Emission and Absorption Line Diagnostics

Emission and absorption line diagnostics use the strengths, ratios, and profiles of spectral lines to infer the temperature, density, abundances, and ionization state of astronomical gas.

Definition

Line diagnostics are the methods that translate measured properties of emission and absorption lines, such as equivalent width, flux ratio, and profile shape, into physical parameters of the gas producing them.

Scope

This topic covers the quantitative interpretation of spectral lines: measuring equivalent widths and line profiles, using line ratios to determine electron temperature and density in nebulae, deriving chemical abundances from line strengths, and applying diagnostic diagrams to distinguish excitation sources. It connects observed line measurements to the physical conditions of the emitting or absorbing gas.

Core questions

• How is the equivalent width of a line measured and related to abundance?
• Which line ratios diagnose electron temperature and density in ionized gas?
• How do diagnostic diagrams distinguish star-forming regions from active galactic nuclei?
• What does the shape of a line profile reveal about velocity fields in the gas?

Key theories

Forbidden-line density and temperature diagnostics

Ratios of collisionally excited forbidden lines from the same ion are sensitive to electron density or temperature, allowing these conditions to be read directly from a nebular spectrum.

Excitation diagnostic diagrams

Plotting ratios of strong emission lines separates gas ionized by hot young stars from gas ionized by harder radiation, classifying the excitation mechanism of emission-line objects.

Clinical relevance

Line diagnostics yield the chemical enrichment history of galaxies, the temperatures and densities of nebulae and the interstellar medium, star-formation rates from recombination lines, and the identification of active galactic nuclei.

History

The interpretation of nebular forbidden lines, once mysterious 'nebulium' features, was resolved by Bowen's identification of collisionally excited transitions, and was systematized in the diagnostic frameworks of Osterbrock and in emission-line ratio diagrams.

Seminal works

• osterbrockFerland2006
• baldwin1981
• gray2005

Frequently asked questions

What is an equivalent width?

It is the width of a rectangular strip of continuum that has the same area as the line, providing a continuum-normalized measure of line strength that is independent of spectral resolution.

Why are forbidden lines useful diagnostics?

Forbidden lines arise from low-density gas and their relative strengths depend sensitively on temperature and density, making their ratios direct probes of the physical conditions in nebulae.