Comparing Methods of Finding z~0 Dwarf AGN using the RESOLVE and ECO Surveys

Finding AGN in metal-poor, gas-rich, star-forming dwarfs is an ongoing challenge, with complementary detection methods sensitive to different dwarf AGN subpopulations. In this study, we compare the statistics and properties of z~0 dwarf AGN in the volume- and mass-limited RESOLVE and ECO surveys found by three methods: (1) comprehensive optical emission-line classification simultaneously using the [O III]/Hb vs. [N II]/Ha (BPT plot), [S II]/Ha, and [O I]/Ha diagnostic plots, (2) a redefined demarcation line for the BPT plot, and (3) mid-IR colour selection. The comprehensive emission-line method finds a new category of AGN in metal-poor, gas-rich, and star-forming dwarfs, contributing to a ~3-15x higher dwarf AGN percentage than previously identified with strong emission lines (Polimera et al., 2022). Each of the three methods identifies AGN in ~1-2% of dwarfs in the full galaxy population. AGN hosts identified by the comprehensive classification method vs. by mid-IR colours appear similar — mostly dwarfs with high star formation activity, high gas content, and low metallicity — but there is virtually no overlap in the identified objects. Relatively few AGN hosts identified by the redefined BPT plot are dwarfs, and these few dwarfs differ from those identified by the comprehensive classification method or mid-IR colours in having modest star formation activity, low gas content, and high metallicity. All methods together yield a full-population dwarf AGN percentage of ~3-5%, which falls in the middle of the range of dwarf AGN occupation fractions predicted by current AGN simulations.