RESOLVE and ECO: Finding Low-Metallicity $z\sim0$ Dwarf AGN Candidates
  Using Optimized Emission-Line Diagnostics
  Existing star-forming/AGN classification schemes using optical emission-line
diagnostics mostly fail for low-metallicity and/or highly star-forming
galaxies, missing AGN in typical $z\sim0$ dwarfs. To recover AGN in dwarfs with
strong emission lines, we present a classification scheme optimizing the use of
existing optical diagnostics. We use SDSS emission-line catalogs overlapping
the volume- and mass-limited RESOLVE and ECO surveys to determine the AGN
percentage in strong emission line dwarfs. Our photoionization grids show that
the [O III]/H$\beta$ versus [S II]/H$\alpha$ diagram (SII plot) and [O
III]/H$\beta$ versus [O I]/H$\alpha$ diagram (OI plot) are less metallicity
sensitive and more successful in identifying dwarf AGN than the popular [O
III]/H$\beta$ versus [N II]/H$\alpha$ diagnostic (NII plot or "BPT diagram").
We identify a new category of "Star Forming-AGN" (SF-AGN) classified as
star-forming by the NII plot but as AGN by the SII and/or OI plots. Including
SF-AGN, we find the $z\sim0$ AGN percentage in dwarfs with strong emission
lines to be $\sim$3-15%, far exceeding most previous optical estimates
($\sim$1%). The large range in our dwarf AGN percentage reflects differences in
spectral fitting methodologies between catalogs. The highly complete nature of
RESOLVE and ECO allows us to normalize strong emission-line galaxy statistics
to the full galaxy population, reducing the dwarf AGN percentage to $\sim$1-2%.
The newly identified SF-AGN are mostly gas-rich dwarfs with halo mass $ <
10^{11.5} M_\odot$, where highly efficient cosmic gas accretion is expected.
Almost all SF-AGN also have low metallicities (Z $\lesssim 0.4$ Z$_\odot$),
demonstrating the advantage of our method.
