Discovery of a large ∼200 kpc gaseous nebula at z≈2.7 with the Spitzer Space Telescope

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-24 mu m=0.86 mJy) was first detected in observations made using the Spitzer Space Telescope. Existing broadband 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 B-W 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 Ly alpha nebula has a luminosity of L-Ly alpha approximate to 1.7x10(44) ergs s(-1) and an extent of at least 20" (160 kpc). Its central approximate to 8" shows an ordered, monotonic velocity profile; interpreted as rotation, this region encloses a mass M approximate to 6x10(12) M circle dot. Several sources lie within the nebula. The central region of the nebula shows narrow (approximate to 365 km s(-1)) emission lines of C (IV) and He (II). The mid-infrared source is a compact object lying within the nebula but offset from the center by a projected distance of approximate to 2".5 ( 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.