Danielle Berg (UT Austin)

JWST has revealed galaxies with extreme emission-line spectra and unexpected abundance patterns only a few hundred million years after the Big Bang. These observations offer a window into early star formation and chemical enrichment, but their interpretation depends critically on gas conditions and the sources of ionizing radiation. In this talk, I will combine JWST observations of distant galaxies with detailed studies of nearby analogs to explore what drives these extreme spectra. I will present extremely metal-poor “imposters,” whose high gas densities bias abundance measurements, and show how weakly accreting intermediate-mass black holes can explain high-ionization emission in nearby extreme emission-line galaxies. I will also discuss a nitrogen-enhanced lensed galaxy at z ≈ 6 with Wolf–Rayet star signatures, whose unusual abundance pattern is consistent with rapid, localized enrichment by massive-star winds. Together, these results show how dense gas, massive-star feedback, and black-hole accretion shape galaxy spectra. Disentangling these effects is essential for using chemical abundances as clocks to reconstruct the earliest episodes of galaxy formation.