most massive galaxies (sigma greater than or similar to 300 km s(-1)) ismuch stronger than predicted from synthetic spectra with Milky Way-like IMFs and solar abundances. Reproducing Na I 1.14 mu m at high s 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] approximate to +0.5 and a steep IMF (single-slope-equivalent x approximate to 3.2, where x = 2.35 for Salpeter), for the high-sigma galaxies. At lower mass (sigma = 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-sigma sample on average, these galaxies have strong Na I 1.14 mu m; 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-sigma sample, recovers an IMF less heavy than Salpeter, but under-predicts the Na I 1.14 mu m line at the 5 sigma 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 IMFstudies.