Letter to the Editor

The neutral gas phase nearest to supermassive black holes

Massive neutral-atom- and molecule-rich broad line regions in active galactic nuclei

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse, 85741 Garching, Germany
² Department of Physics, Section of Astrophysics, Astronomy and Mechanics, Aristotle University of Thessaloniki, 54124 Thessaloniki, Greece
³ Research Center for Astronomy, Academy of Athens, Soranou Efesiou 4, 11527 Athens, Greece
⁴ School of Physics and Astronomy, Cardiff University, Queens Buildings, The Parade, Cardiff CF24 3AA, UK

Received: 8 March 2024
Accepted: 9 July 2024

Abstract

Context. Broad line regions (BLRs) are known to contain gravitationally bound gas within a r∼(few) × (10² − 10³) Schwarszchild radii (R_S) near supermassive black holes (SMBHs) in active galactic nuclei (AGNs). Photo-ionized by a strong non-stellar AGN continuum, this gas emits luminous ultraviolet/optical/near-infrared lines from ionized hydrogen (and other multi-ionized atoms) that have the widest velocity profiles observed in galaxies, uniquely indicating the deep gravitational wells of SMBHs.

Aims. Nearly all BLR studies focus on its ionized gas phase (hereafter BLR⁺), with typical masses of only ∼(few) × (10–100) M_⊙, despite strong indications of neutral BLR gas reservoirs (hereafter, BLR⁰) with M_BLR⁰ ∼ 10⁵⁻⁶ M_⊙.

Methods. We used the photoionization code CLOUDY, with its chemistry augmented using three-body reactions, to explore 1D models of dustless BLRs, focusing on the BLR⁰ conditions and the abundances of its most prevalent neutral atoms and molecules.

Results. A (neutral-atom-) and molecule-rich BLR⁰ gas phase is found to be underlying the BLR⁺. The latter occupies only a thin outer layer of AGN-irradiated gas column densities, while the former contains the bulk of the BLR gas mass. Atomic carbon and oxygen as well as the CO molecule can reach substantial abundances in the BLR⁰, while their lines at infrared (IR) and submillimeter (submm) wavelengths can yield new probes of the BLR physical conditions and dynamics, unhindered by the dust absorption from outer AGN tori that readily absorb the BLR⁺ optical and far-ultraviolet (FUV) lines.

Conclusions. We find that neutral-atom-rich and even molecule-rich gas can exist in the BLR⁰. The corresponding spectral lines from neutral atoms and molecules promise a new spectral window of gas dynamics in the vicinity of SMBHs unhindered by dust absorption. This may even offer the prospect of conducting novel tests of general relativity in strongly curved spacetime.