Lyman alpha Emitting Galaxies in the Nearby Universe

Abstract: The Lya emission line of HI is intrinsically the brightest feature in the spectrum of astrophysical nebulae, making it a very attractive observational tool with which to survey galaxies. Moreover as a UV resonance line, Lya possesses several unique characteristics that make it useful to study the ISM and ionizing stellar population at all cosmic epochs. In this review I present a summary of Lya observations of galaxies in the nearby universe. At UV magnitudes reachable with current facilities, only ~5% of the local galaxy population shows a Lya equivalent width (EW_Lya) that exceeds 20\AA. This fraction increases dramatically at higher z, but only in the local universe can we study galaxies in detail and assemble unprecedented multi-wavelength datasets. I discuss many local Lya observations, showing that when galaxies show net Lya emission, they ubiquitously produce large halos of scattered Lya, that dominate the integrated luminosity. We discuss how global EW_Lya and the Lya escape fraction (fescLya) are higher (EW_Lya >~ 20\AA\ and fescLya> 10%) in galaxies that represent the less massive and younger end of the distributions for local objects. This is connected with various properties, such that Lya-emitters have lower metallicities (median value of 12+log(O/H) ~ 8.1) and dust reddening. However, the presence of galactic outflows is also vital to Doppler shift the Lya line out of resonance with the HI, as high EW_Lya is found only among galaxies with winds faster than ~50km/s. The 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 fluid instabilities, which reduce the scattering of Lya. Concluding remarks take the form of perspectives upon the most pressing questions that can be answered by observation.