Optical and JWST Mid-IR Emission Line Diagnostics for Simultaneous IMBH and Stellar Excitation in z~0 Dwarf Galaxies

TL;DR

This paper explores optical and mid-IR emission line diagnostics to detect intermediate mass black holes in dwarf galaxies, accounting for physical uncertainties, and provides tools for future JWST observations.

Contribution

It introduces photoionization simulations to identify robust emission line diagnostics for IMBHs in dwarf galaxies, considering various physical conditions and uncertainties.

Findings

[O III]/Hβ remains bright for IMBH-powered dwarf AGN.

Mid-IR lines [O IV] and [Ar II] are less sensitive to uncertainties.

Diagnostic diagrams effectively separate starbursts and AGN at low redshift.

Abstract

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.