Optical and JWST Mid-IR Emission Line Diagnostics for Simultaneous IMBH
  and Stellar Excitation in z~0 Dwarf Galaxies
  Current observational facilities have yet to conclusively detect $10^3 - 10^4
M_{\odot}$ intermediate mass black holes (IMBHs) that fill in the evolutionary
gap between early universe seed black holes and $z \sim 0$ 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 vs. non-coincident mixing of
IMBH and starlight excitation, open vs. closed surrounding gas cloud
geometries, and different AGN SED shapes. 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$\beta$ typically remains bright for dwarf AGN powered by IMBHs down to
$10^3 M_{\odot}$. 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$\mu$m and [Ar II] 6.98$\mu$m are less sensitive to physical
uncertainties than are optical diagnostics. Based on these emission lines, we
provide several 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
$z\sim0.1$, 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.
