Mid-infrared IRS spectroscopy of NGC 7331:: A first look at the Spitzer Infrared Nearby Galaxies Survey (SINGS) Legacy

The nearby spiral galaxy NGC 7331 was spectrally mapped from 5 to 38 mum using all modules of Spitzer's Infrared Spectrograph (IRS). A strong new dust emission feature, presumed due to polycyclic aromatic hydrocarbons, was discovered at 17.1 mum. The feature's intensity is nearly half that of the ubiquitous 11.3 mum band. The 7-14 mum spectral maps revealed significant variation in the 7.7 and 11.3 mum PAH features between the stellar ring and nucleus. Weak [O IV] 25.9 mum line emission was found to be centrally concentrated in the nucleus, with an observed strength of over 10% of the combined neon line flux, indicating an AGN or unusually active massive star photoionization. Two [S III] lines fix the characteristic electron density in the H II regions at n(e) less than or similar to 200 cm(-3). Three detected H-2 rotational lines, tracing warm molecular gas, together with the observed IR continuum, are difficult to match with standard photodissociation region (PDR) models. Either additional PDR heating or shocks are required to simultaneously match lines and continuum.