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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. (2019) Initial mass function variations cannot explain the ionizing spectrum of low metallicity starbursts. Astronomy and Astrophysics, 621 . A105. doi:10.1051/0004-6361/201834359

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Official URL: https://doi.org/10.1051/0004-6361/201834359

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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, Engineering and Medicine > 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: 18 January 2019
Dates:
DateEvent
18 January 2019Published
9 November 2018Available
8 November 2018Accepted
Volume: 621
Article Number: A105
DOI: 10.1051/0004-6361/201834359
Status: Peer Reviewed
Publication Status: Published
Reuse Statement (publisher, data, author rights): 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
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