Discovery of a Very Large ( 20 kpc) Galaxy at z = 3.72 This paper is based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the National Research Council (Canada), CONICYT (Chile), Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina), and Ministério da Ciência, Tec

We report the discovery and spectroscopic confirmation of a very large star-forming galaxy, G6025, at . In the rest frame 2100 , G6025 subtends 24 kpc in physical extent when measured from the 1.5 isophote, in agreement with the parametric size measurements that yield a half-light radius of 4.9 0.5 kpc and a semimajor axis of 12.5 0.1 kpc. It is also very UV-luminous ( ) and young ( 140 60 Myr). Despite its unusual size and luminosity, the stellar population parameters and dust reddening ( and E(B − V) ∼ 0.18 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. It lies close to but not within a known massive protocluster at z = 3.78. We find four companions within 6 Mpc from G6025, two of which lie within 1.6 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.