Keck/Palomar Cosmic Web Imagers (KCWI/PCWI) Reveal an Enormous
  Ly$\alpha$ Nebula in an Extremely Overdense QSO Pair Field at $z=2.45$
  Enormous Ly$\alpha$ nebulae (ELANe) represent the extrema of Ly$\alpha$
nebulosities. They have detected extents of $>200$ kpc in Ly$\alpha$ and
Ly$\alpha$ luminosities $>10^{44}$ erg s$^{-1}$. The ELAN population is an
ideal laboratory to study the interactions between galaxies and the
intergalactic/circumgalactic medium (IGM/CGM) given their brightness and sizes.
The current sample size of ELANe is still very small, and the few $z\approx2$
ELANe discovered to date are all associated with local overdensities of active
galactic nuclei (AGNs). Inspired by these results, we have initiated a survey
of ELANe associated with QSO pairs using the Palomar and Keck Cosmic Web
Imagers (PCWI/KCWI). In this letter, we present our first result: the discovery
of ELAN0101+0201 associated with a QSO pair at $z=2.45$. Our PCWI discovery
data shows that, above a 2-$\sigma$ surface brightness of $1.2\times10^{-17}$
\sbunit, the end-to-end size of ELAN0101+0201 is $\gtrsim 232$ kpc. We have
conducted follow-up observations using KCWI, resolving multiple Ly$\alpha$
emitting sources within the rectangular field-of-view of $\approx 130\times165$
projected kpc$^2$, and obtaining their emission line profiles at high
signal-to-noise ratios. Combining both KCWI and PCWI, our observations confirm
that ELAN0101+0201 resides in an extremely overdense environment. Our
observations further support that a large amount of cool ($T\sim10^4$K) gas
could exist in massive halos (M$\gtrsim10^{13}$M$_\odot$) at $z\approx2$.
Future observations on a larger sample of similar systems will provide
statistics of how cool gas is distributed in massive overdensities at
high-redshift and strongly constrain the evolution of the intracluster medium
(ICM).
