ALMA resolves molecular clouds in metal-poor Magellanic Bridge A^★

¹ Departamento de Astronomía, Universidad de Chile, Santiago, Chile
e-mail: maria.valdivia@ug.uchile.cl
² University of Maryland, MD, USA
³ Observatorio Astronómico de Córdoba, UNC, Argentina
⁴ Joint Alma Observatory (JAO), Alonso de Córdova 3107, Vitacura, Santiago, Chile

Received: 2 December 2019
Accepted: 22 June 2020

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 α_CO 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.