1998ApJ...500..181S

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 µm. These maps show a prominent circumnuclear ring of radius 0'.25x0'.75 (1.1x3.3 kpc) encircling an extended central source. The 7.7 and 11.3 µm dust emission features are strong in this galaxy, contributing approximately one-third of the total IRAS 12 µm broadband flux. In contrast to starburst galaxies, the 15 µm 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 nor a large increase in the 15 µm continuum in the star-forming ring. In the bulge there is some enhancement of the LW2 (6.75 µm) flux, probably because of contributions from photospheric light; however, the 11.3 µm dust feature is also seen, showing additional emission from interstellar or circumstellar dust.