2026ApJ...996...70N

We analyze a sample of ∼4500 photometrically selected Hα emitter galaxies at redshifts z ∼ 2 and z ∼ 4-6 selected from James Webb Space Telescope Near-Infrared Camera medium-band images in the Great Observatories Origins Deep Survey South. The bulk (80%) of the galaxies in our sample have stellar masses lower than 10⁸M_⊙, with a median stellar mass of ∼10^7.3M_⊙. We derive Hα rest-frame equivalent widths (EW${}_{0}^{{\rm{H}}\alpha }$), line fluxes, and star formation rates (SFRs) using a robust photometric excess technique tested against spectroscopic measurements, being sensitive to EW${}_{0}^{{\rm{H}}\alpha }\gt 75\,$Å. Both ${{\rm{EW}}}_{0}^{{\rm{H}}\alpha }$ and sSFR(Hα) exhibit an anticorrelation with stellar mass. At fixed stellar mass, low-mass galaxies (M_⋆ < 10⁸M_⊙) display a steep increase in sSFR(Hα) with redshift, following a relation sSFR(Hα) ∝ (1 + z)^2.4 ± 0.3^. In contrast, more massive galaxies (M_⋆ > 10⁸M_⊙) show a significantly flatter evolution, with sSFR(Hα) ∝ (1 + z)^1.0 ± 0.2^. By comparing the Hα and rest-frame UV-derived SFRs, we probe the star formation histories of our galaxies in the past 100 Myr. The fraction of low-mass galaxies with signs of bursty star formation from their SFR(Hα)/SFR(UV) is ∼46%. It quickly drops to ∼25% for M_⋆ > 10⁸M_⊙. This is consistent with the results from sSFR(Hα), showing 72% and 10%, respectively. SFR(Hα)/SFR(UV) is a stricter criterion than those based on the galaxy sSFR(Hα), as it only identifies starbursts that are at the initial phases of a bursty star formation episode.