This chapter reviews the properties of extended Lyα emission in the high redshift universe (z > 2) and their connection with gas flows in and around galaxies and their halos. In particular, the attention will be focused on giant Lyα nebulae, i.e. emission with sizes exceeding 100 kpc (unless otherwise noticed, I will always use "physical" units in this chapter) and in general on Lyα emission extending on scales that are significantly larger than individual galaxies. In terms of energetics, these systems have integrated Lyα luminosities (LLyα) larger than 1043 erg s−1. For space reason, smaller and lower luminosities Lyα halos found around individual Lyman α emitters (LAE), e.g. Wisotzki et al. (2016), or in stacking analysis, e.g. Steidel et al. (2011) will not be covered in this chapter although their origin in some cases may be connected with larger nebulae (see e.g., Borisova et al. 2016 for discussion). Similarly, I will not review here the detection and study of giant nebulae in metal emission line in the low redshift universe (e.g., Bergeron et al. 1983, Veilleux et al. 2003).
The chapter is organized as follows. In section 2, I provide an observationally-oriented and historical overview describing the discovery and characterization of high-redshift Lyα Nebulae. In section 3, a detailed discussion about the origin of the Lyα emission is provided in terms of the atomic processes responsible for the production of Lyα photons. In section 4, I review the kinematics of the nebulae and the connection with the physical origin and fate of the gas including accretion onto galaxies and their halos. I summarize the chapter in section 5. Throughout this chapter, a ΛCDM cosmology with Ωm = 0.3, ΩΛ = 0.7 and H = 70 km s−1 is assumed. For reference, one arcsec at z = 3 corresponds to about 7.6 kpc with these cosmological parameters.