Keck/Palomar Cosmic Web Imagers Reveal an Enormous Lyα Nebula in an Extremely Overdense Quasi-stellar Object Pair Field at z = 2.45

Enormous Lyα nebulae (ELANe) represent the extrema of Lyα nebulosities. They have detected extents of >200 kpc in Lyα and Lyα luminosities >10⁴⁴ 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 ≈ 2 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 quasi-stellar object (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 2σ surface brightness of 1.2 × 10^-17 erg s^-1 cm^-2 arcsec^-2, the end-to-end size of ELAN0101+0201 is ≳232 kpc. We have conducted follow-up observations using KCWI, resolving multiple Lyα emitting sources within the rectangular field of view of ≈130 × 165 projected kpc², and obtaining their emission line profiles at high signal-to-noise ratios (S/Ns). 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 ∼ 10⁴ K) gas could exist in massive halos (M ≳10¹³ M _o) at z ≈ 2. 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.