Emission-line Diagnostics for IMBHs in Dwarf Galaxies: Accounting for Black Hole Seeding and Ultraluminous X-Ray Source Excitation

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Abstract
Dwarf active galactic nuclei (AGN) serve as the ideal systems for identifying intermediate-mass black holes (IMBHs) down to the most elusive regimes (∼10 3 –10 4 M ⊙ ). However, the ubiquitously metal-poor nature of dwarf galaxies gives rise to ultraluminous X-ray sources (ULXs) that can mimic the spectral signatures of IMBH excitation. We present a novel photoionization model suite that simultaneously incorporates IMBHs and ULXs in a metal-poor, highly star-forming environment. We account for changes in M BH according to formation seeding channels and metallicity, and changes in ULX populations with poststarburst age and metallicity. We find that broadband X-rays and UV emission lines are insensitive to M BH and largely unable to distinguish between ULXs and IMBHs. Many optical diagnostic diagrams cannot correctly identify dwarf AGN. The notable exceptions include He II λ 4686 and [O I ] λ 6300, for which we redefine typical demarcations to account for ULX contributions. Emission lines in the mid-IR show the most promise in separating stellar, ULX, IMBH, and shock excitation while presenting sensitivity to M BH and f AGN . Overall, our results expose the potential biases in identifying and characterizing dwarf AGN purely on strong-line ratios and diagnostic diagrams rather than holistically evaluating the entire spectrum. As a proof of concept, we argue that recently discovered overmassive black holes (BHs) in high- z JWST AGN might not represent the overall BH population, with many galaxies in these samples potentially being falsely classified as purely star forming.

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