Keck/Palomar Cosmic Web Imagers (KCWI/PCWI) Reveal an Enormous Ly$\hat{I}\pm$ Nebula in an Extremely Overdense QSO Pair Field at $z=2.45$

Abstract

Enormous Ly$\hat{I}\pm$ nebulae (ELANe) represent the extrema of Ly$\hat{I}\pm$ nebulosities. They have detected extents of $>200$ kpc in Ly$\hat{I}\pm$ and Ly$\hat{I}\pm$ 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 $z\approx 2$ 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 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-$\ddot{I}ƒ$ surface brightness of $1.2\times 10^{-17}$ \sbunit, the end-to-end size of ELAN0101+0201 is $\gtrsim 232$ kpc. We have conducted follow-up observations using KCWI, resolving multiple Ly$\hat{I}\pm$ emitting sources within the rectangular field-of-view of $\approx 130\times 165$ 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 ($T\sim 10^4$K) gas could exist in massive halos (M$\gtrsim 10^{13}$M${}_{\odot }$) at $z\approx 2$. 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 (ICM).

Submitted to Astrophysical Journal Letter, 9 pages, 4 figures, Comments Welcome