Analysis of ~1500 Massive High-Redshift Galaxies in the Hawaiʻi Twenty Square Degree Survey

The first massive galaxies to form after the Big Bang demonstrate the unique evolution and early properties of our universe. The search for massive high-redshift galaxies at z > 3.5 (less than 2 billion years post-Big Bang) has thus far been limited by a lack of adequate coverage and depth in long-wavelength imaging. The H20 survey covers two 10 deg² contiguous fields and combines Subaru Hyper Suprime-Cam grizy optical imaging with Spitzer [3.6 micron] + [4.5 micron] deep-IR imaging. This survey provides some of the first robust characterizations of populations of massive galaxies at these redshifts. In our analysis, we use the ultra-deep H20 survey to identify a sample of ~1500 galaxies at 3.5 < z < 5.8 with masses > 10^10.5 M_☉, which have well-defined physical properties. With these sources, we extend previous work investigating the size-mass relation. We find that sizes quantified by effective radii generally increase with mass out to z ~ 4.5-4.7. We also identify candidate quiescent, star-forming, and post-starburst galaxies, and find that the fractions of post-starburst and quenched galaxies rapidly decrease above z = 3.8 and z = 4.1 respectively. Through preliminary visual classification, which distinguishes isolated and disturbed sources, we estimate a major-merger fraction of ~40% at 3.5 < z < 3.8, which agrees with predictions from recent cosmological simulations.