RESOLVE and ECO: Finding Low-Metallicity $z\sim0$ Dwarf AGN Candidates Using Optimized Emission-Line Diagnostics

Abstract

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 ∼0 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 masslimited RESOLVE and ECO surveys to determine the AGN percentage in strong emission line dwarfs. Our photoionization grids show that the [O III]/Hβ versus [S II]/Hα diagram (SII plot) and [O III]/Hβ versus [O I]/Hα diagram (OI plot) are less metallicity sensitive and more successful in identifying dwarf AGN than the popular [O III]/Hβ versus [N II]/Hα 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 ∼0 AGN percentage in dwarfs with strong emission lines to be ∼3-15%, far exceeding most previous optical estimates (∼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 ∼1-2%. The newly identified SF-AGN are mostly gas-rich dwarfs with halo mass < 10 11.5 M , where highly efficient cosmic gas accretion is expected. Almost all SF-AGN also have low metallicities (Z 0.4 Z ), demonstrating the advantage of our method.

Hundreds of Low-mass Active Galaxies in the Galaxy And Mass Assembly (GAMA) Survey

“…The active fraction using the He II/Hβ ratio and [Fe X]-emitting galaxies are each ∼ 0.2%, and the fraction of detectable [Ne V]-emitting galaxies is just ∼ 0.01%. While accurate comparisons to other spectroscopic searches for AGNs in the low-mass regime are complicated by various selection criteria and the differing survey characteristics, the values we find are in approximate agreement with prior work (Reines et al 2013;Moran et al 2014;Sartori et al 2015;Molina et al 2021b;Polimera et al 2022).…”