Discovery of a Large ∼200 kpc Gaseous Nebula atz≈ 2.7 with theSpitzer Space Telescope

We report the discovery of a very large, spatially extended Lyα-emitting nebula at z = 2.656 associated with a luminous mid-infrared source. The bright mid-infrared source ( F 24 μm = 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 Lyα nebula has a luminosity of L Lyα ≈ 1.7 × 10 44 ergs s -1 and an extent of at least 20'' (160 kpc). Its central ≈8'' shows an ordered, monotonic velocity profile; interpreted as rotation, this region encloses a mass M ≈ 6 × 10 12 M ☉ . Several sources lie within the nebula. The central region of the nebula shows narrow (≈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 ≈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.