Witnessing galaxy assembly in an extended z~3 structure
  We present new observations acquired with the Multi Unit Spectroscopic
Explorer instrument on the Very Large Telescope in a quasar field that hosts a
high column-density damped Ly{\alpha} absorber (DLA) at z~3.25. We detect
Ly{\alpha} emission from a nebula at the redshift of the DLA with line
luminosity (27+/-1)x1e41 erg/s, which extends over 37+/-1 kpc above a surface
brightness limit of 6x1e-19 erg/s/cm2/arcsec2 at a projected distance of
30.5+/-0.5 kpc from the quasar sightline. Two clumps lie inside this nebula,
both with Ly{\alpha} rest-frame equivalent width > 50 A and with relative
line-of-sight velocities aligned with two main absorption components seen in
the DLA spectrum. In addition, we identify a compact galaxy at a projected
distance of 19.1+/-0.5 kpc from the quasar sightline. The galaxy spectrum is
noisy but consistent with that of a star-forming galaxy at the DLA redshift. We
argue that the Ly{\alpha} nebula is ionized by radiation from star formation
inside the two clumps, or by radiation from the compact galaxy. In either case,
these data imply the presence of a structure with size >>50 kpc inside which
galaxies are assembling, a picture consistent with galaxy formation in groups
and filaments as predicted by cosmological simulations such as the EAGLE
simulations.
