The neutral gas phase nearest to supermassive black holes

Abstract

Broad Line Regions (BLRs) are known to contain gravitationally-bound gas within r~(few) 10^2-10^3 Schwarszchild radii (Rs) near supermassive black holes (SMBHs) in Active Galactic Nuclei (AGNs). Photo-ionized by 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. Nearly all BLR studies focus on its ionized gas phase (hereafter BLR+), with typical masses of only a few 10-100 M_Sun, despite strong indications of neutral BLR gas reservoirs (hereafter BLR0) with M_BLR0~10^{5-6} M_Sun. We used the photoionization code CLOUDY, with its chemistry augmented using three-body reactions, to explore 1-D models of dustless BLRs, focusing on the BLR0 conditions, and the abundances of its most prevalent neutral atoms and molecules. A neutral-atom-) and molecule-rich BLR0 gas phase is found underlying the BLR+, the latter occupying 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 and submillimeter 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/far-ultraviolet (FUV) lines. We find that neutral-atom-rich 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 spacetimes.

arXiv commentAccepted for publication in A&A Letter after language correction

References in corpus (22)

• The UMIST database for astrochemistry 2012
• A PDR-Code Comparison Study
• Spatially resolved rotation of the broad-line region of a quasar at sub-parsec scale
• What the Optical Recombination Lines Can Tell Us About the Broad Line Regions of Active Galactic Nuclei
• Molecular hydrogen formation in the interstellar medium
• On the Disappearance of the Broad-Line Region in Low-Luminosity Agns
• What broad emission lines tell us about how active galactic nuclei work
• Dust inflated accretion disc as the origin of the Broad Line Region in Active Galactic Nuclei
• High-Velocity Bipolar Molecular Emission from an AGN Torus
• Cold Accretion Disks and Lineless Quasars
• Radiation pressure confinement - II. Application to the broad line region in active galactic nuclei
• The Mass of Quasar Broad Emission Line Regions
• A physically-based model of the ionizing radiation from active galaxies for photoionization modeling
• H2 formation on PAHs in photodissociation regions: a high-temperature pathway to molecular hydrogen
• Evidence for a hot dust-free inner disk around 51 Oph
• Detection of steam in the circumstellar disk around a massive Young Stellar Object
• Spatially resolving the hot CO around the young Be star 51 Ophiuchi
• Nature of the gas and dust around 51 Oph
• First observation of double-peaked O I emission in the near-infrared spectrum of an active galaxy
• AstroSat/UVIT observations of IC4329A: Constraining the accretion disc inner radius
• A recent update of gas-phase chemical reactions and molecular lines in CLOUDY: its effects on millimeter and sub-millimeter molecular line predictions
• Fractal signatures of non-Kerr spacetimes in the shadow of light-ring bifurcations