The nearby spiral galaxy NGC 7331 was spectrally mapped from 5–38 µm using all modules of Spitzer ’ s IRS spectrograph .
A strong new dust emission feature , presumed due to PAHs , was discovered at 17.1 µm .
The feature ’ s intensity is nearly half that of the ubiquitous 11.3 µm band .
The 7–14 µm spectral maps revealed significant variation in the 7.7 and 11.3 µm PAH features between the stellar ring and nucleus .
Weak [ O iv ] 25.9 µm line emission was found to be centrally concentrated in the nucleus , with an observed strength over 10 % of the combined neon line flux , indicating an AGN or unusually active massive star photo-ionization .
Two [ S iii ] lines fix the characteristic electron density in the H ii regions at n _ { e } \lesssim 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 PDR models .
Either additional PDR heating or shocks are required to simultaneously match lines and continuum .