We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium ( ISM ) to investigate ultraviolet-line diagnostics of stars , the ionized and the neutral ISM in star-forming galaxies .
We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters .
In doing so , we find that neglecting stochastic sampling of the stellar initial mass function in these young ( \sim 10 –100 Myr ) , low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass .
Then , we proceed and develop a simple approach , based on an idealized description of the main features of the ISM , to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies .
Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds , as well as for radiative transfer through a diffuse intercloud medium .
We use this approach to explore the entangled signatures of stars , the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies .
We find that , aside from a few notable exceptions , most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM , which increases with metallicity .
We also identify several nebular-emission and interstellar-absorption features , which stand out as particularly clean tracers of the different phases of the ISM .