The Spitzer View of Low-Metallicity Star Formation. I. Haro 3

Abstract

We present Spitzer observations of the blue compact dwarf galaxy 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 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 4.5 μm traces extended stellar photospheric emission from the body of the galaxy and hot dust continuum coming mainly from star-forming regions, 8 μm probes extended PAH emission coming mainly from the general ISM, MIPS 24 and 70 μm images map compact small-grain warm dust emission associated with active star formation, and 160 μm reflects cooler extended dust associated with older stellar populations. We have derived the optical-to-radio SED of the brightest star-forming region A in Haro 3. The best-fit DUSTY model of the SED gives a total luminosity of 2.8×10⁹ L_solar and a mass of 2.8×10⁶ M_solar 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.