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 \sim 10 ^ { 6 } cm ^ { -3 } .
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 ] 25.89 µm line .
The nucleus is located within an exponential disk with colors consistent with a single stellar population of age \gtrsim 1 Gyr .
The infrared morphology of Mrk 996 changes with wavelength .
IRAC 4.5 µm images show extended stellar photospheric emission from the body of the galaxy , and an extremely red nuclear point source , indicative of hot dust ; IRAC 8 µm 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 \sim 580 pc with densities declining from \sim 10 ^ { 6 } at the center to a few hundreds cm ^ { -3 } at the outer radius , where most of the optical lines arise ; and a H ii region with a density of \sim 300 cm ^ { -3 } that is hidden in the optical but seen in the MIR .
We suggest that 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 an interstellar medium .
A hidden population of Wolf-Rayet stars of type WNE-w or a hidden AGN as sources of hard ionizing radiation are less likely possibilities .