Initial mass function variations cannot explain the ionizing spectrum of low metallicity starbursts

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 ionizi...

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Bibliographic Details
Published in:Astronomy and astrophysics (Berlin) 2019-01, Vol.621, p.A105
Main Authors: Stanway, E. R., Eldridge, J. J.
Format: Article
Language:English
Subjects:
Astronomical models
binaries: general
Binary stars
Fluctuations
Flux
Galaxies
galaxies: high-redshift
galaxies: stellar content
Helium
Initial mass function
Ionization
Metallicity
methods: numerical
Population
Population (statistical)
Red shift
Starbursts
Stars & galaxies
stars: evolution
Stellar age
Stellar evolution
Synthesis
ultraviolet: general
Universe
Wavelengths
Wolf-Rayet stars
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Summary: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 IMF is fundamentally a statistical construct, and that stellar population synthesis models are most effective when considering entire galaxy populations rather than individual objects.
ISSN:0004-6361
1432-0746
DOI:10.1051/0004-6361/201834359