ALMA observations of cool dust in a low-metallicity starburst, SBS 0335−052^⋆,⋆⋆

¹ INAF − Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
e-mail: hunt@arcetri.astro.it
² ESO, Karl Schwarzschild str. 2, 85748 Garching bei München, Germany
³ INAF − Istituto di Radioastronomia & Italian ALMA Regional Centre, via Gobetti 101, 40129 Bologna, Italy
⁴ Observatorio Astronómico Nacional (OAN)-Observatorio de Madrid, Alfonso XII 3, 28014 Madrid, Spain
⁵ Observatoire de Paris, LERMA (CNRS:UMR8112), 61 Av. de l’Observatoire, 75014 Paris, France
⁶ Universidad Andrés Bello, Deparamento de Ciencias Físicas, República 252, 8370134 Santiago, Chile
⁷ School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
⁸ Max-Planck-Institut für Radioastronomie, auf dem Hügel 69, 53121 Bonn, Germany
⁹ Astronomy Department, King Abdulaziz University, PO Box 80203, Jeddah, Saudia Arabia
¹⁰ Cavendish Laboratory, University of Cambridge, 19 J.J. Thomson avenue, Cambridge CB3 0HE, UK
¹¹ CEA, Laboratoire AIM, Irfu/SAp, Orme des Merisiers, 91191 Gif-sur-Yvette, France

Received: 23 September 2013
Accepted: 21 November 2013

Abstract

We present Atacama Large Millimeter/submillimeter Array (ALMA) Cycle 0 Band 7 observations of an extremely metal-poor dwarf starburst galaxy in the Local Universe, SBS 0335−052 (12 + log (O/H) ~ 7.2). With these observations, dust is detected at 870 μm (ALMA Band 7), but 87% of the flux in this band is due to free-free emission from the starburst. We have compiled a spectral energy distribution (SED) of SBS 0335−052 that spans almost 6 orders of magnitude in wavelength and fit it with a spherical dust shell heated by a single-age stellar population; the best-fit model gives a dust mass of (3.8    ±    0.6) × 10⁴ M_⊙. We have also constructed a SED including Herschel archival data for I Zw 18, another low-metallicity dwarf starburst (12 + log (O/H) ~ 7.17), and fit it with a similar model to obtain a dust mass of (3.4    ±    1.0) × 10² M_⊙. It appears that for such low-metallicity dwarf galaxies, the ratio of their stellar mass to their dust mass is within the range of values found for spirals and other star-forming galaxies. However, compared with their atomic gas mass, the dust mass of SBS 0335−052 far exceeds the prediction of a linear trend of dust-to-gas mass ratio with metallicity, while I Zw 18 falls far below. We use gas scaling relations to assess a putative missing gas component in both galaxies and find that the missing, possibly molecular, gas in SBS 0335−052 is a factor of 6 times higher than the value inferred from the observed Hi column density; in I Zw 18 the missing component is much smaller. Finally, we constrain the H₂ surface density conversion factor α_CO with our upper limit for CO J = 3−2 line in SBS 0335−052, and find that this is consistent with a linear or even super-linear trend of increasing α_CO with decreasing metallicity. Ultimately, despite their similarly low metallicity, the differences in gas and dust column densities in SBS 0335−052 and I Zw 18 suggest that metal abundance does not uniquely define star-formation processes. At some level, self-shielding and the survival of molecules may depend just as much on gas and dust column density as on metallicity. The effects of low metallicity may at least be partially compensated for by large column densities in the interstellar medium.