  The recent discovery by Cantalupo et al. (2014) of the largest (~500 kpc) and
luminous Ly-alpha nebula associated with the quasar UM287 (z=2.279) poses a
great challenge to our current understanding of the astrophysics of the halos
hosting massive z~2 galaxies. Either an enormous reservoir of cool gas is
required $M\simeq10^{12}$ $M_{\odot}$, exceeding the expected baryonic mass
available, or one must invoke extreme gas clumping factors not present in
high-resolution cosmological simulations. However, observations of Ly-alpha
emission alone cannot distinguish between these two scenarios. We have obtained
the deepest ever spectroscopic integrations in the HeII and CIV lines with the
goal of detecting extended line emission, but detect neither line to a
3$\sigma$ limiting SB $\simeq10^{-18}$ erg/s/cm$^2$/arcsec$^2$. We construct
models of the expected emission spectrum in the highly probable scenario that
the nebula is powered by photoionization from the central hyper-luminous
quasar. The non-detection of HeII implies that the nebular emission arises from
a mass $M_{\rm c}\lesssim6.4\times10^{10}$ $M_{\odot}$ of cool gas on ~200 kpc
scales, distributed in a population of remarkably dense ($n_{\rm H}\gtrsim3$
cm$^{-3}$) and compact ($R\lesssim20$ pc) clouds, which would clearly be
unresolved by current cosmological simulations. Given the large gas motions
suggested by the Ly-alpha line ($v\simeq$ 500 km/s), it is unclear how these
clouds survive without being disrupted by hydrodynamic instabilities. Our study
serves as a benchmark for future deep integrations with current and planned
wide-field IFU such as MUSE, KCWI, and KMOS. Our work suggest that a $\simeq$
10 hr exposure would likely detect ~10 rest-frame UV/optical emission lines,
opening up the possibility of conducting detailed photoionization modeling to
infer the physical state of gas in the CGM.
