The Detection of Molecular Gas in the Outskirts of NGC 6946

TL;DR

This study detects molecular gas in the outer regions of NGC 6946, revealing its distribution, properties, and the factors influencing its presence beyond the optical disk.

Contribution

It provides new observational evidence of molecular gas in galaxy outskirts and examines the factors affecting its detectability, challenging previous assumptions.

Findings

Molecular gas detected up to 1.3 R$_{25}$, tentative at 1.4 R$_{25}$

Molecular gas column densities range from 0.3 to 9×10^{20} cm$^{-2}$

No single factor like HI column, metallicity, or heating rate solely determines molecular gas presence

Abstract

We present the results of a search for molecular gas emission via the CO line in the far outer disk of the nearby spiral, NGC 6946. The positions targeted were chosen to lie on or near previously-identified outer disk HII regions. Molecular gas was clearly detected out to 1.3 R$_{25}$, with a further tentative detection at 1.4 R$_{25}$. The CO detections show excellent agreement with the HI velocities and imply beam-averaged column densities of $0.3-9\times 10^{20}$ cm$^{-2}$ and molecular gas masses of (2-70)$\times 10^{5}$ M$_{\sun}$ per 21$''$ beam (560pc). We find evidence for an abrupt decrease in the molecular fraction at the edge of the optical disk, similar to that seen previously in the azimuthally-averaged areal star formation rate. Our observations provide new constraints on the factors that determine the presence and detectability of molecular gas in the outskirts of galaxies, and suggest that neither the HI column, the metallicity or the local heating rate alone plays a dominant role.