A super-star cluster in NGC 253: Mid-infrared properties

We observed the nearby starburst galaxy NGC 253 in the mid-infrared to obtain a three-dimensional data set with arcsecond angular resolution and 0.2 mu m spectral resolution. The observations show the major spectral features in the upper half of the mid-IR window: the 11.3 mu m polycyclic aromatic hydrocarbon (PAH) line and the 12.8 mu m [Ne II] line as well as the broad silicate absorption feature at 9.7 mu m. We use the [Ne II] line to determine the emission measure of the ionized gas, and in combination with radio observations to predict the thermal and nonthermal contributions to the radio continuum. The amount of ionized gas is related to the rate of star formation. Based on the mid-IR spectra, we identify three major components in the nucleus of NGC 243: an AGN in the center of the galaxy, a super-star cluster also seen in optical images, and a larger scale diffuse envelope composed of an older population of supernova remnants and lower mass stars.