The Hunt for Intermediate-mass Black Holes in the JWST Era

Abstract

Intermediate-mass black holes (IMBHs), with masses between 100–10 5 M ⊙ , fill the gap between stellar mass black holes and the supermassive black holes that reside in galaxy centers. While IMBHs are crucial to our understanding of black hole seed formation, black holes of less than ≈10 4 M ⊙ have so far eluded detection by traditional searches. Observations of the infrared coronal lines (CLs) offer us one of the most promising tools to discover IMBHs in galaxies. We have modeled the infrared emission line spectrum that is produced by gas photoionized by an active galactic nucleus (AGN) radiation field and explored, for the first time, the dependence of the infrared CL spectrum on black hole mass over the range of 10 2 –10 8 M ⊙ . We show that as the black hole mass decreases, the hardening of the spectral energy distribution of the accretion disk causes infrared coronal lines with the highest ionization potentials to become prominent, revealing a powerful probe of black hole mass in AGNs. We identify key emission line ratios that are most sensitive to black hole mass in the 1–30 μ m range. With the unprecedented sensitivity of the James Webb Space Telescope , a large number of CLs will be detectable for the first time, providing important insight into the existence and properties of IMBHs in the local universe, and potentially revolutionizing our understanding of this class of object.