2025ApJ...993..154R

Dwarf active galactic nuclei (AGN) serve as the ideal systems for identifying intermediate-mass black holes (IMBHs) down to the most elusive regimes (∼10³-10⁴M_⊙). 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 IIλ4686 and [O I] λ6300, 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.