Interpretation of the infrared emission lines from starburst galaxies and active galactic nuclei based on photoionization models

The evolution of galaxies is a wide field in astrophysics that can be studied through several approaches, being one of them the analysis of the chemical content of the interstellar medium (ISM). Big Bang Nucleosynthesis predicts a well constrained composition for the ISM, and any deviation from these initial conditions must be attributed to the different star formation processes that govern galaxy evolution and which are eventually shaped by hydrodynamical processes (inflows, outflows, ...) within galaxies. The ionized gas offers a unique opportunity to analyze the chemical composition of the gas surrounding the most energetic events within galaxies such as star formation, Active Galactic Nuclei (AGN) or shocks, allowing us to understand the present-day composition of the gas that fuels and witnesses these events. Specifically, emission lines from the gas-phase ISM are key to constrain their physical and chemical properties, being collisionally excited lines (CELs) the best resource due to the their brightness in comparison to recombination lines (RLs). Studies analyzing chemical properties, essentially traced by the oxygen content, of the ionized gas have been historically performed on galaxies whose activity is dominated by star formation (starforming galaxies, SFGs) and by means of optical spectroscopic observations, which are easily retrieved from ground-based observatories for low-redshift galaxies. In the recent decades, these studies have been complemented with the analysis of AGNs, also relying on optical observations. High-redshift galaxies have been targeted as well, although in this case optical observatories (enhanced by spacebased missions allowing deeper surveys) retrieved rest-frame ultraviolet (UV) spectra, using a similar approach to that developed for optical observations. With the advent of JWST and its potential for galaxies up to redshift (𝑧∼ 10), we are now able to analyze chemical enrichment in the early Universe. Optical and UV studies present several disadvantages. Both of them, specially the UV, are extremely affected by dust extinction and attenuation, and dusty regions might remain unobserved by optical and UV tracers. Optical and UV CELs are strongly dependent on the physical properties of the ISM, being the electron temperature a very important factor in their emissivity, and establishing a temperature threshold below which these emissions lines are too faint to be observed. Overall, optical [3000Å − 9600Å] and UV [900Å − 3000Å] studies are biased towards regions that match the proper conditions for a proper analysis. This incomplete picture of the chemical enrichment must be complemented with infrared (IR) observations [9600Å − 700𝜇m]. IR emission lines are almost insensitive to interstellar dust, given us a unique window for the analysis of dusty regions in galaxies. Due to the involved atomic transitions, temperature effects are mostly mitigated, allowing us to observe cold regions, and avoiding the problem of having a proper constrained for the temperature and density conditions within the ionized gas. The great variety of emission lines observable in the IR regime not only allows us to perform chemical diagnostics of the ISM; but also robust constraints on the dominant ionizing sources. This thesis presents a detailed analysis on the use of IR emission lines to perform chemical evolution studies based on the composition of the gas-phase ISM. By means of the large amounts of archival data from past IR missions, we show the potential of IR emission line studies for ongoing (JWST, ALMA) and future (METIS) missions. We present our technique that we have developed based on photoionization models (HII-CHI-Mistry-IR), which follows a robust methodology whose validity has already been probed in other spectral regimes (optical and UV), and that can be applied for both SFGs and AGNs. We have structured the thesis as a transition from the optical to the infrared regime. In the first chapters, we show the limitations of optical studies to assess the proper ionizing nature of the gas-phase ISM, using integral field spectroscopic data to analyze a sample of low-luminosity AGNs, whose ionizing nature is still puzzling. Our analysis of the nuclear regions reveals that even with the limitations of optical studies, an analysis of the chemical enrichment history of these galaxies can be performed if a robust methodology that analyzes the chemical composition as traced by a primary (oxygen) and secondary (nitrogen) elements is used. Moreover, the analysis of the chemical abundance radial gradients reinforces this conclusion. In the remaining chapters, we show the use of our methodology for IR emission lines, showing that IR regime allows for a robust estimation not only of oxygen and nitrogen, but also of sulfur which is supposed to be less affected by dust depletion and, its relative abundance to oxygen is useful to assess the impact of dust depletion and whether nucleosynthesis of sulfur and oxygen follows the expected trend (i.e. a constant ratio). We also apply our methodology to a sample of (Ultra)-Luminous Infrared Galaxies [(U)LIRGs] which are characterized by large amounts of dust that shield their star formation processes. We obtain that, contrary to what it is inferred from optical studies, the majority of them do not deviate from the standard relations reported in the local Universe for SFGs and that the nitrogen content is essential to determine the chemical enrichment history in galaxies that suffer from strong hydrodynamical processes (as it is the case for the deep-diving phase). Overall, these results highlight the importance of IR studies in the future years to complement our picture of chemical evolution in galaxies, understanding the possible differences among them depending on their dominant ionizing activity.

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