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Daniel Dale, G. Helou, E. Valjavec, N. A. Silbermann, S. Malhotra, C. A. Beichman (IPAC), H.L. Dinerstein (U. Texas), D. J. Hollenbach (NASA Ames), D.A. Hunter (Lowell Obs.), K.Y. Lo (U. Illinois), S.D. Lord, N.Y. Lu (IPAC), R.H. Rubin (NASA Ames), G. Stacey (Cornell U.), H.A. Thronson, Jr. (NASA HQ), M.W. Werner (JPL)
A color and surface brightness study is performed on NGC 1313, one of the nearby systems in the US-Infrared Space Observatory Key Project on Normal Galaxies. ISOCAM images taken using the LW2 (6.75 \mum) and LW3 (15 \mum) filters generally recover the spiral structure seen optically in this mildly inclined and barred Sd galaxy. Though the global LW3/LW2 flux ratio is typical of normal late-type galaxies, its trend with surface brightness differs from that seen in NGC 6946, an Scd galaxy similarly mapped with ISOCAM. The run of LW3/LW2 color with surface brightness increases more dramatically in NGC 1313 than seen in the disk of NGC 6946. This discrepancy illuminates the range of drivers of surface brightness variations in normal galaxies: brighter locations in NGC 1313 represent regions of increased heating intensity, whereas surface filling factors govern surface brightness fluctuations in NGC 6946.