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Intermediate-mass black holes (IMBHs) with masses between $100 -10^5M_{\odot}$ are crucial to our understanding of black hole seed formationand are the prime targets for LISA, yet black holes in this mass range haveeluded detection by traditional optical spectroscopic surveys aimed at findingactive galactic nuclei (AGNs). In this paper, we have modeled for the firsttime the dependence of the optical narrow emission line strengths on the blackhole mass of accreting AGN over the range of $100-10^8M_{\odot}$. We show thatas the black hole mass decreases, the hardening of the spectral energydistribution from the accretion disk changes the ionization structure of thenebula. The enhanced high energy emission from IMBHs results in a more extendedpartially ionized zone compared with models for higher mass black holes. Thiseffect produces a net decrease in the predicted [OIII]/H$\beta$ and[NII]/H$\alpha$ emission line ratios. Based on this model, we demonstrate thatthe standard optical narrow emission line diagnostics used to identify massiveblack holes fail when black hole mass falls below $\approx10^4M_{\odot}$ forhighly accreting IMBHs and for radiatively inefficient IMBHs with active starformation. Our models call into question the ability of common opticalspectroscopic diagnostics to confirm AGN candidates in dwarf galaxies, andindicate that the low-mass black hole occupation fraction inferred from suchdiagnostics will be severely biased.

The Limitations of Optical Spectroscopic Diagnostics in Identifying AGNs in the Low Mass Regime