Discovery of an Enormous Lyα Nebula in a Massive Galaxy Overdensity at z = 2.3

Enormous Lyα nebulae (ELANe), unique tracers of galaxy density peaks, are predicted to lie at the nodes and intersections of cosmic filamentary structures. Previous successful searches for ELANe have focused on wide-field narrowband surveys or have targeted known sources such as ultraluminous quasi-stellar objects (QSOs) or radio galaxies. Utilizing groups of coherently strong Lyα absorptions, we have developed a new method to identify high-redshift galaxy overdensities and have identified an extremely massive overdensity, BOSS1441, at z=2{--}3. In its density peak, we discover an ELAN that is associated with a relatively faint continuum. To date, this object has the highest diffuse Lyα nebular luminosity of {L}_{nebula}=5.1+/- 0.1× {10}⁴⁴ erg s^-1. Above the 2σ surface brightness limit of SB{}_{Lyα }=4.8× {10}^-18 erg s^-1 cm^-2 arcsec^-2, this nebula has an end-to-end spatial extent of 442 kpc. This radio-quiet source also has extended C IV λ 1549 and He II λ 1640 emission on ≳ 30 {kpc} scales. Note that the Lyα, He II, and C IV emissions all have double-peaked line profiles. Each velocity component has an FWHM of ≈700-1000 km s^-1. We argue that this Lyα nebula could be powered by shocks due to an active galactic nucleus-driven outflow or photoionization by a strongly obscured source.