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Abstract We show the variations of the COJ= 2–1/1–0 line ratio (R_21/10) across the barred spiral galaxy M83, using the 46 pc resolution data from the Atacama Large Millimeter/submillimeter Array. TheR_21/10map clearly evidences the systematic large-scale variations as a function of galactic structures. Azimuthally, it starts from lowR_21/10≲ 0.7 in the interarm regions and becomes high ≳0.7 in the bar and spiral arms, suggesting that the density and/or kinetic temperature of molecular gas increase by about a factor of 2–3. This evolution is seen even in the parts of spiral arms without star formation, andR_21/10is often elevated even higher to ∼0.8–1.0 when Hiiregions exist in the vicinity. Radially,R_21/10starts very high ≳1.0 at the galactic center, remains low ≲0.7 in the bar region, increases to ≳0.7 around the bar end, and again decreases to ≲0.7 in the rest of disk where the spiral arms dominate. The evolutionary sequence is synchronized with galactic rotation, and therefore, it is determined largely by the galactic structures and dynamics and is governed by the galactic rotation timescales. TheR_21/10map also shows that the influence of stellar feedback is localized and limited. Massive, large, and non-star-forming molecular structures have lowR_21/10, which also suggests that the bulk molecular gas in the disk is not regulated by stellar feedback, but more likely by galactic structures and dynamics. These results are consistent with suggestions by the earlier studies of the Milky Way and other barred spiral galaxies, and thus, are likely general among barred spiral galaxies in the local Universe. 

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. 

Abstract We present a catalog of clouds identified from the¹²CO (1–0) data of M83, which was observed using the Atacama Large Millimeter/submillimeter Array with a spatial resolution of ∼46 pc and a mass sensitivity of ∼10⁴M_⊙(3σ). The almost full-disk coverage and high sensitivity of the data allowed us to sample 5724 molecular clouds with a median mass of ∼1.9 × 10⁵M_⊙, which is comparable to the most frequently sampled mass of giant molecular clouds by surveys in the Milky Way (MW). About 60% of the total CO luminosity in M83's disk arises from clouds more massive than 10⁶M_⊙. Such massive clouds comprise 16% of the total clouds in number and tend to concentrate toward the arm, bar, and center, while smaller clouds are more prevalent in interarm regions. Most >10⁶M_⊙clouds have peak brightness temperaturesT_peakabove 2 K with the current resolution. Comparing the observed cloud properties with the scaling relations determined by P. M. Solomon et al. (1987, hereafter S87),T_peak> 2 K clouds follow the relations, butT_peak< 2 K clouds, which are dominant in number, deviate significantly. Without considering the effect of beam dilution, the deviations would suggest modestly high virial parameters (medianα_vir∼ 2.7) and low surface mass densities (median Σ ∼ 22M_⊙pc⁻²) for the entire cloud samples, which are similar to values found for the MW clouds by T. S. Rice et al. (2016) and M.-A Miville-Deschênes et al. (2017). However, once beam dilution is taken into account, the observedα_virand Σ for a majority of the clouds (mostlyT_peak<2 K) can be potentially explained with intrinsic Σ of ∼100M_⊙pc⁻²andα_virof ∼1, which are similar to the clouds of S87. 

Abstract We report a CO(J= 3−2) detection of 23 molecular clouds in the extended ultraviolet (XUV) disk of the spiral galaxy M83 with the Atacama Large Millimeter/submillimeter Array. The observed 1 kpc²region is at about 1.24 times the optical radius (R₂₅) of the disk, where CO(J= 2–1) was previously not detected. The detection and nondetection, 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 8.2 × 10²to 2.3 × 10⁴M_⊙and from 2.7 × 10²to 7.5 × 10³M_⊙, 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 common. 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 common cloud mass structure also justifies the use of high-JCO transitions to trace the total gas mass of clouds, or galaxies, even in the high-zuniverse. This study is the first demonstration that CO(3−2) is an efficient tracer of molecular clouds even in low-metallicity environments. 

Abstract We present Atacama Large Millimeter/submillimeter Array (ALMA) imaging of molecular gas across the full star-forming disk of the barred spiral galaxy M83 in CO( J = 1–0). We jointly deconvolve the data from ALMA’s 12 m, 7 m, and Total Power arrays using the MIRIAD package. The data have a mass sensitivity and resolution of 10 4 M ⊙ (3 σ ) and 40 pc—sufficient to detect and resolve a typical molecular cloud in the Milky Way with a mass and diameter of 4 × 10 5 M ⊙ and 40 pc, respectively. The full disk coverage shows that the characteristics of molecular gas change radially from the center to outer disk, with the locally measured brightness temperature, velocity dispersion, and integrated intensity (surface density) decreasing outward. The molecular gas distribution shows coherent large-scale structures in the inner part, including the central concentration, offset ridges along the bar, and prominent molecular spiral arms. However, while the arms are still present in the outer disk, they appear less spatially coherent, and even flocculent. Massive filamentary gas concentrations are abundant even in the interarm regions. Building up these structures in the interarm regions would require a very long time (≳100 Myr). Instead, they must have formed within stellar spiral arms and been released into the interarm regions. For such structures to survive through the dynamical processes, the lifetimes of these structures and their constituent molecules and molecular clouds must be long (≳100 Myr). These interarm structures host little or no star formation traced by H α . The new map also shows extended CO emission, which likely represents an ensemble of unresolved molecular clouds.