We report the discovery and spectroscopic confirmation of a very large star-forming Lyman Break galaxy , G6025 , at z _ { spec } = 3.721 \pm 0.003 .
In the rest-frame \approx 2100Ã , G6025 subtends \approx 24Â kpc in physical extent when measured from the 1.5 \sigma isophote , in agreement with the parametric size measurements which yield the half-light radius of 4.9 \pm 0.5 Â kpc and the semi-major axis of 12.5 \pm 0.1 Â kpc .
G6025 is also very UV-luminous ( \approx 5 L ^ { * } _ { UV,z \sim 4 } ) and young ( \approx 140 \pm 60 Â Myr ) .
Despite its unusual size and luminosity , the stellar population parameters and dust reddening ( M _ { star } \sim M ^ { * } _ { z \sim 4 } , and E ( B - V ) \sim 0.18 \pm 0.05 ) estimated from the integrated light , are similar to those of smaller galaxies at comparable redshifts .
The ground-based morphology and spectroscopy show two dominant components , both located off-center , embedded in more diffuse emission .
We speculate that G6025 may be a scaled-up version of chain galaxies seen in deep HST imaging , or alternatively , a nearly equal-mass merger involving two super- L ^ { * } galaxies in its early stage .
G6025 lies close to but not within a known massive protocluster at z = 3.78 .
We find four companions within 6 Â Mpc from G6025 , two of which lie within 1.6Â Mpc .
While the limited sensitivity of the existing spectroscopy does not allow us to robustly characterize the local environment of G6025 , it likely resides in a locally overdense environment .
The luminosity , size , and youth of G6025 make it uniquely suited to study the early formation of massive galaxies in the universe .