``Observational Benchmarks of Galaxy Evolution from CNO Emission in Extreme Emission Line Galaxies''

 

Emission lines produced by the ionized gas in galaxies encode numerous properties that are important to galaxy evolution, including the current physical conditions and chemical abundances of the emitting gas, the hardness of the stellar radiation field, and the escape of ionizing photons. In turn, these properties are shaped by the star formation history of a galaxy. Since stars produce different metals, e.g., nitrogen, carbon, and oxygen, at different times, the relative abundances of C/O and N/O act as clocks benchmarking important episodes throughout the star formation history. For example, N/O can be interpreted as a tracer of the relative ratio of moderately old stars (>100 Myr) to very young stars (<10 Myr). Recently, high-redshift (z > 6) James Webb Space Telescope (JWST) observations have found galaxies with extremely strong high-energy C, N, and O emission lines that are indicative of high-energy ionizing sources and super-solar relative abundance ratios. This poses a contradiction: how did such young galaxies so early in the universe become enriched with metals from older stars and are they contributing too many ionizing photons? I will discuss our current understanding of these so-called extreme emission line galaxies and what we are learning from rest-UV, optical, and infrared spectra taken with the Hubble Space Telescope (HST) and JWST of low- to high-redshift galaxies. Some of these observations are revising our interpretation of the relative gas abundances and their implications for the chemical evolution, star formation histories, and feedback prescriptions of early galaxies. Such detailed studies of the mechanisms that produce the extreme emission lines are critical to interpreting high-redshift spectroscopic samples and understanding the chemical enrichment of the first galaxies and their contributions to reionization.