2025ApJ...982..200R

Precise tracking of the growth in galaxy size and the evolution of merger fractions with redshift is vital for understanding the formation history of submillimeter galaxies (SMGs). This study investigates this evolution over a broad redshift range (1 < z ≲ 6) using a sample of 222 SMGs with a median redshift of $z=2.6{1}_{-0.82}^{+0.89}$ identified by the Atacama Large Millimeter/submillimeter Array and the James Clerk Maxwell Telescope, enhanced by the advanced imaging capabilities of the JWST/NIRCam and MIRI. We find significant evolution in effective radii (R_e) in the rest-frame V band (R_e ∝ (1 + z)^-0.87 ± 0.08^) and near-infrared (NIR) band (R_e ∝ (1 + z)^-0.88 ± 0.11^), with the NIR size evolution resembling that of massive star-forming galaxies (SFGs) at lower redshift. Visual inspections reveal a major merger fraction of 24.3% ± 3.7% and an interaction fraction of up to 48.4% ± 11.1%. The major merger fraction exhibits an increase from 14.7% ± 9.1% at z = 1 to 26.6% ± 8.4% at z = 3, after which it remains approximately constant across the redshift range 3 < z < 6. In contrast, the interaction fraction remains relatively stable across the range 2 < z < 5. Our results indicate that late-stage major mergers are not the primary formation mechanism for SMGs at z < 3, while interactions appear to play a significant role across the broader redshift range of 1 < z < 6. Additionally, Hubble Space Telescope-based major merger identifications may overestimate the true fraction by a factor of 1.7 at z ∼ 2. These findings highlight the varying roles of mergers and interactions in driving the formation of massive, dusty SFGs across different redshifts.