%0 Generic %1 smith2015imfsensitive %A Smith, Russell J. %A Alton, Padraig %A Lucey, John R. %A Conroy, Charlie %A Carter, David %D 2015 %K early-type galaxies imf sodium variation %T The IMF-sensitive 1.14-micron Na I doublet in early-type galaxies %U http://arxiv.org/abs/1509.01253 %X We present J-band spectroscopy of passive galaxies focusing on the Na I doublet at 1.14 \mum. Like the Na I 0.82 \mum 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 \mum increases steeply with increasing velocity dispersion, \sigma, and for the most massive galaxies (> 300 km/s) is much stronger than predicted from synthetic spectra with Milky-Way-like IMFs and solar abundances. Reproducing Na I 1.14 \mum at high \sigma 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 $\approx$ +0.5 and a steep IMF (single-slope-equivalent x $\approx$ 3.2, where x = 2.35 for Salpeter), for the high-galaxies. At lower mass (\sigma = 50-100 km/s), the line strengths are compatible with 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 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-sample, recovers an IMF less heavy than Salpeter, but under-predicts the Na I 1.14 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 IMF studies.

@misc{smith2015imfsensitive, abstract = {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, {\sigma}, and for the most massive galaxies (\sigma > 300 km/s) is much stronger than predicted from synthetic spectra with Milky-Way-like IMFs and solar abundances. Reproducing Na I 1.14 {\mu}m at high {\sigma} 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] $\approx$ +0.5 and a steep IMF (single-slope-equivalent x $\approx$ 3.2, where x = 2.35 for Salpeter), for the high-\sigma galaxies. At lower mass ({\sigma} = 50-100 km/s), the line strengths are compatible with 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 IMF studies.}, added-at = {2015-09-07T11:38:13.000+0200}, author = {Smith, Russell J. and Alton, Padraig and Lucey, John R. and Conroy, Charlie and Carter, David}, biburl = {https://www.bibsonomy.org/bibtex/29097ab1ee3e643c13b23c3a7dec42768/miki}, description = {[1509.01253] The IMF-sensitive 1.14-micron Na I doublet in early-type galaxies}, interhash = {56c4f6bf1c74e1ed21924f7c066d3e91}, intrahash = {9097ab1ee3e643c13b23c3a7dec42768}, keywords = {early-type galaxies imf sodium variation}, note = {cite arxiv:1509.01253Comment: MNRAS accepted}, timestamp = {2015-09-07T11:38:13.000+0200}, title = {The IMF-sensitive 1.14-micron Na I doublet in early-type galaxies}, url = {http://arxiv.org/abs/1509.01253}, year = 2015 }