Spatially resolved Spitzer spectroscopy of the starburst nucleus in NGC 5253

We present new Spitzer Space Telescope data on the nearby, low-metallicity starburst galaxy NGC 5253, from the Infrared Array Camera (IRAC) and the Infrared Spectrograph (IRS). The mid-IR luminosity profile of NGC 5253 is clearly dominated by an unresolved cluster near the center, which outshines the rest of the galaxy at longer wavelengths. We find that the [ Ne III]/[ Ne II] ratio decreases from similar to 8.5 at the center to similar to 2.5 at a distance of similar to 250 pc. The [ S IV]/[ S III] follows the [ Ne III]/[ Ne II] ratio remarkably well, being about 4 - 5 times lower at all distances. Our spectra reveal for the first time a PAH emission feature at 11.3 mu m, and its equivalent width increases significantly with distance from the center. The good anticorrelation between the PAH strength and the product between hardness and luminosity of the UV radiation field suggests photodestruction of the PAH molecules in the central region. The high-excitation [O IV] 25.91 mu m line was detected at 0.42 x 10(-20) W cm(-2). Our results demonstrate the importance of spatially resolved mid-IR spectroscopy.