Investigating Active Galactic Nuclei in Dwarf and Giant Galaxies in the Nearby Universe

Understanding the occurrence and properties of actively accreting black holes, known as Active Galactic Nuclei (AGN), is crucial for unraveling the co-evolution of black holes and galaxies across different mass scales. However, existing classification systems for identifying AGN based on optical emission-line diagnostics are primarily optimized for giant galaxies with stellar masses, M*> 10^9.5 M_solar. This poses a significant challenge in detecting AGN in low-metallicity and highly star-forming galaxies, particularly typical dwarfs in the local Universe. In this work, we uncover AGN in dwarfs that have strong emission lines using a multi-diagnostic classification system that optimizes the use of existing optical diagnostics. Our multi-diagnostic system is more robust and systematic, providing a consistent categorization of galaxies that lack unique classifications. This systematic approach enables the identification of a hidden population of AGN candidates, referred to as Star Forming-AGN (SF-AGN), in metal-poor, gas-rich, star-forming local dwarfs. We apply this system to the RESOLVE and ECO surveys and discover that the percentage of optical AGN in dwarfs with strong emission lines ranges from 3-15%, which significantly exceeds previous estimates of ~1%.

To investigate the frequency of actively accreting black holes in the nearby Universe, we compile a comprehensive inventory of AGN in the RESOLVE and ECO surveys, using optical emission line diagnostics, including our multi-diagnostic approach for identifying SF-AGN, and mid-IR color-selection using WISE colors. The statistical completeness of the RESOLVE and ECO surveys allows us to calculate accurate AGN occupation fractions, representing the number of AGN relative to the total number of galaxies within the survey volume, including those that did not meet the criteria for any AGN search sample. Among the entire galaxy population, the percentage of optical and mid-IR AGN in dwarfs is approximately 3-5%, whereas in giants it ranges from 20-33%. Notably, the dwarf AGN percentage is at least double the previous estimates, while the AGN occurrence in giants is consistent with earlier studies. A key finding of this research is that AGN hosted by typical local dwarfs are predominantly classified as SF-AGN or WISE AGN.

Subsequently, we extend the optical multi-diagnostic AGN classification system to a large archival dataset based on the SDSS footprint, encompassing 10x the number of galaxies. Similar to the RESOLVE and ECO surveys, we observe that over half of the dwarf AGN in the archival sample fall into the newly identified SF-AGN category. Leveraging the statistical power of this archival dataset, we investigate the relationship between galaxy compactness and AGN activity as a function of mass. Our analysis reveals a contrast in the compactness-AGN relation between dwarfs and giants.

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