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 4.5 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 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 \times 10 ^ { 9 } L _ { \sun } and a mass of 2.8 \times 10 ^ { 6 } M _ { \sun } for the ionizing clusters .
We infer an extinction A _ { V } \lesssim 3 , intermediate between the optical A _ { V } \sim 0.5 and the radio A _ { V } \sim 8 , consistent with the picture that longer wavelength observations probe more deeply into star-forming regions .