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The IMF-sensitive 1.14-micron Na I doublet in early-type galaxies | | Russell J. Smith
; Padraig Alton
; John R. Lucey
; Charlie Conroy
; David Carter
; | | Date: |
3 Sep 2015 | | 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. | | Source: | arXiv, 1509.1253 | | Services: | Forum | Review | PDF | Favorites | |
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