Early Galaxy Formation Near and Far Probed with SDSS and JWST Data

speaker: Moka Nishigaki

Extremely metal poor galaxies (EMPGs), which are defined by the gas-phase metallicity of < 0.1 Zsun, are important to understand the early phase of galaxy formation. Although EMPGs are actively investigated over decades, there are no EMPGs with metallicities below < 0.01 Zsun found so far, which is called the metallicity floor. In addition, the physical origin of EMPGs in the local universe are not fully understood. We search for the metal-poor galaxy candidates in the local universe below the known possible metallicity floor of < 0.01 Zsun, selecting photometric candidates by broadband color excess and machine-learning techniques with the SDSS photometric data. We remove stellar contaminants from the photometric candidates by shallow spectroscopy with Seimei and Nayuta telescopes. We then conduct deep spectroscopy with Magellan/MagE for faint [OIII]4363 lines and confirm that three candidates are EMPGs with 0.05-0.1 Zsun. We also discuss the physical origin of EMPGs from their clustering and chemical properties. Extending the broadband color excess technique to a high-z EMPG search, we select 17 candidates of z ~ 4–5 EMPGs with the deep near-infrared JWST/NIRCam images obtained by Early Release Observations (ERO) and Early Release Science (ERS) programs. We find galaxy candidates with negligible [OIII]4959,5007 emission weaker than the local EMPGs and known high-z galaxies, suggesting that some of these candidates may fall in 0–0.01 Zsun, which potentially break the lowest metallicity limit known to date.