Astronomy in Focus - XXX Proceedings IAU Symposium No. XXX, 2018 M. T. Lago, ed. c  International Astronomical Union 2020 doi:10.1017/S1743921319004186 The CHAOS Survey Danielle A. Berg 1 , Richard W. Pogge 1 , Evan D. Skillman 2 , Kevin V. Croxall 3 , John Moustakas 4 and Ness Mayker 1 1 Dept. of Astronomy, The Ohio State University, 140 W 18th Ave., Columbus, OH, 43210 2 Institute for Astrophysics, University of Minnesota, 116 Church St., Minneapolis, MN 55455 3 Illumination Works LLC, 5650 Blazer Parkway, Suite 152, Dublin OH 43017 4 Dept. of Physics & Astronomy, Siena College, 515 Loudon Road, Loudonville, NY 12211 Keywords. galaxies: abundances, galaxies: evolution, galaxies: ISM, galaxies: spira l, ISM: HII regions, ISM: abundances, ISM: lines and bands 1. Introduction Our very best understanding of the abundances in spiral galaxies comes from detailed spectroscopic studies and measurements of their faint, sensitive emission lines using either temperature-sensitive auroral collisionally-excited lines (CELs) or recombination lines (RLs). However, the RL method is known to produce systematically higher oxygen abundance measurements than the CEL-method (e.g., Garc´ıa-rojas & Esteban 2007), and which method is more accurate is still debated in the astronomical community. Because of the inherent faintness of both the auroral CELs (∼ 10 2 × fainter than Hβ) and the RLs (∼ 10 3 × fainter than Hβ), extragalactic abundance measurements are commonly based on empirical calibrations to stronger emission lines or photoionization modeling. Unfortunately, these so-called strong-line calibrations, which are the typical method of determining galactic abundances in lower S/N spectra, including IFU surveys and high redshift targets, demonstrate large systematic discrepancies amongst different methods (Kewley & Ellison 2008). Thus, all ISM abundances derived from emission-line spec- tra, especially those based on strong-line calibrations, are fundamentally limited by the current lack of an accurate absolute abundance scale. The CHemical Abundances of Spirals (CHAOS) survey leverages the combined power of the dual 8.4m mirrors on the Large Binocular Telescope (LBT) with the broad spec- tral range (3400 ˚ A <λ< 1μm) and sensitivity of the Multi Object Double Spectrograph (MODS; Pogge et al. 2010) to measure the physical conditions and abundance gradients of nearby spiral galaxies. With a field of view comparable to the angular diameters of the nearest spiral galaxies, we have used laser-machined slit masks to obtain multi-object spectroscopy of ∼ 650 H ii regions across 11 face-on spiral galaxies. To date, 5 galaxies have been analyzed with an unprecedented number of temperature-sensitive auroral CEL detections: NGC 628 (46 regions; Berg et al. 2015), NGC 5194 (30 regions; Croxall et al. 2015), NGC 5457 (75 regions; Croxall et al. 2016), NGC 3184 (32 regions; in prep), and M 33 (in prep). 2. First Results from CHAOS CHAOS has produced the richest data set to date of direct elemental abundances of Hii regions in nearby bright spiral galaxies derived from electron temperature (T e ) measure- ments. In Berg et al. (2015) we observed significant auroral line detections from one or 246 https://doi.org/10.1017/S1743921319004186 Published online by Cambridge University Press