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

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 ∼ 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 Sloan Digital Sky Survey emission-line catalogs overlapping the volume- and mass-limited REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntex (ECO) surveys to determine the AGN percentage in SEL dwarfs. Our photoionization grids show that the [O iii]/Hβ versus [S ii]/Hα diagram (S ii plot) and [O iii]/Hβ versus [O i]/Hα diagram (O i plot) are less metallicity sensitive and more successful in identifying dwarf AGN than the popular [O iii]/Hβ versus [N ii]/Hα diagnostic (N ii plot or “BPT diagram”). We identify a new category of “star-forming AGN” (SF-AGN) classified as star-forming by the N ii plot but as AGN by the S ii and/or O i plots. Including SF-AGN, we find the z ∼ 0 AGN percentage in dwarfs with SELs to be ∼3%–16%, 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 ∼0.6%–3.0%. The newly identified SF-AGN are mostly gas-rich dwarfs with halo mass <1011.5 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.

CLASSY. VIII. Exploring the Source of Ionization with UV Interstellar Medium Diagnostics in Local High-z Analogs*

“…In this section, we present the most frequently used classification methods in the optical, showing and discussing where the CLASSY galaxies are located in different diagnostic diagrams. In particular, we compared different criteria to discriminate between ionization processes, sometimes finding that there is no one-to-one agreement in classification, especially for low-mass and low-metallicity galaxies (e.g., Groves et al 2006;Shirazi & Brinchmann 2012;Sartori et al 2015;Nakajima & Maiolino 2022;Polimera et al 2022). In Table 2, we summarize the explored optical diagnostics and the corresponding classifications for each galaxy.…”

“…On the one hand, [N II]/Hα is particularly sensitive to metallicity (Denicoló et al 2002;Kewley & Dopita 2002;Pettini & Pagel 2004;Kewley & Ellison 2008), with lower [N II]/Hα at decreasing metallicity (see, e.g., the CLASSY galaxies J1323-0132 and J0808+3948 in Paper V Figure 2). Since higher [N II]/ Hα line ratios are required to classify galaxies as AGN, the [N II]-BPT by itself is not sufficient for identifying AGN in typical lowmetallicity star-forming z ∼ 0 galaxies (e.g., Groves et al 2006;Reines et al 2020;Molina et al 2021;Polimera et al 2022). For instance, Polimera et al (2022) showed that the [O I]-BPT 38 can identify a theoretical dwarf AGN with a spectrum characterized by ∼90% SF contribution and classified as star-forming according to the [N II]-BPT (see also Hogarth et al 2020).…”

“…Since higher [N II]/ Hα line ratios are required to classify galaxies as AGN, the [N II]-BPT by itself is not sufficient for identifying AGN in typical lowmetallicity star-forming z ∼ 0 galaxies (e.g., Groves et al 2006;Reines et al 2020;Molina et al 2021;Polimera et al 2022). For instance, Polimera et al (2022) showed that the [O I]-BPT 38 can identify a theoretical dwarf AGN with a spectrum characterized by ∼90% SF contribution and classified as star-forming according to the [N II]-BPT (see also Hogarth et al 2020). Indeed, [O I]/Hα is highly sensitive to hard radiation fields, particularly shock emission in the neutral ISM (e.g., Osterbrock & Ferland 2006;Allen et al 2008), and is not very dependent on metallicity.…”

“…They have been found useful to discriminate between H II-like sources and objects photoionized by a harder radiation field (e.g., power-law continuum by an AGN or shock excitation). However, existing BPT diagrams can be less effective in discriminating mechanisms other than SF in metalpoor galaxies (e.g., Groves et al 2006;Reines et al 2020;Polimera et al 2022), since, on the one hand, the involved line ratios can be dependent on metallicity and, on the other hand, the hot metal-poor stellar populations have a hard spectrum to irradiate the gas (e.g., Feltre et al 2016;Byler et al 2018;Xiao et al 2018;Wofford et al 2021). In addition, these diagnostics are not optimal in discriminating shocks from other ionization sources.…”

Overmassive Black Holes at Cosmic Noon: Linking the Local and the High-redshift Universe

“…These studies triggered a quest for actively accreting intermediate-mass BHs in dwarf galaxies, where they can be detected in large numbers as low-mass (M BH  10 6 M e ) AGN (e.g., see review by Reines 2022). The use of optical spectroscopy has thus far yielded detections of hundreds of AGN in dwarf galaxies and provided estimates of the AGN occupation fraction (taken as a proxy for BH occupation fraction) ranging from <1% (Reines et al 2013;Salehirad et al 2022) to ∼20% (Polimera et al 2022;Mezcua & Domínguez Sánchez 2020. AGN variability studies of large samples of nearby sources are also providing interesting constraints on the local BH occupation fractions (Burke et al 2024).…”