RESOLVE and ECO: Finding Low-metallicity z ∼ 0 Dwarf AGN Candidates Using Optimized Emission-line Diagnostics

Existing star-forming vs. active galactic nucleus (AGN) classification schemes using optical emission-line diagnostics mostly fail for low-metallicity and/or highly galaxies, missing AGN in typical $z\sim0$ dwarfs. To recover dwarfs with strong emission lines (SELs), we present a scheme optimizing the use of existing diagnostics. We SDSS catalogs overlapping volume- and mass-limited RESOLVE ECO surveys to determine percentage SEL Our photoionization grids show that [O III]/H$\beta$ versus [S II]/H$\alpha$ diagram (SII plot) I]/H$\alpha$ (OI are less metallicity sensitive more successful identifying dwarf than popular [N diagnostic (NII plot or "BPT diagram"). identify new category "star-forming AGN" (SF-AGN) classified as by NII but SII OI plots. Including SF-AGN, find SELs be $\sim$3-16\%, far exceeding most previous estimates ($\sim$1\%). The large range our reflects differences spectral fitting methodologies between catalogs. complete nature allows us normalize galaxy statistics full population, reducing $\sim$0.6-3.0\%. newly identified SF-AGN gas-rich halo mass $ < 10^{11.5} M_\odot$, where efficient cosmic gas accretion is expected. Almost all also have low metallicities (Z $\lesssim 0.4$ Z$_\odot$), demonstrating advantage method.