Collaborative Research: Establishing the Physical Picture for Rest-Optical Emission Lines in Star-Forming Galaxies from Cosmic Noon to Cosmic Dawn

The co-PIs have conducted the MOSFIRE Deep Evolution Field (MOSDEF) survey, which includes rest- optical spectra for ∼ 1500 galaxies at z ∼ 1.5−3.5. Based on MOSDEF, there is an emerging consensus on the physical picture underlying the rest-optical emission-line spectra of distant star-forming galaxies. This consensus includes a radiation Nield dominated by massive stars with alpha-enhanced chemical abundance patterns, and lacking emission from diffuse ionized gas (DIG) – both in contrast to local star-forming galaxies. The co-PIs have identiNied several outstanding directions required to complete this picture and carried out several corresponding follow-up spectroscopic programs with Keck instruments (MOSFIRE, LRIS, OSIRIS, and DEIMOS). With these Keck data already in hand, they will perform: (1) the Nirst analysis of neutral oxygen emission in the ISM of z ∼ 2 star-forming galaxies; (2) the Nirst joint in- depth analysis of a z ∼ 2 galaxies covered simultaneously by rest-UV and rest- optical spectra and integral Nield unit spectral maps of the ionized ISM; (3) a Nirst joint analysis of doubly- and singly-ionized sulfur and oxygen emission at z ∼ 1.5; (4) analysis of the largest sample of new and existing direct oxygen abundance measurements at z ∼ 1.5 − 2.5. In addition, the co-PIs will publicly release all new Keck spectroscopic datasets described here, and will also build the public software tool, Spectral Modeler of Integrated Line Emission (SMILE). This tool will incorporate all results from MOSDEF about the nature of the ionizing spectrum, the physical density, and ISM architecture at high redshift, and enable users to model their rest-optical emission-line spectra to obtain galaxy physical properties. Over the past decade, advances in near-infrared spectroscopic instrumentation have enabled the measurement of rest-optical emission lines out to z ∼ 3.5 from the ground for large samples of galaxies. However, there are important questions regarding the translation of rest-optical emission line properties to physical ones (e.g., star-formation rate, gas-phase oxygen abundance, ionization parameter) in distant galaxies. Such uncertainties limit the ability to trace the evolution of galaxy scaling laws over cosmic time such as the mass-metallicity and fundamental metallicity relations. The co-PIs propose to marshal a rich and multi-faceted Keck spectroscopic dataset for a deNinitive sample of z = 1.5 − 2.5 star-forming galaxies drawn from the MOSDEF survey, establish the translation between galaxy emission-line and physical properties, and build and publicly release a software tool for robust modeling of the rest-optical emission-line spectra of distant galaxies all the way back to cosmic dawn (z > 6). Collectively, these products will provide a lasting legacy as we enter into a new era for near-IR spectroscopy with great observatories in space.