A local window on galaxy formation with ALMA

Both dust and metals are products of stellar evolution: they are created through nucleosynthesis in stars and expelled into the interstellar medium via winds and supernovae. Large amounts of dust are present in the young, still chemically unevolved Universe. Thus, very short timescales of star formation and consequent chemical enrichment in the early Universe are required to convert the primordial dust- and metal-free environtment into a dusty, metal-rich one, as the present-day Universe.

Studying the connection between dust content, metallicity and star formation at very high redshift is observationally extremely challenging. Highly star-forming, metal-poor dwarf galaxies in the vicinity of the Milky Way offer an unique window over the conditions of primordial galaxy formation and evolution. One such system is SBS0335-052, a galaxy 10000 times smaller than the Milky Way but forming stars at the same level as our galaxy and with a metallicity of only 3% solar.

Hubble Space Telescope optical image of SBS0335-052. The ALMA map at 870μm is overlaid with white contours. Red contours show the radio emission from the Very Large Array. The star formation and dust emission in SBS0335-052 occurs mainly in two luminous compact star clusters.

Leslie Hunt, associated researcher at the Observatory of Arcetri, and her collaborators from Italy, France, Germany, Spain and UK have observed this galaxy at submillimiter wavelength with the Atacama Large Millimeter/submillimiter Array (ALMA) to study its dust and gas content with an unprecedented spatial resolution of 90pc, as part of Early Science Cycle 0 with Band 7 at 870μm. Emission at these wavelengths can come from cool dust, thermal free-free emission from ionized gas and synchrotron emission from supernova remnants. Aided by multiwavelegth data, including far Infrared data from Herschel, the authors could disentangle the different contributions and estimate a mass of cool dust of 38000 solar masses.

Contrasting these observations with those of another well-studied dwarf starburst galaxy, I ZW 18, they noticed that, despite the similar stellar mass, gas mass and metallicity, SBS0335-052 has more than 100 times higher amount of dust than I ZW 18 and 20 times higher than expected from its atomic gas mass. This suggests that a significant fraction of the molecular hydrogen is not traced by the emission from CO molecules, detectable with ALMA, and is thus missing. The similar metallicity but different dust content of the two dwarf galaxies indicates that the role of metallicity in the production and lifetime of dust is not as clear as commonly thought. Gas column density and the compactness of the star forming regions seem to have a comparably important influence on the survival of molecules. Similar observations of other extreme metal-poor galaxies can tell us whether these are general conditions and to which extent these systems can be used as proxies for high-redshift star formation process.

The results of this work are published in the article "ALMA observations of cool dust in a low-metallicity starburst, SBS0335-052", Hunt L. et al, Astronomy & Astrophysics (in press)

The Atacama Large Millimeter/submillimiter Array (ALMA) is an interferometer that operates in the millimeter/submillimeter regime. At its completion it will consist of three different arrays (fifty 12-m antennas, twelve 7-m antennas and four 12-m Total Power antennas. It is located at an altitude of 5000m in northern Chile. The primary science goals of submillimeter astronomy with ALMA are: the high-redshift Universe and Cosmology, galaxies and AGNs, star formation and interstellar medium, exoplanets and stellar evolution. Image credit: ALMA(ESO/NAOJ/NRAO), R. Hills

Edited by Anna Gallazzi and Leslie Hunt