SAPPHIRES: Extremely Metal-Poor Galaxy Candidates with $12+\log (O/H)<7.0$ at $z\sim 5-7$ from Deep JWST/NIRCam Grism Observations

Abstract

Population III stars, the hypothetical first generation metal-free stars, have yet to be discovered. Even after three years of successful JWST operations, studies have shown that most galaxies identified to date at $z>5$ exhibit a metallicity floor of $Z\gtrsim 2\%Z_{\odot }$, possibly due to unknown selection biases toward bright galaxies or rapid metal enrichment. To address this question, we search for galaxies with low R3$=$[OIII]$\lambda$5008/H$\beta$ ratios as part of the JWST Cycle-3 large treasury program, the Slitless Areal Pure-Parallel HIgh-Redshift Emission Survey (SAPPHIRES). Using deep NIRCam Wide-Field Slitless Spectrscopy (WFSS) data, we report the discovery of seven extremely metal-poor galaxy candidates in the SAPPHIRES Early Data Release field, with estimated $12+\log (O/H)<7.0$ at $z\sim 5-7$, including two sources with $Z<1\%Z_{\odot }$, significantly breaking the metallicity floor observed both locally and at high redshift. These candidates appear extremely faint ($\sim 28-30$ F200W AB mag) and low-mass ($\log (M_{\ast }/M_{\odot })\sim 6.8-7.8$), as expected from the mass-metallicity relation. They also exhibit very blue UV slopes ($-2.6\lesssim \beta \lesssim -2.0$), likely due to low dust content $A_V\lesssim 0.2\mathrm{mag}$ or young stellar ages $\sim 5-20\mathrm{Myr}$. Compared to galaxies at similar redshift, they appear exceptionally bursty in their star formation activity. Our results highlight the power of NIRCam/WFSS in identifying extremely metal-poor galaxies, from just a single pointing, with more data to come in SAPPHIRES. This underscores the potential of pure-parallel programs towards achieving JWST's primary science goal: discovering the first pristine stars and galaxies. Deep JWST/NIRSpec follow-up observations will also be essential to confirm their nature and perform detailed chemical abundance analyses.

18 pages, 7 figures, submitted