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

Abstract

Intermediate mass black holes (IMBHs), with masses between 100 − 10 5 M , represent the link between stellar mass black holes and the supermassive black holes that reside in galaxy centers. While these IMBHs are crucial to our understanding of black hole seed formation, black holes of less than ≈ 10 4 M 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 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 M − 10 8 M . We show that infrared CLs are expected to be prominent in the spectra of accreting IMBHs and can potentially be a powerful probe of the black hole mass in AGNs. In particular, emission line ratios involving ions with the highest ionization potentials with respect to those with lower ionization potentials, such as [SiXI]/[SiVI], [SiIX]/[SiVI], and [FeXIII]/[FeVI], vary by as much as seven orders of magnitude over the mass range explored in our calculations, with the highest ratios corresponding to the lowest mass black holes. We identify key emission line ratios that are most sensitive to black hole mass spanning six orders of magnitude in black hole mass. While variations in accretion rate and the physical parameters of the gas can also affect the CL spectrum, we demonstrate that the effect of black hole mass is likely to be the most dramatic over the mass range explored in our models. With the unprecedented sensitivity of JWST, a large number of CLs will be detected for the first time in numerous galaxies, providing important insight into the existence and properties of IMBHs in the local universe, potentially revolutionizing our understanding of this class of object.

Multiwavelength Observations of SDSS J105621.45+313822.1, a Broad-line, Low-metallicity AGN

“…Cann et al 2020, in preparation). The infrared coronal line fluxes are also enhanced when the black hole mass decreases, highlighting the need for infrared spectroscopic observations in the hunt for intermediatemass black holes (Cann et al 2018).…”

“…. This technique can be a powerful avenue in which black holes in this "mass desert" can finally be discovered using future high sensitivity JWST observations (Cann et al 2018). While coronal lines were also seen in the SDSS spectrum of J1056+3138, the predicted fluxes of optical coronal lines are a factor of at least five times less than the infrared coronal lines even in the absence of any extinction (J.…”

The Limitations of Optical Spectroscopic Diagnostics in Identifying Active Galactic Nuclei in the Low-mass Regime

“…In this paper, we model the optical emission line ratios with a simple photoionization model assuming the ionizing radiation field is produced exclusively by an AGN continuum. The details of this model, using XSPEC (Arnaud 1996) and Cloudy c17 (Ferland et al 2017), are discussed in Cann et al (2018). Briefly, the AGN is modeled as a simple geometrically thin, optically thick Shakura-Sunyaev (Shakura and Sunyaev 1973) accretion disk with Comptonized X-ray radiation in the form of a power law and a soft-excess component.…”

“…We did not include the effects of shocks. We point the reader to Cann et al (2018) for details on model parameters and assumptions adopted. We computed a total of 5, 070 models, where ionization parameter was varied between log U = −1 to log U = −4 in increments of 0.25 dex, the hydrogen gas density between log n H /cm −3 = 1.5 to 3.5 in increments of 1.0 dex, the Eddington ratio between 10 −4 − 1 in increments of 1.0 dex, and mass between 100 − 10 8 M in increments of 0.5 dex.…”

A New Sample of (Wandering) Massive Black Holes in Dwarf Galaxies from High-resolution Radio Observations

“…Moreover, the resolution (6 − 12 for WISE) is not sufficient to disentangle potential AGN emission from star formation related emission. The detection of infrared coronal lines with the James Webb Space Telescope may prove to be a useful diagnostic for probing low-mass BHs (Cann et al 2018), however large surveys of dwarf galaxies intended to identify new lowmass AGNs will likely be prohibitively expensive.…”