Spatially Resolved Spitzer Spectroscopy of the Starburst Nucleus in NGC 5253

Abstract

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 [NeIII]/[NeII] ratio decreases from $\sim8.5$ at the center to $\sim2.5$ at a distance of $\sim250$ pc. The [SIV]/[SIII] follows the [NeIII]/[NeII] ratio remarkably well, being about 4-5 times lower at all distances. Our spectra reveal for the first time PAH emission feature at 11.3$\mu$m and its equivalent width increases significantly with distance from the center. The good anti-correlation between the PAH strength and the product between hardness and luminosity of the UV radiation field suggests photo-destruction of the PAH molecules in the central region. The high-excitation [OIV]25.91$\mu$m line was detected at $0.42\times10^{-20}$W cm$^{-2}$. Our results demonstrate the importance of spatially resolved mid-IR spectroscopy.