Compact and high excitation molecular clumps in the extended ultraviolet disk of M83

¹ Observatoire de Paris, LERMA, PSL Univ., CNRS, Sorbonne Univ., Paris, France
² Department of Physics and Astronomy, Stony Brook University, Stony Brook, NY 11794-3800, USA
³ Observatoire de Paris, LERMA, Collège de France, PSL Univ., CNRS, Sorbonne Univ., Paris, France
⁴ Departamento de Astronomia, Universidad de Chile, Casilla 36-D, Santiago, Chile
⁵ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁶ Argelander-Institut für Astronomie, Universität Bonn, Auf dem Hügel 71, D-53121 Bonn, Germany
⁷ Departamento de Física de la Tierra y Astrofísica, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁸ Instituto de Física de Partículas y del Cosmos IPARCOS, Facultad de CC. Físicas, Universidad Complutense de Madrid, 28040 Madrid, Spain
⁹ National Centre for Nuclear Research, Pasteura 7, 02-093 Warsaw, Poland
¹⁰ Department of Astronomy, University of Massachusetts Amherst, 710 North Pleasant Street, Amherst, MA 01003, USA
¹¹ National Radio Astronomy Observatory (NRAO), 520 Edgemont Road, Charlottesville, VA 22903, USA
¹² Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
¹³ National Astronomical Observatory of Japan, Mitaka, Tokyo 181-8588, Japan
¹⁴ Aix Marseille Univ., CNRS, CNES, Laboratoire d’Astrophysique de Marseille, Marseille, France

^⋆ Corresponding author; jin.koda@stonybrook.edu

Received: 15 July 2024
Accepted: 23 September 2024

Abstract

Context. The extended ultraviolet (XUV) disks of nearby galaxies show ongoing massive-star formation, but their parental molecular clouds remain mostly undetected despite searches in CO(1–0) and CO(2–1). The recent detection of 23 clouds in the higher excitation transition CO(3–2) within the XUV disk of M83 thus requires an explanation.

Aims. We test the hypothesis introduced to explain the non-detections and recent detection simultaneously: The clouds in XUV disks have a clump-envelope structure similar to those in Galactic star-forming clouds, having dense star-forming clumps (or concentrations of multiple clumps) at their centers, which predominantly contribute to the CO(3–2) emission and are surrounded by less dense envelopes, where CO molecules are photo-dissociated due to the low-metallicity environment there.

Methods. We utilize new high-resolution ALMA CO(3–2) observations of a subset (11) of the 23 clouds in the XUV disk of M83.

Results. We confirm the compactness of the CO(3–2)-emitting dense clumps (or their concentrations), finding clump diameters below the spatial resolution of 6–9 pc. This is similar to the size of the dense gas region in the Orion A molecular cloud, a local star-forming cloud with massive-star formation.

Conclusions. The dense star-forming clumps are common between normal and XUV disks. This may also indicate that once the cloud structure is set, the process of star formation is governed by the cloud’s internal physics rather than by external triggers. This simple model explains the current observations of clouds with ongoing massive-star formation, although it may require some adjustment, for example including the effect of cloud evolution, to describe star formation in molecular clouds more generally.