Professor Andrew Bunker

Abstract:

Abstract JWST is beginning to uncover a population of extremely metal-poor galaxies (EMPGs, $Z < 1~{{\ \rm per\ cent}}~\mathrm{Z}_\odot$) at redshift z > 3, mostly through serendipitous NIRSpec discoveries and blind slitless spectroscopy. To accelerate our understanding of pristine star formation, we further develop a methodology to identify EMPG candidates from photometry, utilizing the extensive deep NIRCam medium-band imaging from JADES. Our EMPG candidates at 2.5 < z < 6.5 exhibit strong photometric boosts by Hα, yet correspondingly weak boosts by [O iii] + Hβ, likely indicating extremely low metallicity to explain their lack of [O iii] emission. We further demand our EMPG candidates to have strong Balmer jumps, as revealed by medium-band imaging, to ensure that they are young starbursts, as opposed to broad-line AGN/LRDs, though contamination by dusty/dense-gas starbursts and highly-obscured AGN remains a concern. SED-fitting with near-pristine models (~0.1–$1~{{\ \rm per\ cent}}~\mathrm{Z}_\odot$) indicates that our 14 EMPG candidates are low-mass (median M* ≈ 106.7 M⊙), faint dwarf galaxies (MUV ≈ −16.9), with high ionizing photon production efficiencies (log (ξion, obs/(Hz erg−1)) ≈ 26.0). Hence these are plausible sites of near-pristine star formation, constituting ~0.02–0.4% of 2.5 < z < 6.5 galaxies at −19 < MUV < −16. We discuss this extremely metal-poor extension to the mass–metallicity relation. We forecast that deep (~28 h) NIRCam slitless spectroscopy can identify bright EMPGs through strong Hβ but lack of [O iii] emission, or secure the redshifts of fainter systems through Hα detections. Highly-multiplexed NIRSpec spectroscopy offers an alternate route to discovering the faintest pristine galaxies out to z = 10, without requiring deep NIRCam medium-band and/or MIRI imaging to identify secure candidates.

Abstract:

Abstract Recent observations revealed that distant galaxies have bursty star formation histories, regulated by stellar or active galactic nuclei (AGN) feedback and gas inflows. According to theoretical models, feedback preferentially removes metal-rich gas, while subsequent starbursts are triggered by mergers and newly accreted gas that is generally less enriched than the galaxy’s interstellar medium (ISM). Therefore, gas-phase metallicity provides key insights into the baryonic processes shaping early galaxies. We present the first NIRSpec/IFU study of spatially resolved ISM properties in the MACS0647-JD system (z = 10.17). The system consists of two stellar components detected in NIRSpec/IFU and NIRCam photometry. The main component (log (M*/M⊙) = 7.77 ± 0.09; $12+\log \left(\rm O/H\right)=7.89 \pm 0.16$) is more massive and significantly more metal-rich compared to its companion (log (M*/M⊙) = 7.42 ± 0.07; $12+\log \left(\rm O/H\right)=7.47 \pm 0.20$), suggesting an older stellar population and a prolonged chemical enrichment history. We find that the Hγ line emission centroid is spatially offset by ~0.1″ (150 pc in the source plane) from the stellar continuum centroid; the latter coincides with the location of the main stellar component. This offset provides possible evidence of a merger-driven starburst in this system. By comparing the spatial distributions of the metallicity, velocity dispersion, and the burstiness of star formation history, we infer the presence of turbulent, metal-poor gas outside the stellar components. This metal-poor, dynamically unstable gas is likely responsible for the enhanced recent star formation in the north-east region of the system.