DEEP HE II AND C IV SPECTROSCOPY OF A GIANT LY α NEBULA: DENSE COMPACT GAS CLUMPS IN THE CIRCUMGALACTIC MEDIUM OF A z ∼ 2 QUASAR

The recent discovery by Cantalupo et al. of the largest (similar to 500 kpc) luminous (L similar or equal to 1.43 x 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 similar to 2 galaxies. Either an enormous reservoir of cool gas is required M similar or equal to 10(12) M-circle dot, 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 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 similar or equal to 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 He II implies that the nebular emission arises from a mass Mc less than or similar to 6.4 x 10(10) M-circle dot of cool gas on similar to 200 kpc scales, distributed in a population of remarkably dense (nH greater than or similar to 3 cm(-3)) and compact (R less than or similar to 20 pc) clouds, which would clearly be unresolved by current cosmological simulations. Given the large gas motions suggested by the Ly alpha line (v similar or equal to 500 km s(-1)), 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 similar or equal to 10 hr exposure would likely detect similar to 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.