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

Mugdha S. Polimera; Sheila J. Kannappan; Chris T. Richardson; Ashley S. Bittner; Carlynn Ferguson; Amanda J. Moffett; Kathleen D. Eckert; Jillian M. Bellovary; Mark A. Norris

doi:10.3847/1538-4357/ac6595

paper

RESOLVE and ECO: Finding Low-Metallicity $z \sim 0$ Dwarf AGN Candidates Using Optimized Emission-Line Diagnostics

arXiv:2204.03633 · doi:10.3847/1538-4357/ac6595

Abstract

Existing star-forming vs. active galactic nucleus (AGN) classification schemes using optical emission-line diagnostics mostly fail for low-metallicity and/or highly star-forming galaxies, missing AGN in typical $z \sim 0$ dwarfs. To recover AGN in dwarfs with strong emission lines (SELs), 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 SEL dwarfs. Our photoionization grids show that the [O III]/H $β$ versus [S II]/H $α$ diagram (SII plot) and [O III]/H $β$ versus [O I]/H $α$ diagram (OI plot) are less metallicity sensitive and more successful in identifying dwarf AGN than the popular [O III]/H $β$ versus [N II]/H $α$ 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 $z \sim 0$ AGN percentage in dwarfs with SELs to be $\sim$ 3-16\%, 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 $\sim$ 0.6-3.0\%. The newly identified SF-AGN are mostly gas-rich dwarfs with halo mass $< 1 0^{11.5} M_{⊙}$ , 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.

31 pages, 16 figures, 3 tables. Accepted by ApJ on April 3 2022

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

Abstract

References in corpus (31)

RESOLVE and ECO: Finding Low-Metallicity $z \sim 0$ Dwarf AGN Candidates Using Optimized Emission-Line Diagnostics