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Initial mass function variations cannot explain the ionizing spectrum of low metallicity starbursts
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Stanway, Elizabeth R. and Eldridge, J. J. (2018) Initial mass function variations cannot explain the ionizing spectrum of low metallicity starbursts. Astronomy and Astrophysics . doi:10.1051/0004-6361/201834359 (In Press)
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Official URL: https://doi.org/10.1051/0004-6361/201834359
Abstract
Aims. Observations of both galaxies in the distant Universe and local starbursts are showing increasing evidence for very hard ionizing
spectra that stellar population synthesis models struggle to reproduce. Here we explore the effects of the assumed stellar initial mass
function (IMF) on the ionizing photon output of young populations at wavelengths below key ionization energy thresholds.
Methods. We use a custom set of binary population and spectral synthesis (BPASS) models to explore the effects of IMF assumptions
as a function of metallicity, IMF slope, upper mass limit, IMF power law break mass and sampling.
Results. We find that while the flux capable of ionizing hydrogen is only weakly dependent on IMF parameters, the photon flux
responsible for the He II and O VI lines is far more sensitive to assumptions. In our current models this flux arises primarily from
helium and Wolf-Rayet stars which have partially or fully lost their hydrogen envelopes. The timescales for formation and evolution
of both Wolf Rayet stars and helium dwarfs, and hence inferred population age, are affected by choice of model IMF. Even the most
extreme IMFs cannot reproduce the He II ionizing flux observed in some high redshift galaxies, suggesting a source other than stellar
photospheres.
Conclusions. We caution that detailed interpretation of features in an individual galaxy spectrum is inevitably going to be subject
to uncertainties in the IMF of its contributing starbursts. We remind the community that the initial mass function is fundamentally a
statistical construct, and that stellar population synthesis models are most effective when considering entire galaxy populations rather
than individual objects.
| Item Type: | Journal Article | ||||||
|---|---|---|---|---|---|---|---|
| Subjects: | Q Science > QB Astronomy | ||||||
| Divisions: | Faculty of Science > Physics | ||||||
| Library of Congress Subject Headings (LCSH): | Galaxies , Stars -- Initial mass function , Stars -- Luminosity function, Double stars | ||||||
| Journal or Publication Title: | Astronomy and Astrophysics | ||||||
| Publisher: | EDP Sciences | ||||||
| ISSN: | 0004-6361 | ||||||
| Official Date: | 9 November 2018 | ||||||
| Dates: |
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| Date of first compliant deposit: | 13 November 2018 | ||||||
| DOI: | 10.1051/0004-6361/201834359 | ||||||
| Status: | Peer Reviewed | ||||||
| Publication Status: | In Press | ||||||
| Publisher Statement: | The material cannot be used for any other purpose without further permission of the publisher, and is for private use only. | ||||||
| Access rights to Published version: | Restricted or Subscription Access | ||||||
| Copyright Holders: | EDP Sciences | ||||||
| Funder: | New Zealand eScience Infrastructure (NeSI), New Zealand. Ministry of Business, Innovation & Employment | ||||||
| Related URLs: | |||||||
| Open Access Version: |
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