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 micron 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 4.5 micron traces extended stellar photospheric emission from the body of the galaxy and hot dust continuum coming mainly from star-forming regions; 8 micron probes extended PAH emission coming mainly from the general ISM; MIPS 24 and 70 micron images map compact small-grain warm dust emission associated with active star formation, and 160 micron 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 2.8e9 Lsun and a mass of 2.8e6 Msun for the ionizing clusters. We infer an extinction A(V)<3, intermediate between the optical A(V)~0.5 and the radio A(V)~8, consistent with the picture that longer wavelength observations probe more deeply into star-forming regions. 
L. K. Hunt (1), T. X. Thuan (2), M. Sauvage (3), Y. I. Izotov (4) ((1)   INAF-Istituto di Radioastronomia-Sez., Firenze, Ita...