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.
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