Lyman Alpha and MgII as Probes of Galaxies and their Environments

TL;DR

This review discusses how Ly{eta} emission, Ly{eta} and MgII absorption lines serve as key tools for studying neutral hydrogen across different cosmic environments, shedding light on galaxy formation and evolution at various redshifts.

Contribution

It synthesizes observational and theoretical insights on Ly{eta} and MgII lines as probes of gas physics in galaxy environments, highlighting their roles in understanding galaxy evolution.

Findings

Ly{eta} emission traces diffuse gas outside galaxies.

MgII absorption reveals metal-enriched gas in galaxy halos.

High-redshift Ly{eta} and MgII observations inform galaxy formation models.

Abstract

Ly{\alpha} emission, Ly{\alpha} absorption and MgII absorption are powerful tracers of neutral hydrogen. Hydrogen is the most abundant element in the universe and plays a central role in galaxy formation via gas accretion and outflows, as well as being the precursor to molecular clouds, the sites of star formation. Since 21cm emission from neutral hydrogen can only be directly observed in the local universe, we rely on Ly{\alpha} emission, and Ly{\alpha} and MgII absorption to probe the physics that drives galaxy evolution at higher redshifts. Furthermore, these tracers are sensitive to a range of hydrogen densities that cover the interstellar medium, the circumgalactic medium and the intergalactic medium, providing an invaluable means of studying gas physics in regimes where it is poorly understood. At high redshift, Ly{\alpha} emission line searches have discovered thousands of star-forming galaxies out to z = 7. The large Ly{\alpha} scattering cross-section makes observations of this line sensitive to even very diffuse gas outside of galaxies. Several thousand more high-redshift galaxies are known from damped Ly{\alpha} absorption lines and absorption by the MgII doublet in quasar and GRB spectra. MgII, in particular, probes metal-enriched neutral gas inside galaxy haloes in a wide range of environments and redshifts (0.1 < z < 6.3), including the so-called redshift desert. Here we review what observations and theoretical models of Ly{\alpha} emission, Ly{\alpha} and MgII absorption have told us about the interstellar, circumgalactic and intergalactic medium in the context of galaxy formation and evolution.