SDSS-IV MaNGA: Identification of active galactic nuclei in optical
  integral field unit surveys
  In this paper, we investigate 2727 galaxies observed by MaNGA as of June 2016
to develop spatially resolved techniques for identifying signatures of active
galactic nuclei (AGN). We identify 303 AGN candidates. The additional spatial
dimension imposes challenges in identifying AGN due to contamination from
diffuse ionized gas, extra-planar gas and photoionization by hot stars. We show
that the combination of spatially-resolved line diagnostic diagrams and
additional cuts on H$\alpha$ surface brighness and H$\alpha$ equivalent width
can distinguish between AGN-like signatures and high-metallicity galaxies with
LINER-like spectra. Low mass galaxies with high specific star formation rates
are particularly difficult to diagnose and routinely show diagnostic line
ratios outside of the standard star-formation locus. We develop a new
diagnostic -- the distance from the standard diagnostic line in the line-ratios
space -- to evaluate the significance of the deviation from the star-formation
locus. We find 173 galaxies that would not have been selected as AGN candidates
based on single-fibre spectral measurements but exhibit photoionization
signatures suggestive of AGN activity in the MaNGA resolved observations,
underscoring the power of large integral field unit (IFU) surveys. A complete
census of these new AGN candidates is necessary to understand their nature and
probe the complex co-evolution of supermassive black holes and their hosts.
