What drives the Coronal Lines? Resolving the forbidden, high-ionization emission regions in a sample of AGNs

TL;DR

This study investigates the physical conditions and ionization mechanisms of coronal line regions in AGNs using high-resolution spectroscopy, establishing a new black hole mass relation and preparing for JWST observations to further refine these insights.

Contribution

It introduces a novel black hole mass scaling relation with coronal lines and demonstrates photoionization as the primary process, paving the way for future high-resolution spatial studies with JWST.

Findings

Established a black hole mass scaling relation with [Si VI] line

Photoionization by accretion disk continuum is the main ionization mechanism

Prepared for JWST to spatially resolve coronal line regions in AGNs

Abstract

Emission-line studies in the active galactic nuclei (AGNs), particularly those utilizing high spatial resolution, provide the most accurate method to determine critical quantities of the central engine and of the gas a few tens of parsecs away. Using seeing-limited data with spectroscopy, we have explored the extended narrow-line region for a sample of active galactic nuclei (AGNs) with strong, forbidden emission lines that have high-ionization potentials (IP $\gtrsim$ 100 eV). We have studied the optical and near-infrared spectra for these AGNs, extracted and compared their spectral energy distributions, and put constraints on the physical conditions of the region producing the coronal lines. We have realized a novel black hole mass scaling relation with one such prominent coronal line - [Si VI] 1.963 microns, over the 10$^6$ - 10$^8$ M$_{\odot}$ interval, that suggests photoionization by the continuum produced by the accretion disk is the primary physical process at play here. We perform a detailed parameter space study to optimize the emission from these coronal lines in terms of fundamental black hole parameters and test predictions that can be used to measure the kinematics of the extended X-ray emission gas. With the successful launch and first light of the JWST, we are well-poised to refine our findings using the superb angular resolution of the telescope that will allow us to map the inner few parsecs to the central supermassive black holes. This opens up the study of the higher ionization lines that will be spatially resolved by JWST, expanding our sample to tens of hundreds of AGNs, and putting firmer constraints on the physical conditions in the coronal line region.