The Detection of Molecular Gas in the Outskirts of NGC 6946

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${}^{\prime \prime }$ 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.

arXiv comment12 pages, 3 figures, accepted in ApJ Letters

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Cited by in corpus (16)

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• Unusually Luminous Giant Molecular Clouds in the Outer Disk of M33
• PHANGS-MeerKAT and MHONGOOSE HI observations of nearby spiral galaxies: physical drivers of the molecular gas fraction, $R_{\mathrm{mol}}$
• PHANGS-ALMA: Arcsecond CO(2-1) Imaging of Nearby Star-Forming Galaxies
• Rise and fall of molecular clouds across the M33 disk
• First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83
• ALMA CO(2-1) observations in the XUV disk of M83
• A detailed view of a Molecular Cloud in the far outer disk of M33
• Submillimeter Array CO(2-1) Imaging of the NGC 6946 Giant Molecular Clouds
• Compact and High Excitation Molecular Clumps in the Extended Ultraviolet Disk of M83