Discovery of an Enormous Ly$\alpha$ nebula in a massive galaxy
  overdensity at $z=2.3$
  Enormous Ly$\alpha$ 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 Ly$\alpha$ absorptions (CoSLAs), we have developed a new method to
identify high-redshift galaxy overdensities and have identified an extremely
massive overdensity, BOSS1441, at $z=2-3$ (Cai et al. 2016a). In its density
peak, we discover an ELAN that is associated with a relatively faint continuum.
To date, this object has the highest diffuse Ly$\alpha$ nebular luminosity of
$L_{\rm{nebula}}=5.1\pm0.1\times10^{44}$ erg s$^{-1}$. Above the 2$\sigma$
surface brightness limit of SB$_{\rm{Ly\alpha}}= 4.8\times10^{-18}$ 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 \civ\ $\lambda1549$ and
\heii\ $\lambda1640$ emission on $\gtrsim30$ kpc scales. Note that the
Ly$\alpha$, \heii\ and \civ\ emission all have double-peaked line profiles.
Each velocity component has a full-width-half-maximum (FWHM) of $\approx700 -
1000$ km s$^{-1}$. We argue that this Ly$\alpha$ nebula could be powered by
shocks due to an AGN-driven outflow or/and photoionization by a strongly
obscured source.
