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Modelling emission lines in star-forming galaxies

Baugh, C M and Lacey, Cedric G and Gonzalez-Perez, Violeta and Manzoni, Giorgio (2022) 'Modelling emission lines in star-forming galaxies.', Monthly Notices of the Royal Astronomical Society, 510 (2). pp. 1880-1893.


We present a new model to compute the luminosity of emission lines in star-forming galaxies and apply this in the semi-analytical galaxy formation code GALFORM. The model combines a pre-computed grid of H II region models with an empirical determination of how the properties of H II regions depend on the macroscopic properties of galaxies based on observations of local galaxies. The new model gives a very good reproduction of the locus of star-forming galaxies on standard line ratio diagnostic diagrams. The new model shows evolution in the locus of star-forming galaxies with redshift on this line ratio diagram, with a good match to the observed line ratios at z = 1.6. The model galaxies at high redshift have gas densities and ionisation parameters that are predicted to be ≈2–3 times higher than in local star-forming galaxies, which is partly driven by the changing selection with redshift to mimic the observational selection. Our results suggest that the observed evolution in emission line ratios requires other H II region properties to evolve with redshift, such as the gas density, and cannot be reproduced by H II model grids that only allow the gas metallicity and ionisation parameter to vary.

Item Type:Article
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Publisher statement:© The Author(s) 2021. Published by Oxford University Press on behalf of Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (, which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Date accepted:25 November 2021
Date deposited:10 February 2022
Date of first online publication:03 December 2021
Date first made open access:10 February 2022

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