In this thesis I focus on understanding the physical properties of extended Lyman-alpha emitting gas at high redshift, from the circumgalactic medium (CGM) and intergalactic medium (IGM). First, I investigate the physics of giant, luminous Lyman-alpha nebulae known as Lyman alpha blobs (LABs). The mechanism powering the emission of LABs is poorly understood, although they are now often associated with active galactic nuclei (AGN). Our poor understanding results primarily from the lack of information beyond the Lyman-alpha line. Targeting 13 LABs, I thus conduct a deep search for the HeII1640 and CIV1549 lines, which probe the volume density, the metallicity, and the ionization level within the nebulae. Although I did not detect any emission down to unprecedented surface brightness levels, I show that LABs could be still consistent with photoionization from an obscured AGN. Second, I led a narrow-band imaging survey (FLASHLIGHT) targeting the Lyman-alpha line around 25 quasars at redshift of about 2. FLASHLIGHT is the deepest line imaging study ever undertaken around quasars, and aims to uncover the emission from their CGM. During this campaign, I took part in the discovery of the largest (about 500 kpc) Lyman-alpha nebula known at high redshift: UM287. Its bright large scale emission is in tension with our current understanding of the physical state of gas in massive dark matter halos. As for the LABs, I obtained even deeper HeII and CIV spectroscopy of UM287, and again failed to detect emission. Using photoionization modeling I show that the extended Lyman-alpha emission is likely arising from remarkably dense and compact clouds, which are clearly unresolved in current cosmological simulations. Lastly, by stacking the FLASHLIGHT data, I obtain the first measurement of the average Lyman-alpha emission from the typical quasar CGM. Combined with absorption line measurements on the total CGM gas mass, this provides the first measurement of the gas density in the quasar CGM. Given this low surface brightness, the next generation of telescopes is probably needed to routinely detect CGM emission around individual quasars. Nevertheless, this thesis paves the way for the understanding of the CGM in emission, which is timely given the upcoming instruments
