Compact [O III] Emission-line Regions ("Green Seeds") in Hα Emitters at Cosmic Noon from JWST Observations

Abstract

We present a rest-frame optical, spatially resolved analysis of more than 100 Hα emitters (HAEs) at z ∼ 2.2 in the ZFOURGE-CDFS field using NIRCam imaging from the JWST Advanced Deep Extragalactic Survey. The ultradeep, high-resolution data give us maps of the resolved emission-line regions of HAEs with stellar mass ranging from 10⁸ M_⊙ to 10¹⁰ M_⊙. An [O III] emission-line map of each HAE is created from the flux excess in the F150W filter, leading to the discovery of a population of kiloparsec-scale compact emission-line regions ("Green Seeds") with high equivalent widths (EW). We obtain a sample of 128 Green Seeds from 68 HAEs with rest-frame EW_{[O III]} > 200 Å. Moreover, 17 of them have extremely large EW_{[O III]} > 1000 Å, suggesting the possible Lyman continuum leakage from these emission-line regions. Embedded within the host galaxy, many Green Seeds correspond to UV star-forming clumps and H II regions, indicating elevated starburst activity in them, with specific star formation rates several times higher than that of the host galaxy. Based on theoretical frameworks, Green Seeds are expected to be formed through gravitational disk instability and/or galaxy mergers. Considering the stellar masses of Green Seeds, we speculate that high-mass Green Seeds may migrate toward the galactic center to build the central bulge, while low-mass Green Seeds are easily disrupted and short lived. Besides, we propose that some Green Seeds could be the progenitors of globular clusters or ultracompact dwarf galaxies observed in the local Universe.