The recent discovery by of the largest ( \sim 500 kpc ) and luminous ( { L \simeq 1.43 \times 10 ^ { 45 } } erg s ^ { -1 } ) 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 \sim 2 galaxies .
Either an enormous reservoir of cool gas is required M \simeq 10 ^ { 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 can not distinguish between these two scenarios .
We have obtained the deepest ever spectroscopic integrations in the He ii \lambda 1640 and C iv \lambda 1549 emission lines with the goal of detecting extended line emission , but detect neither line to a 3 \sigma limiting SB \simeq 10 ^ { -18 } erg s ^ { -1 } cm ^ { -2 } arcsec ^ { -2 } .
We construct simple 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 _ { c } \lesssim 6.4 \times 10 ^ { 10 } M _ { \odot } of cool gas on \sim 200 { kpc } scales , distributed in a population of remarkably dense ( n _ { H } \gtrsim 3 cm ^ { -3 } ) and compact ( R \lesssim 20 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 work serves as a benchmark for future deep integrations with current and planned wide-field IFU spectrographs such as MUSE , KCWI , and KMOS .
Our observations and models suggest that a \simeq 10 { hr } exposure would likely detect \sim 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 circumgalactic medium .