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Session 24 - Disks, Bars & Halos.
Oral session, Wednesday, January 07
International Ballroom East,

[24.04] The Mid-Infrared Colors of Normal Galaxies

G. Helou, E. Valjavec, N. Silbermann, C. A. Beichman (IPAC/Caltech), H. L. Dinerstein (U. Texas), D. J. Hollenbach (NASA/Ames), D. A. Hunter (Lowell Obs.), K. Y. Lo (U. Illinois), S. Lord, N. Y. Lu, S. Malhotra (IPAC/Caltech), R. H. Rubin (NASA/Ames), G. Stacey (Cornell U.), H. A. Thronson Jr. (U. Wyoming/NASA HQ), M. W. Werner (JPL/Caltech)

In a color-color diagram combining IRAS (60 and 100\mum) and ISOCAM (7 and 15\mum) data, star forming galaxies display a well defined pattern that highlights the difference between quiescent and active galaxies. Quiescent galaxies have a roughly constant ISOCAM 7-to-15\mum color ratio even though the IRAS 60-to-100\mum ratio varies by a factor 2. Active star forming galaxies however show a steeply decreasing mid-infrared color ratio as the 60-to-100\mum ratio continues to increase. The knee appears associated with a radiation intensity about a thousand times greater than the interstellar radiation intensity near the Sun, and an equilibrium grain temperature of about 100K. This behavior can be well understood in terms of the increasing heating intensity pushing the thermally stable dust population to 100K so this population increasingly dominates the 15\mum band, while at lower heating intensities the fluctuating grains dominate both of the ISO-CAM mid-infrared bands.

Based on observations with ISO, an ESA project with instruments funded by ESA Member States (especially the PI countries: France, Germany, the Netherlands and the United Kingdom) with the participation of ISAS and NASA. This research was supported in part by ISO Data Analysis funding from the National Aeronautics and Space Administration.

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