A double molecular disc in the triple-barred starburst galaxy NGC 6946: structure and stability

TL;DR

This study analyzes the molecular gas structure and stability in NGC 6946, revealing a double exponential density profile, a flat stability parameter, and the influence of various factors on its dynamics, especially within the central kiloparsec.

Contribution

It introduces a detailed stability analysis using the full radial profile of velocity dispersion, highlighting the role of multiple factors in the galaxy's molecular gas dynamics.

Findings

Double exponential surface density profile with distinct scale lengths

Flat Q stability parameter above unity across the disc

Signatures of nuclear starburst and inner bar in instability wavelengths

Abstract

The late-type spiral galaxy NGC 6946 is a prime example of molecular gas dynamics driven by "bars within bars". Here we use data from the BIMA SONG and HERACLES surveys to analyse the structure and stability of its molecular disc. Our radial profiles exhibit a clear transition at distance R ~ 1 kpc from the galaxy centre. In particular, the surface density profile breaks at R ~ 0.8 kpc and is well fitted by a double exponential distribution with scale lengths R_1 ~ 200 pc and R_2 ~ 3 kpc, while the 1D velocity dispersion sigma decreases steeply in the central kpc and is approximately constant at larger radii. The fact that we derive and use the full radial profile of sigma rather than a constant value is perhaps the most novel feature of our stability analysis. We show that the profile of the Q stability parameter traced by CO emission is remarkably flat and well above unity, while the characteristic instability wavelength exhibits clear signatures of the nuclear starburst and inner bar within bar. We also show that CO-dark molecular gas, stars and other factors can play a significant role in the stability scenario of NGC 6946. Our results provide strong evidence that gravitational instability, radial inflow and disc heating have driven the formation of the inner structures and the dynamics of molecular gas in the central kpc.