Overturning the Case for Gravitational Powering in the Prototypical
  Cooling Lyman-alpha Nebula
  The Nilsson et al. (2006) Lyman-alpha nebula has often been cited as the most
plausible example of a Lyman-alpha nebula powered by gravitational cooling. In
this paper, we bring together new data from the Hubble Space Telescope and the
Herschel Space Observatory as well as comparisons to recent theoretical
simulations in order to revisit the questions of the local environment and most
likely power source for the Lyman-alpha nebula. In contrast to previous
results, we find that this Lyman-alpha nebula is associated with 6 nearby
galaxies and an obscured AGN that is offset by $\sim$4"$\approx$30 kpc from the
Lyman-alpha peak. The local region is overdense relative to the field, by a
factor of $\sim$10, and at low surface brightness levels the Lyman-alpha
emission appears to encircle the position of the obscured AGN, highly
suggestive of a physical association. At the same time, we confirm that there
is no compact continuum source located within $\sim$2-3"$\approx$15-23 kpc of
the Lyman-alpha peak. Since the latest cold accretion simulations predict that
the brightest Lyman-alpha emission will be coincident with a central growing
galaxy, we conclude that this is actually a strong argument against, rather
than for, the idea that the nebula is gravitationally-powered. While we may be
seeing gas within cosmic filaments, this gas is primarily being lit up, not by
gravitational energy, but due to illumination from a nearby buried AGN.
