Understanding Galaxy Evolution through Emission Lines

TL;DR

This paper reviews how emission lines across UV, optical, and infrared spectra are used to understand galaxy evolution, introduces new diagnostic calibrations, and discusses future prospects with upcoming telescopes.

Contribution

It provides new self-consistent emission-line diagnostic calibrations covering UV to infrared and introduces advanced 2D and 3D diagnostics for galaxy analysis.

Findings

Emission lines probe different nebular zones and conditions.

New diagnostic calibrations enable comprehensive galaxy property analysis.

Diffuse ionized gas impacts metallicity and ionization measurements.

Abstract

We review the use of emission-lines for understanding galaxy evolution, focusing on excitation source, metallicity, ionization parameter, ISM pressure and electron density. We show that the UV, optical and infrared contain complementary diagnostics that can probe the conditions within different nebular ionization zones. In anticipation of upcoming telescope facilities, we provide new self-consistent emission-line diagnostic calibrations for complete spectral coverage from the UV to the infrared. These diagnostics can be used in concert to understand how fundamental galaxy properties have changed across cosmic time. We describe new 2D and 3D emission-line diagnostics to separate the contributions from star formation, AGN and shocks using integral field spectroscopy. We discuss the physics, benefits, and caveats of emission-line diagnostics, including the effect of theoretical model uncertainties, diffuse ionized gas, and sample selection bias. Accounting for complex density gradients and temperature profiles is critical for reliably estimating the fundamental properties of H ii regions and galaxies. Diffuse ionized gas can raise metallicity estimates, flatten metallicity gradients, and introduce scatter in ionization parameter measurements. We summarize with a discussion of the challenges and major opportunities for emission-line diagnostics in the coming years.