The Lyman alpha emission line ( Ly \alpha ) of neutral hydrogen ( H i ) is intrinsically the brightest emission feature in the spectrum of astrophysical nebulae , making it a very attractive observational feature with which to survey galaxies .
Moreover as an ultraviolet ( UV ) resonance line , Ly \alpha possesses several unique characteristics that make it useful to study the properties of the interstellar medium ( ISM ) and ionizing stellar population at all cosmic epochs .
In this review I present a summary of Ly \alpha observations of galaxies in the nearby universe .
By UV continuum selection , at the magnitudes reachable with current facilities , only \approx 5 % of the local galaxy population shows a Ly \alpha equivalent width ( W _ { \mathrm { Ly } \alpha } ) that exceeds 20 Å .
This fraction increases dramatically at higher redshifts , but only in the local universe can we study galaxies in detail and assemble unprecedented multi-wavelength datasets .
I discuss many local Ly \alpha observations , showing that when galaxies show net Ly \alpha emission , they ubiquitously also produce large-scale halos of scattered Ly \alpha , that dominate the integrated luminosity .
Concerning global measurements , we discuss how W _ { \mathrm { Ly } \alpha } and the Ly \alpha escape fraction ( f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } ) are higher ( W _ { \mathrm { Ly } \alpha } \gtrsim 20 Å and f _ { \mathrm { esc } } ^ { \mathrm { Ly } \alpha } \gtrsim 10 % ) in galaxies that represent the less massive and younger end of the distribution for local objects .
This is connected with various properties , such that Ly \alpha -emitting galaxies have lower metal abundances ( median value of 12 + \log ( \mathrm { O / H } ) \sim 8.1 ) and dust reddening .
However , the presence of galactic outflows/winds is also vital to Doppler shift the Ly \alpha line out of resonance with the atomic gas , as high W _ { \mathrm { Ly } \alpha } is found only among galaxies with winds faster than \sim 50 km s ^ { -1 } .
The empirical evidence is then assembled into a coherent picture , and the requirement for star formation driven feedback is discussed in the context of an evolutionary sequence where the ISM is accelerated and/or subject to hydrodynamical instabilities , which reduce the scattering of Ly \alpha .
Concluding remarks take the form of perspectives upon future developments , and the most pressing questions that can be answered by observation .