ALMA resolves molecular clouds in the 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 mu m 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 (CO)-C-12(2-1) line emission at subparsec resolution. This region was also mapped in continuum at 870 mu m and in (CO)-C-12(2-1) line emission at similar to 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 (CO)-C-12(2-1) line cubes to study the molecular gas emission.

Results. Magellanic Bridge A breaks up into two distinct molecular clouds in dust and (CO)-C-12(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 approximate to 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 mu m) similar to 7 and E(870 mu m) similar to 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 (CO)-C-12(2-1) emission is concentrated in two parsec-sized clouds with virial masses of around 400 and 700 M-circle dot. Their bulk volume densities are n(H-2) similar to 0.7-2.6 x 10(3) cm(-3), higher than typical bulk densities of Galactic molecular clouds. The (CO)-C-12 luminosity to H-2 mass conversion factor alpha(CO) is 6.5 and 15.3 M-circle dot (K km s(-1) pc(2))(-1) for the North and South clouds, calculated using their respective virial masses and (CO)-C-12(2-1) luminosities. Gas mass estimates from our MBB fits to dust emission yields masses M similar to 1.3 x 10(3)M(circle dot) and 2.9 x 10(3)M(circle dot) for North and South, respectively, a factor of similar to 4 higher than the virial masses we infer from (CO)-C-12.