ALMA Resolves Molecular Clouds in the Metal Poor Magellanic Bridge A

M. T. VALDIVIA-MENA; M. RUBIO; A. D. BOLATTO; H. P. SALDAÑO; C. VERDUGO

ASTRONOMY AND ASTROPHYSICS

EDP SCIENCES S A

Lugar: Paris; Año: 2020

0004-6361

Context. The Magellanic Bridge is a tidal feature located between both 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 with sub-parsec resolution a 3´ field of view centeredon the Magellanic Bridge A molecular cloud, in 1.3 mm continuum emission and 12CO(2-1) line emission. 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. Magallanic Bridge A breaks up into two distinct molecular clouds in dust and 12CO(2-1) emission. Dust emission in the North source, according to our best parameters from fitting the FIR 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(870μm) ∼ 7 and E(870μm) ∼ 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 around 400 and 700 Msun each. Their bulk volume densities are n(H2) ∼ 0.7 - 2.6 × 10^3 cm-3 , larger 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 pc^2 )^-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 × 10^3 Msun and 2.9 × 10^3 Msun for North and South respectively, a factor of ∼ 4 larger than the virial masses we infer from 12CO.