Low-mass and High-sSFR H$α$ Emitters at $z \sim 2$ in the ZFOURGE-COSMOS Field

Abstract: We present a near-infrared spectroscopic follow-up study of seven H$α$ emitter candidates at $z\sim2$ in the ZFOURGE-COSMOS field, selected via the $K_{\rm s}$-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 ($\log(M_\star/M_\odot) < 9$) with high specific SFRs, while four are high-mass systems ($\log(M_\star/M_\odot) > 10$) on the star-forming main sequence. The low-mass galaxies show consistent H$α$ flux measurements between the $K_{\rm s}$-band flux excess method and the spectroscopy, whereas the high-mass galaxies show higher H$α$ flux inferred from the $K_{\rm s}$-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 ($\mathrm{EW}_{\mathrm{rest}}$) ranging from 480 to 1064 $\mathring{\mathrm{A}}$. 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 $z \gtrsim 6$ 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.