ISOCAM MIDINFRARED IMAGING OF THE QUIESCENT SPIRAL GALAXY NGC-73311

Using the mid-infrared camera (ISOCAM) on the Infrared Space Observato ry (ISO), the Sb LINER galaxy NGC 7331 has been imaged in two broadban d and four narrowband filters between 6.75 and 15 mu m. These maps sho w a prominent circumnuclear ring of radius 0'.25 X 0'.75 (1.1 X 3.3 kp c) encircling an extended central source. The 7.7 and 11.3 mu m dust e mission features are strong in this galaxy, contributing approximately one-third of the total IRAS 12 mu m broadband flux. In contrast to st arburst galaxies, the 15 mu m continuum is weak in NGC 7331. The mid-i nfrared spectrum does not vary dramatically with position in this quie scent galaxy, showing neither large-scale destruction of the carriers of the emission bands nor a large increase in the 15 mu m continuum in the star-forming ring. In the bulge there is some enhancement of the LW2 (6.75 mu m) flux, probably because of contributions from photosphe ric light; however, the 11.3 mu m dust feature is also seen, showing a dditional emission from interstellar or circumstellar dust.