ISOCAM Mid-Infrared Imaging of the Quiescent Spiral Galaxy NGC 7331
  Using the mid-infrared camera (ISOCAM) on the Infrared Space Observatory (ISO), the Sb LINER galaxy NGC 7331 has been imaged in two broadband and four narrowband filters between 6.75 and 15 microns. These maps show a prominent circumnuclear ring of radius 0.25 arcminutes X 0.75 arcminutes (1.1 X 3.3 kpc) encircling an extended central source. The 7.7 and 11.3 micron dust emission features are strong in this galaxy, contributing approximately 1/3 of the total IRAS 12 micron broadband flux from this galaxy. In contrast to starburst galaxies, the 15 micron continuum is weak in NGC 7331. The mid-infrared spectrum does not vary dramatically with position in this quiescent galaxy, showing neither large-scale destruction of the carriers of the emission bands or a large increase in the 15 micron continuum in the star forming ring. In the bulge, there is some enhancement of the 6.75 micron flux, probably because of contributions from photospheric light, however, the 11.3 micron dust feature is also seen, showing additional emission from interstellar or circumstellar dust. 
Beverly J. Smith (IPAC/Caltech)