The Dense Molecular Gas in the Circumnuclear Disk of NGC 1068

TL;DR

This study uses broadband molecular spectra and Bayesian analysis to characterize the dense, warm gas in NGC 1068's circumnuclear disk, supporting the dominance of AGN-driven X-ray processes.

Contribution

It provides new broadband molecular measurements and Bayesian modeling to constrain physical conditions and molecular abundances in the galaxy's central region.

Findings

Molecules trace warm gas with T > 100 K and densities 10^4.2-10^4.9 cm^-3.

Line ratios are consistent with X-ray dominated region models.

Strong constraints on molecular column densities and total mass in the disk.

Abstract

We present a 190-307 GHz broadband spectrum obtained with Z-Spec of NGC 1068 with new measurements of molecular rotational transitions. After combining our measurements with those previously published and considering the specific geometry of this Seyfert 2 galaxy, we conduct a multi-species Bayesian likelihood analysis of the density, temperature, and relative molecular abundances of HCN, HNC, CS, and HCO+. We find that these molecules trace warm (T > 100 K) gas of H2 number densities 10^4.2 - 10^4.9 cm^-3. Our models also place strong constraints on the column densities and relative abundances of these molecules, as well as on the total mass in the circumnuclear disk. Using the uniform calibration afforded by the broad Z-Spec bandpass, we compare our line ratios to X-ray dominated region (XDR) and photon-dominated region models. The majority of our line ratios are consistent with the XDR models at the densities indicated by the likelihood analysis, lending substantial support to the emerging interpretation that the energetics in the circumnuclear disk of NGC 1068 are dominated by accretion onto an active galactic nucleus.