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

TL;DR

This study reports the discovery of an enormous Ly$ extalpha$ nebula (ELAN) associated with a quasar pair at redshift 2.45, revealing extensive cool gas in an extremely overdense environment, and demonstrates the effectiveness of using KCWI/PCWI for such detections.

Contribution

First detection of an ELAN associated with a QSO pair at high redshift using KCWI/PCWI, expanding the sample of known ELANe and providing insights into cool gas in dense environments.

Findings

ELAN0101+0201 extends over >232 kpc at 2-sigma surface brightness.

Multiple Ly$ extalpha$ sources resolved within 130x165 kpc$^2$ field.

Supports presence of large amounts of cool gas in massive halos at z=2.45.

Abstract

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).