The optical-to-radio spectral energy distributions of low-metallicity blue compact dwarf galaxies

¹ INAF – Istituto di Radioastronomia/Sez. Firenze, Largo Enrico Fermi, 5, 50125 Firenze, Italy e-mail: [hunt;sbianchi]@arcetri.astro.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo Enrico Fermi, 5, 50125, Firenze, Italy e-mail: maiolino@arcetri.astro.it

Received: 11 October 2004
Accepted: 3 January 2005

Abstract

We present global spectral energy distributions (SEDs) from 0.3 μm to 90 cm for a sample of seven low-metallicity blue compact dwarf galaxies (BCDs). In addition to data compiled from the literature, we report new SCUBA data for the galaxies in the sample, including the two most metal-poor star-forming galaxies known, I Zw 18 and SBS 0335-052. The standard starburst templates, M 82 and Arp 220, do not give a good approximation to any of the BCD SEDs in our sample. Several SEDs are instead characterized by: (i) approximately flat radio spectra suggesting dominant thermal processes; (ii) far-infrared (FIR) spectra which peak at or shortward of 60 μm; (iii) a significant warm dust component and absence of Aromatic Features in Emission (or PAHs) in the mid-infrared (MIR). The SEDs of low-metallicity BCDs do not follow “canonical” scaling relations, and the “standard” star-formation indicators based on radio continuum, FIR, and MIR luminosities can be discrepant by factors of 10. We present new models of the dust SEDs from 5 μm to 1 cm, and derive infrared luminosities, dust distributions, temperatures, and masses. The observed SEDs and dust models are interpreted in terms of the active/passive regimes of star formation and trends with metallicity. Finally, we discuss the implications of our results on the  starburst populations which will be detected by forthcoming submm and radio facilities.