Discovery of a Very Large (≈20 kpc) Galaxy at z = 3.72

We report the discovery and spectroscopic confirmation of a very large star-forming galaxy, G6025, at zspec = 3.721 ±0.003 . In the rest frame 2100 A, G6025 subtends ≈2kpc in physical extent when measured from the 1.5s isophote, in agreement with the parametric size measurements that yield a half-light radius of 4.9±0.5kpc and a semimajor axis of 12.5±0.1kpc. It is also very UV-luminous (5LUV, 4 ∗ z∼ ) and young (140±60 Myr). Despite its unusual size and luminosity, the stellar population parameters and dust reddening (M M star ∼ z∗ ∼4 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 offcenter, 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.