2008ApJ...676..304B

We present high spatial resolution (∼35 pc) 5-38 µm spectra of the central region of M82, taken with the Spitzer Infrared Spectrograph. From these spectra we determined the fluxes and equivalent widths (EWs) of key diagnostic features, such as the [Ne II] 12.8 µm, [Ne III] 15.5 µm, and H₂S(1) 17.03 µm lines, and the broad mid-IR polycyclic aromatic hydrocarbon (PAH) emission features in six representative regions and analyzed the spatial distribution of these lines and their ratios across the central region. We find a good correlation of the dust extinction with the CO 1-0 emission. The PAH emission follows closely the ionization structure along the galactic disk. The observed variations of the diagnostic PAH ratios across M82 can be explained by extinction effects, within systematic uncertainties. The 16-18 µm PAH complex is very prominent, and its EW is enhanced outward from the galactic plane. We interpret this as a consequence of the variation of the UV radiation field. The EWs of the 11.3 µm PAH feature and the H₂S(1) line correlate closely, and we conclude that shocks in the outflow regions have no measurable influence on the H₂emission. The [Ne III]/[Ne II] ratio is on average low, at ∼0.18, and shows little variations across the plane, indicating that the dominant stellar population is evolved (5-6 Myr) and well distributed. There is a slight increase of the ratio with distance from the galactic plane of M82, which we attribute to a decrease in gas density. Our observations indicate that the star formation rate has decreased significantly in the last 5 Myr. The quantities of dust and molecular gas in the central area of the galaxy argue against starvation and for negative feedback processes, observable through the strong extraplanar outflows.