Cake Talk by Amirnezam Amiri
The abundances of chemical elements across cosmic epochs provide unique information on the physical processes driving the evolution of galaxies. Over the past decades many attempts have been made to measure metallicities using emission lines from photoionized gas in galaxies, ranging from direct measurements based on electron temperature and densities (Te-based metallicities), to calibration of relations between line ratios and metallicities, to the use of photoionization models. However, these methods are often based on simplifying assumption which does not adequately describe the complexity of the emitting regions, and their reliability is questioned. I will analyze a set of average galaxy spectra using our new multi-component photoionization models which are able to reproduce all observed emission lines with a very high accuracy, down to a few percent. Our models thus allow for accurate metallicity measurements and to recover the well-known trends between ionization parameter and metallicity, and N/O versus Oxygen abundance. I will show that while our models do reproduce all observed line ratios, hence providing the same Te-based metallicities as observations, the true model metallicities are sometimes very different, questioning the use of Te-based metallicities. I present newly calibrated metallicity estimators based on ratios between strong emission lines. Finally, I'll demonstrate how the new method is critical for determining and exploring Te-based metallicity at high redshift.