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

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 has the peculiarity of possessing an extraordinarily dense nuclear star-forming region, with a central density of ∼ 10 cm. The nuclear region of Mrk 996 is characterized by several unusual properties: a very red color J −K = 1.8, broad and narrow emission-line components, and ionizing radiation as hard as 54.9 eV, as implied by the presence of the [O iv] 25.89μm line. The nucleus is located within an exponential disk with colors consistent with a single stellar population of age & 1 Gyr. The infrared morphology of Mrk 996 changes with wavelength. IRAC 4.5μm images show extended stellar photospheric emission from the body of the galaxy, and an extremely red nuclear point source, indicative of hot dust; IRAC 8μm images show extended PAH emission from the surrounding ISM and a bright nucleus; MIPS 24 and 70 images consist of bright point sources associated with the warm nuclear dust; and 160 μm images map the cooler extended dust associated with older stellar populations. The IRS spectrum shows strong narrow Polycyclic Aromatic Hydrocarbon (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 which 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 of radius ∼ 580 pc with densities declining from ∼ 10 at the center to a few hundreds cm at the outer radius, where most of the optical lines arise; and a H ii region with a density of ∼ 300 cm that is hidden in the optical but seen in the MIR. Astronomy Department, University of Virginia, P.O. Box 400325, Charlottesville, VA 22904-4325, USA: [email protected] INAF-Istituto di Radioastronomia-Sez. Firenze, L.go Fermi 5, I-50125 Firenze, Italy; [email protected]. Main Astronomical Observatory, National Academy of Sciences of Ukraine, 03680 Kiev, Ukraine; [email protected]