2008ApJ...689..897T

We present new Spitzer, UKIRT, and MMT observations of the blue compact dwarf galaxy (BCD) Mrk 996, with an oxygen abundance of 12+log(O/H)=8.0. This galaxy possesses an extraordinarily dense nuclear star-forming region, with a central density of ∼10⁶/cm3, a very red color J-K=1.8, broad- and narrow-line components, and ionizing radiation as hard as 54.9 eV, as implied by the presence of the [O IV] 25.89 µm line. The infrared morphology of Mrk 996 changes with wavelength, showing extended stellar photospheric emission at 4.5 µm, extended polycyclic aromatic hydrocarbon (PAH) emission at 8.0 µm, and cool extended dust emission at 160 µm. The IRS spectrum shows strong narrow PAH emission, with narrow-line widths and equivalent widths that are high for the metallicity of Mrk 996. Gaseous nebular fine-structure lines are also seen. A CLOUDY model that accounts for both the optical and mid-infrared (MIR) lines requires that they originate in two distinct H II regions: a very dense H II region where most of the optical lines arise, with densities declining from ∼10⁶ at the center to a few hundred/cm3 at the outer radius of ∼580 pc, and a H II region with a density of ∼300/cm3 that is hidden in the optical but seen in the MIR. The infrared lines arise mainly in the optically obscured H II region, while they are strongly suppressed by collisional deexcitation in the optically visible one. The hard ionizing radiation needed to account for the [O IV] 25.89 µm line is most likely due to fast radiative shocks propagating in a dense interstellar medium.