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The Nilsson et al. (2006) Lyman-alpha nebula has often been cited as the mostplausible example of a Lyman-alpha nebula powered by gravitational cooling. Inthis paper, we bring together new data from the Hubble Space Telescope and theHerschel Space Observatory as well as comparisons to recent theoreticalsimulations in order to revisit the questions of the local environment and mostlikely power source for the Lyman-alpha nebula. In contrast to previousresults, we find that this Lyman-alpha nebula is associated with 6 nearbygalaxies and an obscured AGN that is offset by $\sim$4"$\approx$30 kpc from theLyman-alpha peak. The local region is overdense relative to the field, by afactor of $\sim$10, and at low surface brightness levels the Lyman-alphaemission appears to encircle the position of the obscured AGN, highlysuggestive of a physical association. At the same time, we confirm that thereis no compact continuum source located within $\sim$2-3"$\approx$15-23 kpc ofthe Lyman-alpha peak. Since the latest cold accretion simulations predict thatthe brightest Lyman-alpha emission will be coincident with a central growinggalaxy, we conclude that this is actually a strong argument against, ratherthan for, the idea that the nebula is gravitationally-powered. While we may beseeing gas within cosmic filaments, this gas is primarily being lit up, not bygravitational energy, but due to illumination from a nearby buried AGN.

Overturning the Case for Gravitational Powering in the Prototypical Cooling Lyman-alpha Nebula