First Detection of the Molecular Cloud Population in the Extended Ultraviolet (XUV) Disk of M83

TL;DR

This study reports the first detection of molecular clouds in the extended ultraviolet disk of galaxy M83 using ALMA, revealing that CO(3-2) effectively traces molecular gas even in low-metallicity environments.

Contribution

It provides the first observational evidence of molecular clouds in the XUV disk of M83 using CO(3-2), demonstrating the universality of cloud mass structure and the effectiveness of high-J CO lines in low-metallicity regions.

Findings

Detected 23 molecular clouds in the XUV disk of M83.

Cloud masses range from 820 to 23,000 solar masses.

CO(3-2) is an efficient tracer of molecular gas in low-metallicity environments.

Abstract

We report a CO(3-2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with ALMA. The observed 1kpc^2 region is at about 1.24 times the optical radius (R25) of the disk, where CO(2-1) was previously not detected. The detection and non-detection, as well as the level of star formation (SF) activity in the region, can be explained consistently if the clouds have the mass distribution common among Galactic clouds, such as Orion A -- with star-forming dense clumps embedded in thick layers of bulk molecular gas, but in a low-metallicity regime where their outer layers are CO-deficient and CO-dark. The cloud and clump masses, estimated from CO(3-2), range from 8.2x10^2 to 2.3x10^4 Msun and from 2.7x10^2 to 7.5x10^3 Msun, respectively. The most massive clouds appear similar to Orion A in star formation activity as well as in mass, as expected if the cloud mass structure is universal. The overall low SF activity in the XUV disk could be due to the relative shortage of gas in the molecular phase. The clouds are distributed like chains up to 600 pc (or longer) in length, suggesting that the trigger of cloud formation is on large scales. The universal cloud mass structure also justifies the use of high-J CO transitions to trace the total gas mass of clouds, or galaxies, even in the high-z universe. This study is the first demonstration that CO(3-2) is an efficient tracer of molecular clouds even in low-metallicity environments.