The Spitzer View of Low‐Metallicity Star Formation. II. Mrk 996, a Blue Compact Dwarf Galaxy with an Extremely Dense Nucleus

Abstract

We present new Spitzer, UKIRT, and MMT observations of the blue compactdwarf galaxy (BCD) Mrk 996, with an oxygen abundance of 12+log(O/H)=8.0.This galaxy possesses an extraordinarily dense nuclear star-formingregion, with a central density of ~10^{6} cm^{-3}, avery red color J-K=1.8, broad- and narrow-line components, and ionizingradiation as hard as 54.9 eV, as implied by the presence of the [O IV]25.89 {μ}m line. The infrared morphology of Mrk 996 changes withwavelength, showing extended stellar photospheric emission at 4.5 {μ}m,extended polycyclic aromatic hydrocarbon (PAH) emission at 8.0 {μ}m,and cool extended dust emission at 160 {μ}m. The IRS spectrum showsstrong narrow PAH emission, with narrow-line widths and equivalentwidths that are high for the metallicity of Mrk 996. Gaseous nebularfine-structure lines are also seen. A CLOUDY model that accounts forboth the optical and mid-infrared (MIR) lines requires that theyoriginate in two distinct H II regions: a very dense H II region wheremost of the optical lines arise, with densities declining from~10^{6} at the center to a few hundred cm^{-3} at theouter radius of ~580 pc, and a H II region with a density of ~300cm^{-3} that is hidden in the optical but seen in the MIR. Theinfrared lines arise mainly in the optically obscured H II region, whilethey are strongly suppressed by collisional deexcitation in theoptically visible one. The hard ionizing radiation needed to account forthe [O IV] 25.89 {μ}m line is most likely due to fast radiative shockspropagating in a dense interstellar medium.