ALMA resolves molecular clouds in metal-poor Magellanic Bridge A

Abstract

Context. The Magellanic Bridge is a tidal feature located between the Magellanic Clouds, containing young stars formed in situ. Its proximity allows high-resolution studies of molecular gas, dust, and star formation in a tidal low-metallicity environment. Aims. Our goal is to characterize gas and dust emission in Magellanic Bridge A, the source with the highest 870 μm excess of emission found in single-dish surveys. Methods. Using the ALMA telescope including the Morita Array, we mapped a 3′ field of view centered on the Magellanic Bridge A molecular cloud, in 1.3 mm continuum emission and 12CO(2−1) line emission at subparsec resolution. This region was also mapped in continuum at 870 μm and in 12CO(2−1) line emission at ~6 pc resolution with the APEX telescope. To study its dust properties, we also use archival Herschel and Spitzer data. We combine the ALMA and APEX 12CO(2−1) line cubes to study the molecular gas emission. Results. Magellanic Bridge A breaks up into two distinct molecular clouds in dust and 12CO(2−1) emission, which we call North and South. Dust emission in the North source, according to our best parameters from fitting the far-infrared fluxes, is ≈3 K colder than in the South source in correspondence to its less developed star formation. Both dust sources present large submillimeter excesses in LABOCA data: according to our best fits the excess over the modified blackbody (MBB) fit to the Spitzer/Herschel continuum is E(870 μm) ~ 7 and E(870 μm) ~ 3 for the North and South sources, respectively. Nonetheless, we do not detect the corresponding 1.3 mm continuum with ALMA. Our limits are compatible with the extrapolation of the MBB fits, and therefore we cannot independently confirm the excess at this longer wavelength. The 12CO(2−1) emission is concentrated in two parsec-sized clouds with virial masses of around 400 and 700 M⊙. Their bulk volume densities are n(H2) ~ 0.7−2.6 × 103 cm−3, higher than typical bulk densities of Galactic molecular clouds. The 12CO luminosity to H2 mass conversion factor αCO is 6.5 and 15.3 M⊙ (K km s−1 pc2)−1 for the North and South clouds, calculated using their respective virial masses and 12CO(2−1) luminosities. Gas mass estimates from our MBB fits to dust emission yields masses M ~ 1.3 × 103 M⊙ and 2.9 × 103 M⊙ for North and South, respectively, a factor of ~4 higher than the virial masses we infer from 12CO.

Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size-Linewidth Relationship

“…Lower-resolution observations run the risk of small clumps being diluted by large beam sizes, artificially inflating α vir and X CO and also increasing the likelihood of such clumps not being identified at all. Resolved observations of individual pc-scale clumps in distant low-Z environments have only recently become possible and typically yield smaller conversion factors (Muraoka et al 2017;Schruba et al 2017;Saldaño et al 2018;Kalari et al 2020;Valdivia-Mena et al 2020), approaching X CO,M W and in alignment with our expectations for clump f DG .…”

Effects of CO-dark Gas on Measurements of Molecular Cloud Stability and the Size–Linewidth Relationship

“…Lower-resolution observations run the risk of small clumps being diluted by large beam sizes, artificially inflating α vir and X CO , and also increasing the likelihood of such clumps not being identified at all. Resolved observations of individual parsec-scale clumps in distant low-Z environments have only recently become possible and typically yield smaller conversion factors (Muraoka et al 2017;Schruba et al 2017;Saldaño et al 2018;Kalari et al 2020;Valdivia-Mena et al 2020), approaching X CO,MW and in alignment with our expectations for clump f DG .…”

Sequential Star Formation in the Young SMC Region NGC 602: Insights from ALMA

“…The X CO,B ∼ 1.7 X CO,MW we derived is consistent with, albeit slightly lower than, values of X CO,B found in other SMC regions observed at parsec-scales in 12 CO and/or 13 CO. In the Magellanic Bridge, Kalari et al (2020) andValdivia-Mena et al (2020) derived values of X CO of ∼2-4 X CO,MW , while Muraoka et al (2017) found X CO ∼ 4 X CO,MW in the starforming region N83C in the southeast Wing. Jameson et al (2018) similarly found an average X CO ∼ 5 X CO,MW across several star-forming regions in the southwest Bar of the SMC at <3 pc scales.…”