1/13/2024 0 Comments Spectra line formation![]() ![]() As we will see, there is some degeneracy in the information encoded in these line ratios, depending on which part of the BPT diagram the galaxy occupies and the particular H II region model used. In principle, the position of a galaxy on the BPT diagram can be used, along with an H II region model, to infer the ionisation state of the typical H II region in a galaxy and the metallicity of the star-forming gas, once assumptions have been made regarding other properties of the H II regions, such as the ionised gas density. The position of a galaxy on the BPT diagram can be used to distinguish between galaxies in which the emission lines are powered by star formation and those in which the lines are characteristic of the harder ionising spectrum associated with an active galactic nucleus (Kewley et al. Star-forming galaxies occupy a distinct locus on the BPT diagram. Also, since each emission line luminosity is proportional to the rate of production of Lyman continuum photons, the ratio of line luminosities does not depend on this quantity in a simple, direct way (the rate of production of Lyman continuum photons can change the ionisation state of the H II region and hence the line ratios see later). ![]() As the wavelengths of the emission lines in each ratio are close to one another, they will be affected similarly by dust attenuation, so in principle extinction does not change the value of the ratio. The classic emission line diagnostic diagram, introduced by Baldwin, Phillips & Terlevich ( 1981) and referred to as the BPT diagram, shows two ratios of line luminosities, /H β, plotted against /H α. The ratios of various emission line luminosities can be used to deduce the physical properties of H II regions. The luminosity of emission lines depends on several factors, such as the rate of production of Lyman continuum photons which can ionise Hydrogen, thus probing the very recent star formation history in a galaxy, the metallicity of the star-forming, gas and the density of the gas clouds in which young stars form. 2017 for a review of emission lines see Kewley, Nicholls & Sutherland 2019). The nebular emission which results as electrons and nuclei recombine or atoms and ions decay from collisionally excited states adds both continuum and emission line components to the composite stellar spectrum of a galaxy (see e.g. The energetic photons produced predominantly by hot, young stars can ionise the gas close to them, making H II regions. Methods: numerical, H ii regions, galaxies: formation 1 INTRODUCTION 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. 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. 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 new model gives a very good reproduction of the locus of star-forming galaxies on standard line ratio diagnostic diagrams. 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. 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. ![]()
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