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We present a four-step group-finding algorithm for the Gas in Galaxy Groups (G3) initiative, a spin-off of thez∼ 0 REsolved Spectroscopy Of a Local VolumE (RESOLVE) and Environmental COntext (ECO) surveys. In preparation for future comparisons to intermediate redshift (e.g., the LADUMA survey), we design the group finder to adapt to incomplete, shallow, or nonuniform data. We use mock catalogs to optimize the group finder’s performance. Compared to friends-of-friends (with false-pair splitting), the G3 algorithm offers improved completeness and halo-mass recovery with minimal loss of purity. Combining it with the volume-limited Hicensus data for RESOLVE and ECO, we examine the Hicontent of galaxy groups as a function of group halo mass. Group-integrated HimassM_{H I,grp}rises monotonically over halo massesM_halo∼ 10¹¹–10^14.5M_⊙, pivoting in slope atM_halo∼ 10^11.4M_⊙, the gas-richness threshold scale. We present the first measurement of the scatter in this relation, which has a median of ∼0.3 dex and is asymmetric toward lowerM_{H I,grp}. We discuss interesting tensions with theoretical predictions and prior measurements of theM_{H I,grp}–M_halorelation. In an appendix, we release RESOLVE DR4 and ECO DR3, including updates to survey redshifts, photometry, and group catalogs, as well as a major expansion of the ECO Hiinventory with value-added data products.

 

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 typicalz∼ 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 [Oiii]/Hβversus [Sii]/Hαdiagram (Siiplot) and [Oiii]/Hβversus [Oi]/Hαdiagram (Oiplot) are less metallicity sensitive and more successful in identifying dwarf AGN than the popular [Oiii]/Hβversus [Nii]/Hαdiagnostic (Niiplot or “BPT diagram”). We identify a new category of “star-forming AGN” (SF-AGN) classified as star-forming by the Niiplot but as AGN by the Siiand/or Oiplots. Including SF-AGN, we find thez∼ 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 <10^11.5M_⊙, where highly efficient cosmic gas accretion is expected. Almost all SF-AGN also have low metallicities (Z≲ 0.4Z_⊙), demonstrating the advantage of our method.

 

Abstract Current observational facilities have yet to conclusively detect 10 3 –10 4 M ⊙ intermediate-mass black holes (IMBHs) that fill in the evolutionary gap between seed black holes in the early universe and z ∼ 0 supermassive black holes. Dwarf galaxies present an opportunity to reveal active IMBHs amidst persistent star formation. We introduce photoionization simulations tailored to address key physical uncertainties: coincident versus noncoincident mixing of IMBH and starlight excitation, open versus closed geometries of surrounding gas clouds, and different shapes of the spectral energy distribution of active galactic nuclei (AGN). We examine possible AGN emission line diagnostics in the optical and mid-IR, and find that the diagnostics are often degenerate with respect to the investigated physical uncertainties. In spite of these setbacks, and in contrast to recent work, we are able to show that [O iii ]/H β typically remains bright for dwarf AGN powered by IMBHs down to 10 3 M ⊙ . Dwarf AGN are predicted to have inconsistent star-forming and Seyfert/LINER classifications using the most common optical diagnostics. In the mid-IR, [O iv ] 25.9 μ m and [Ar ii ] 6.98 μ m are less sensitive to physical uncertainties than are optical diagnostics. Based on these emission lines, we provide several diagrams of mid-IR emission line diagnostic diagrams with demarcations for separating starbursts and AGN with varying levels of activity. The diagrams are valid over a wide range of ionization parameters and metallicities out to z ∼ 0.1, so will prove useful for future JWST observations of local dwarf AGN in the search for IMBHs. We make our photoionization simulation suite freely available.