CHAOS IV: Gas-Phase Abundance Trends From The First Four CHAOS Galaxies
  The chemical abundances of spiral galaxies, as probed by HII regions across
their disks, are key to understanding the evolution of galaxies over a wide
range of environments. We present LBT/MODS spectra of 52 HII regions in NGC3184
as part of the CHemical Abundances Of Spirals (CHAOS) project. We explore the
direct-method gas-phase abundance trends for the first four CHAOS galaxies,
using temperature measurements from one or more auroral line detections in 190
individual HII regions. We find the dispersion in Te-Te relationships is
dependent on ionization, as characterized by F_5007/F_3727, and so recommend
ionization-based temperature priorities for abundance calculations. We confirm
our previous results that [NII] and [SIII] provide the most robust measures of
electron temperature in low-ionization zones, while [OIII] provides reliable
electron temperatures in high-ionization nebula. We measure relative and
absolute abundances for O, N, S, Ar, and Ne. The four CHAOS galaxies marginally
conform with a universal O/H gradient, as found by empirical IFU studies when
plotted relative to effective radius. However, after adjusting for vertical
offsets, we find a tight universal N/O gradient of alpha_N/O = -0.33 dex/Re
with sigma_tot. = 0.08 for Rg/Re < 2.0, where N is dominated by secondary
production. Despite this tight universal N/O gradient, the scatter in the
N/O-O/H relationship is significant. Interestingly, the scatter is similar when
N/O is plotted relative to O/H or S/H. The observable ionic states of S probe
lower ionization and excitation energies than O, which might be more
appropriate for characterizing abundances in metal-rich HII regions.
