[Paper Review] Molecular Gas Chemistry in AGN I. The IRAM Survey of NGC1068

A. Usero, S. García‐Burillo+3 more|ArXiv.org|Feb 24, 2004

Astrophysics and Star Formation Studies92 citations

TL;DR

This study investigates molecular gas chemistry in the circumnuclear disk (CND) of the Seyfert 2 galaxy NGC 1068 using IRAM 30m telescope observations. It proposes that X-ray irradiation drives anomalous molecular abundances—particularly high SiO and HOC+—through X-ray Dominated Region (XDR) chemistry, with the HCO+/HOC+ ratio of 30–80 being the lowest ever measured in molecular gas.

ABSTRACT

There is observational evidence that nuclear winds and X-rays can heavily influence the physical conditions and chemical abundances of molecular gas in the circumnuclear disks (CND) of Active Galactic Nuclei (AGN). In this paper we probe the chemical status of molecular gas in the CND of NGC1068, a prototypical Seyfert 2 galaxy. Precedent claims that the chemistry of molecular gas in the nucleus of NGC1068 is abnormal by galactic standards were based on the high HCN/CO luminosity ratio measured in the CND. Results from new observations obtained in this survey have served to derive abundances of molecular species such as SiO, CN, HCO+, HOC+, H13CO+ and HCO. These estimates are complemented by a re-evaluation of molecular abundances for HCN, CS and CO, based on previously published single-dish and interferometer observations of NGC1068. We report on the first detection of SiO emission in the CND of NGC1068. The estimated large abundance of SiO in the CND, X(SiO)~5-10x10^{-9}, cannot be attributed to shocks related to star formation, as there is little evidence of a recent starburst in the nucleus of NGC1068. Alternatively, we propose that silicon chemistry is driven by intense X-ray processing of molecular gas. We also report on the first extragalactic detection of the reactive ion HOC+. Most remarkably, the estimated HCO+/HOC+ abundance ratio in the nucleus of NGC1068, ~30-80, is the smallest ever measured in molecular gas. The abundances derived for all molecules that have been the subject of this survey are compared with the predictions of models invoking either oxygen-depletion or X-ray chemistry in molecular gas. Our conclusions favour an overall scenario where the CND of NGC1068 has become a giant X-ray Dominated Region (XDR).

Motivation & Objective

• To determine the chemical state of molecular gas in the circumnuclear disk (CND) of NGC 1068, a prototypical Seyfert 2 galaxy.
• To resolve the discrepancy between observed high HCN/CO ratios and models of oxygen depletion versus X-ray-driven chemistry.
• To test whether X-ray irradiation can explain the enhanced abundances of reactive species like SiO and HOC+ in the CND.
• To compare observed molecular abundances with predictions from XDR and oxygen-depletion models to identify the dominant chemical mechanism.

Proposed method

• Conducted a molecular line survey of NGC 1068 using the IRAM 30m telescope at millimeter wavelengths.
• Measured emission lines of SiO(3–2), SiO(2–1), HOC+, CN, HCO+, H13CO+, HCO, HCN, CS, and CO.
• Combined new single-dish data with previously published interferometric and single-dish observations for abundance calculations.
• Re-evaluated molecular abundances using consistent excitation and optical depth assumptions across all species.
• Compared observed abundance ratios with predictions from XDR models (X(e⁻) ~ 10⁻⁶–10⁻⁴) and oxygen-depletion models.
• Used spatially resolved data to compare chemical differences between eastern and western CND knots, correlating with Chandra X-ray images.

Experimental results

Research questions

• RQ1 What causes the anomalously high HCN/CO luminosity ratio in NGC 1068’s CND, and is it due to X-ray processing or oxygen depletion?
• RQ2 Why is SiO abundance in the CND significantly higher than in star-forming regions, and can shocks from star formation explain this?
• RQ3 What is the origin of the extreme HCO+/HOC+ abundance ratio of 30–80, the lowest ever measured in molecular gas?
• RQ4 Do spatial variations in molecular abundances between the E and W CND knots correlate with differences in X-ray illumination?
• RQ5 Can XDR models simultaneously reproduce the observed abundances of HCN, SiO, HOC+, and CN in the CND?

Key findings

• First detection of SiO(3–2) and SiO(2–1) emission in the CND of NGC 1068, with a global abundance of X(SiO) ~ (5–10) × 10⁻⁹.
• SiO abundance in the CND is 10× higher than in the surrounding starburst ring, ruling out star formation-driven shocks as the origin.
• First extragalactic detection of the reactive ion HOC+ in NGC 1068, with a HCO+/HOC+ abundance ratio of ~30–80, the smallest ever measured.
• The HOC+ line profile is asymmetric, peaking at red velocities, indicating uneven X-ray processing or formation efficiency in the CND.
• XDR models successfully explain the observed HCN/HCO+ and CN/HCN abundance ratios, while oxygen-depletion models fail.
• Spatially resolved data suggest higher X-ray illumination in the western CND knot, consistent with Chandra images showing stronger Fe Kα emission in that region.