iv : a st ro - p h / 06 10 40 5 v 1 1 3 O ct 2 00 6 The Spitzer View of Low - Metallicity Star Formation : I . Haro 3

We present Spitzer observations of the blue compact dwarf galaxy (BCD) Haro 3, with an oxygen abundance of 12+log(O/H)=8.32. These data are part of a larger study of star formation and dust in low-metallicity environments. The IRS spectrum of Haro 3 shows strong narrow Polycyclic Aromatic Hydrocarbon (PAH) emission, with high equivalent widths. Gaseous nebular fine-structure lines are also seen. Despite the absence of optical high-excitation lines, a faint high-ionization [O iv] line at 25.89 μm indicates the presence of radiation as hard as 54.9 eV. A CLOUDY model suggests that the MIR lines originate in two regions: a low-extinction optically-emitting region, and an optically invisible one with much higher extinction. The morphology of Haro 3 changes with wavelength. IRAC 4.5μm traces extended stellar photospheric emission from the body of the galaxy and hot dust continuum coming mainly from star-forming regions; 8μm probes extended PAH emission coming mainly from the general ISM; MIPS 24 and 70μm images map compact small-grain warm dust emission associated with active star formation, and 160μm reflects cooler extended dust associated with older stellar populations. We have derived the optical-to-radio spectral energy distribution (SED) of the brightest star-forming region A in Haro 3. The best-fit DUSTY model of the SED gives a total luminosity of 2.8×10 L⊙ and a mass of 2.8×10 6 M⊙ for the ionizing clusters. We infer an extinction AV.3, intermediate between the optical AV∼ 0.5 and the radio AV∼ 8, consistent with the picture that longer wavelength observations probe more deeply into star-forming regions. Subject headings: Galaxies: individual: Haro 3; Galaxies: compact; Galaxies: dwarf Galaxies: starburst; (ISM:) dust, extinction; ISM: lines and bands; Infrared: galaxies Haro 3 INAF-Istituto di Radioastronomia-Sez. Firenze, L.go Fermi 5, I-50125 Firenze, Italy; [email protected]. Astronomy Department, University of Virginia, P.O. Box 3818, University Station, Charlottesville, VA 22903, USA: [email protected] CEA/DSM/DAPNIA/Service d’Astrophysique, UMR AIM, CE Saclay, 91191 Gif sur Yvette Cedex, France: [email protected] Main Astronomical Observatory, National Academy of Sciences of Ukraine, 03680 Kiev, Ukraine; [email protected]