ALMA resolves molecular clouds in the metal poor Magellanic Bridge A

Abstract

(Abridged)We characterize gas and dust emission in Magellanic Bridge A, which has the highest 870$\mu$m excess of emission found in single dish surveys. Using the ALMA telescope, we mapped the Magellanic Bridge A molecular cloud with sub-parsec resolution, in 1.3 mm continuum and CO(2-1) line emission. We also map the cloud in 870$\mu$m continuum and CO(2-1) line emission at ~6 pc resolution with APEX. We combine the ALMA and APEX CO(2-1) line cubes to study the molecular gas emission. Magellanic Bridge A breaks up into two distinct molecular clouds in dust and CO(2-1) emission, which we call North and South. Dust emission in the North source, according to our best parameters from fitting the far-infrarred 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 are ~7 and ~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(2-1) emission is in two parsec-sized clouds with virial masses around 400 and 700 Mo each. Their volume densities are ~700-2600 cm${}^{-3}$, larger than typical bulk densities of Galactic molecular clouds. The CO-to-H2 conversion factor is 6.5 and 15 M${}_{\odot }$ (K km s${}^{-1}$ pc${}^2$)${}^{-1}$ for the North and South clouds, calculated using their respective virial masses and CO(2-1) luminosities. Gas mass estimates from our MBB fits to dust emission yields masses $M\sim 1.3\times 10^3$ M${}_{\odot }$ and $2.9\times 10^3$ M${}_{\odot }$ for North and South respectively, a factor ~4 larger than the virial masses we infer from CO.

arXiv commentAstronomy & Astrophysics in press. 14 pages, 6 figures, 7 tables

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