IMBH activity in dwarf galaxies: optimizing mid-IR diagnostics for JWST

Intermediate mass black holes (IMBHs) in the range 10³ - 10⁵ M_☉ have notoriously evaded detection, preventing important constraints on black hole seed mechanisms and the black hole occupation fraction in dwarf galaxies. Photoionization simulations are a useful tool for predicting signatures of IMBH activity in local dwarfs and high-z galaxies so IMBHs can be detected by current and future observational facilities. Unfortunately due to nondetection, much uncertainty exists about the physical conditions present in galaxies that harbor IMBHs, including the shape of the IMBH SED, the proper mixing of non-thermal and starlight excitation, and the geometry of the gas surrounding the IMBH. We compute a vast suite of models that account for these often overlooked uncertainties while covering a broad range of metallicities, ionization parameters, and AGN fractions. We show emission line predictions for common AGN diagnostics, like those in the BPT diagram, are not only profoundly affected by these variables in the ~10³ M_☉ regime, but also include degeneracies that make it difficult to impose simple diagnostic relations for AGN activity. In spite of these setbacks, the [O IV] 25.9 micron line remains a promising choice for constraining AGN activity over a wide range of physical conditions with JWST. We propose a series of mid-IR diagnostic diagrams with demarcations based on our simulation suite and dwarf galaxy observations that will prove useful for unveiling IMBHs with JWST.