  We report the discovery of a very large, spatially extended Ly alpha
-emitting nebula at z=2.656 associated with a luminous mid-infrared source. The
bright mid-infrared source (F(24um)=0.86 mJy) was first detected in
observations made using the Spitzer Space Telescope. Existing broad-band
imaging data from the NOAO Deep Wide-Field Survey revealed the mid-infrared
source to be associated with a diffuse, spatially extended, optical counterpart
in the Bw band. Spectroscopy and further imaging of this target reveals that
the optical source is an almost purely line-emitting nebula with little, if
any, detectable diffuse continuum emission. The Lya nebula has a luminosity of
L[Lya] ~ 1.7e44 erg/s and an extent of at least 20 arcsec (160 kpc). Its
central ~8 arcsec shows an ordered, monotonic velocity profile; interpreted as
rotation, this region encloses a mass M = 6e12 Msun. Several sources lie within
the nebula. The central region of the nebula shows narrow (~365 km/s) emission
lines of CIV and HeII. The mid-infrared source is a compact object lying within
the nebula, but offset from the center by a projected distance of ~2.5 arcsec
(20 kpc), and likely to be an enshrouded AGN. A young star-forming galaxy lies
near the northern end of the nebula. We suggest that the nebula is a site of
recent multiple galaxy and AGN formation, with the spatial distribution of
galaxies within the nebula perhaps tracking the formation history of the
system.
