CHAOS I: Direct Chemical Abundances for HII Regions in NGC 628
  The CHemical Abundances of Spirals (CHAOS) project leverages the combined
power of the Large Binocular Telescope (LBT) with the broad spectral range and
sensitivity of the Multi Object Double Spectrograph (MODS) to measure "direct"
abundances in large samples of HII regions in spiral galaxies. We present LBT
MODS observations of 62 HII regions in the nearby NGC628. We measure one or
more auroral lines ([OIII] 4363, [NII] 5755, [SIII] 6312, or [OII] 7320,7330)
in a large number of HII regions (40). Comparing derived temperatures from
multiple auroral line measurements, we find: a strong correlation between
temperatures based on [SIII] and [NII]; and large discrepancies for some
temperatures based on [OII] and [OIII]. These trends are consistent with other
observations in the literature, yet, given the widespread use and acceptance of
[OIII] as a temperature determinant, the magnitude of the T[OIII] discrepancies
still came as a surprise. Based on these results, we conduct a uniform
abundance analysis using the temperatures derived from [SIII] and [NII], and
report the gas-phase abundance gradients for NGC628. Relative abundances of
S/O, Ne/O, and Ar/O are constant across the galaxy, consistent with no
systematic change in the upper IMF over the sampled range in metallicity. These
alpha-element ratios, along with N/O, all show small dispersions consistent
with no intrinsic dispersion (0.05) over 70% of the azimuthally averaged
radius. We interpret these results as an indication that, at a given radius,
the interstellar medium in NGC628 is chemically well-mixed. Unlike the nearly
temperature-independent gradients, O/H abundances have a larger intrinsic
dispersion of ~0.13 dex. We posit that this dispersion represents an upper
limit to the true dispersion in O/H at a given radius and that some of that
dispersion is due to systematic uncertainties arising from temperature
measurements.
