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.
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