Circumnuclear Multi-phase Gas in Circinus Galaxy I: Non-LTE calculations of CO lines

TL;DR

This paper models CO line emissions in the Circinus galaxy using non-LTE radiation transfer simulations based on a radiation-hydrodynamic model, revealing complex, non-uniform gas conditions and variable CO-to-H2 conversion factors.

Contribution

It introduces a detailed non-LTE radiative transfer analysis of CO lines in a galaxy's nucleus, highlighting the non-uniformity and complexity of the molecular gas structure.

Findings

CO lines peak around J=6 and are not thermalized.

X_CO varies significantly, over an order of magnitude.

Physical conditions differ on parsec scales along lines of sight.

Abstract

In this study, we investigate the line emissions from cold molecular gas based on our previous "radiation-driven fountain model" (Wada 2016), which reliably explains the spectral energy distribution of the nearest type 2 Seyfert galaxy, the Circinus galaxy. Using a snapshot of the best-fit radiation-hydrodynamic model for the central r < 16 pc, in which non-equilibrium X-ray-dominated region chemistry is solved, we conduct post-processed, non-local thermodynamic equilibrium radiation transfer simulations for the CO lines. We obtain a spectral line energy distribution with a peak around J=6, and its distribution suggests that the lines are not thermalized. However, for a given line-of-sight, the optical depth distribution is highly non-uniform between $\tau \ll 1$ and $\tau \gg 1$. The CO-to-H2 conversion factor (X_CO), which can be directly obtained from the results, is not a constant and depends strongly on the integrated intensity, and it differs from the fiducial value for local objects. X_CO exhibits a large dispersion of more than one order of magnitude, reflecting the non-uniform internal structure of a "torus." We also found that the physical conditions differ between grid cells on a scale of a few parsecs along the observed lines of sight; therefore, a specific observed line ratio does not necessarily represent a single physical state of the ISM.