Thick disc molecular gas fraction in NGC 6946

TL;DR

This study models the molecular gas distribution in NGC 6946, revealing a largely constant thick disc molecular gas fraction of about 70%, and estimates the amount of extra-planar molecular gas with high spatial resolution.

Contribution

It introduces a novel method to estimate the radial profile of thick disc molecular gas fraction in an external galaxy using spectral width profiles.

Findings

Thick disc molecular gas fraction is approximately 70% across NGC 6946.

About 50% of molecular gas is extra-planar at the galaxy's center.

The method achieves sub-kpc resolution in external galaxy analysis.

Abstract

Several recent studies reinforce the existence of a thick molecular disc in galaxies along with the dynamically cold thin disc. Assuming a two-component molecular disc, we model the disc of NGC 6946 as a four-component system consisting of stars, HI, thin disc molecular gas, and thick disc molecular gas in vertical hydrostatic equilibrium. Following, we set up the joint Poisson-Boltzmann equation of hydrostatic equilibrium and solve it numerically to obtain a three-dimensional density distribution of different baryonic components. Using the density solutions and the observed rotation curve, we further build a three-dimensional dynamical model of the molecular disc and consecutively produce simulated CO spectral cubes and spectral width profiles. We find that the simulated spectral width profiles distinguishably differ for different assumed thick disc molecular gas fractions. Several CO spectral width profiles are then produced for different assumed thick disc molecular gas fractions and compared with the observed one to obtain the best fit thick disc molecular gas fraction profile. We find that the thick disc molecular gas fraction in NGC 6946 largely remains constant across its molecular disc with a mean value of $0.70 \pm 0.09$. We also estimate the amount of extra-planar molecular gas in NGC 6946. We find $\sim 50\%$ of the total molecular gas is extra-planar at the central region, whereas this fraction reduces to $\sim 15\%$ at the edge of the molecular disc. With our method, for the first time, we estimate the thick disc molecular gas fraction as a function of radius in an external galaxy with sub-kpc resolution.