Emission Line Diagnostics for IMBHs in Dwarf Galaxies: Accounting for BH Seeding and ULX Excitation

Chris T. Richardson, Jordan Wels, Kristen Garofali, Julianna M. Levanti, Vianney Lebouteiller, Bret Lehmer, Antara Basu-Zych, Danielle Berg, Jillian M. Bellovary, John Chisholm, Sheila J. Kannappan, Erini Lambrides, Mugdha S. Polimera, Lise Ramambason, Maxime Varese, Thomas Vivona

First listed 2025-05-12 | Last updated 2025-05-12

Abstract

Dwarf AGN serve as the ideal systems for identifying intermediate mass black holes (IMBHs) down to the most elusive regimes ($\sim 10^3 - 10^4 M_{\odot}$). 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 post-starburst 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~II~$λ$4686 and [O~I]~$λ$6300, 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 $f_{\text{AGN}}$. 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 over-massive 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.

Short digest

Builds a multi-wavelength photoionization grid that co-models IMBH accretion and ULX populations in metal-poor, highly star-forming dwarfs, with BH mass tied to seeding channel and metallicity and ULXs evolving with post-starburst age and Z. Finds broadband X-rays and common UV lines carry little leverage on MBH and cannot reliably separate ULXs from IMBHs, while many optical diagrams also fail. Redefines optical cuts using He II λ4686 and [O I] λ6300, and highlights mid-IR ratios (e.g., [Ar II] 6.98μm, [Ne V] 14.3μm, [O IV] 25.9μm) as the cleanest discriminants with sensitivity to MBH and fAGN. Implication: relying on strong-line diagrams alone biases dwarf AGN demographics and may inflate claims of over-massive BHs in some JWST-selected high-z samples.

Key figures to inspect

• SED comparison panel for IMBH vs ULX mixtures: check how decreasing MBH hardens the EUV/soft X-ray and why ULX-inclusive spectra make broadband X-rays and UV lines insensitive to MBH.
• Optical diagnostic diagrams featuring the revised He II λ4686 and [O I] λ6300 demarcations: inspect where ULX-contaminated models land and the minimum fAGN needed for secure IMBH identification.
• Mid-IR line-ratio grids (e.g., [Ne V]14.3μm/[Ne II]12.8μm vs [O IV]25.9μm/[Ar II]6.98μm): look for separation among stellar, ULX, IMBH, and shocks, and how tracks move with MBH and fAGN.
• Parameter-space tracks versus metallicity and post-starburst age: see how evolving ULX populations shift He II, [Ne III], and [O II] ratios and create degeneracies with BH mass.
• Seeding/metallicity scaling figure mapping MBH across the M⋆–Z relation: verify how light vs heavy seeds imprint on emission-line predictions used for dwarf AGN selection.