The Extreme Galaxy Reference Sample: Low Metallicity Galaxies Across Cosmic Time

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Faint, low-mass galaxies are a population of cosmological importance: numerous at high redshifts, they are the most likely source of the photons that re-ionized the universe. Characterization of such galaxies remains one of the key problems in astrophysics. This project will create a publicly-available reference sample of the most highly ionized galaxies, the Extreme Galaxy Reference Sample (EGRS). The EGRS will be used to quantify the diversity in the properties of low-metallicity galaxies and their surrounding gas, in the local universe and at the peak epoch of star formation. The most extensive knowledge of the physical conditions in the galaxies that likely re-ionized the universe comes not from studying reionization-era sources directly, but from detailed examination of plausibly similar objects at low and moderate redshifts. The project includes a multi-layered strategy of sharing science with many audiences, from the general public to undergraduate students, and including curriculum development, outreach, and mentoring of students from under-represented populations.Outstanding questions include the factors that enable the escape of ionizing radiation, the intrinsic ionizing spectra of the most extreme and highly ionized galaxies, and the role of feedback in shaping the properties of the circumgalactic medium (CGM) and enabling ionizing photon escape. The EGRS has two pieces: 1) the Extreme Census of the Local Universe (ExCLU), an imaging survey using four wavelength-adjacent narrowband filters to cover an extraordinarily wide area of 26,470 square degrees that will find tens of thousands of the rarest and most extreme galaxies in the local universe, and enabling studies of their halo masses, evolutionary states, stellar populations, intrinsic ionizing spectra and feedback-related properties, and 2) comprehensive studies of a smaller, carefully selected sample of galaxies at redshift about two, to map the CGM with Lyman-alpha emission and quantify feedback with deep rest-frame ultraviolet imaging, supplemented by higher signal-to-noise, higher resolution studies of gravitationally lensed galaxies. EGRS will provide the community with a baseline for the study of low-mass, low-metallicity galaxies across cosmic time.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.