SDSS-IV MaNGA: Refining Strong Line Diagnostic Classifications Using
  Spatially Resolved Gas Dynamics
  We use the statistical power of the MaNGA integral-field spectroscopic galaxy
survey to improve the definition of strong line diagnostic boundaries used to
classify gas ionization properties in galaxies. We detect line emission from
3.6 million spaxels distributed across 7400 individual galaxies spanning a wide
range of stellar masses, star formation rates, and morphological types, and
find that the gas-phase velocity dispersion sigma_HAlpha correlates strongly
with traditional optical emission line ratios such as [S II]/HAlpha, [N
II]/HAlpha, [O I]/HAlpha, and [O III]/HBeta. Spaxels whose line ratios are most
consistent with ionization by galactic HII regions exhibit a narrow range of
dynamically cold line of sight velocity distributions (LOSVDs) peaked around 25
km/s corresponding to a galactic thin disk, while those consistent with
ionization by active galactic nuclei (AGN) and low-ionization emission-line
regions (LI(N)ERs) have significantly broader LOSVDs extending to 200 km/s.
Star-forming, AGN, and LI(N)ER regions are additionally well separated from
each other in terms of their stellar velocity dispersion, stellar population
age, HAlpha equivalent width, and typical radius within a given galaxy. We use
our observations to revise the traditional emission line diagnostic
classifications so that they reliably identify distinct dynamical samples both
in two-dimensional representations of the diagnostic line ratio space and in a
multi-dimensional space that accounts for the complex folding of the star
forming model surface. By comparing the MaNGA observations to the SDSS
single-fiber galaxy sample we note that the latter is systematically biased
against young, low metallicity star-forming regions that lie outside of the 3
arcsec fiber footprint.
