Low-mass and High-sSFR H alpha Emitters at z about 2 in the ZFOURGE-COSMOS Field

We present a near-infrared spectroscopic follow-up study of seven H α emitter candidates at z about2 in the ZFOURGE-COSMOS field, selected via the K s-band flux excess method. Spectra obtained with Subaru/SWIMS confirm strong H α emission line at z=2.06-2.49 for all the targets. Three galaxies are low-mass systems ((M/M) < 9) with high specific SFRs, while four are high-mass systems ((M/M) > 10) on the star-forming main sequence. The low-mass galaxies show consistent H α flux measurements between the K s-band flux excess method and the spectroscopy, whereas the high-mass galaxies show higher H α flux inferred from the K s-band flux excess than the spectroscopic measurement which may be attributed to their extended star formation distributions. The low-mass galaxies exhibit extreme [O III] lambda5007 emission with high rest-frame equivalent widths (EW rest) ranging from 480 to 1064. They are characterized by high [O III]/H β ratios (>4) and weak [O II] and [N II] emission lines, indicating high ionization parameters and low metallicity. The line ratios are comparable to those of star-forming galaxies at z 6 and their local analogs. JWST/NIRCam imaging reveals a clear mass-dependent color gradient: the low-mass galaxies present positive F115W-F277W radial gradients with blue cores, while the high-mass galaxies exhibit negative gradients with red cores. These results suggest distinct assembly pathways for low- and high-mass galaxies during the cosmic noon epoch.