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Date and Time of the Query: 2019-06-26 T13:02:51 PDT
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For refcode 1999AJ....118.2055D:
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1999AJ....118.2055D TOWARD AN UNDERSTANDING OF THE MID-INFRARED SURFACE BRIGHTNESS OF NORMAL GALAXIES DANIEL A. DALE, GEORGE HELOU, NANCY A. SILBERMANN, AND ALESSANDRA CONTURSI Infrared Processing and Analysis Center, MS 100-22, California Institute of Technology, Pasadena, CA 91125 SANGEETA MALHOTRA Kitt Peak National Observatory, National Optical Astronomy Observatories, P.O. Box 26732, Tucson, AZ 85726 AND ROBERT H. RUBIN NASA Ames Research Center, MS 245-3, Moffett Field, CA 94035 Received 1999 July 12; accepted 1999 August 9 ABSTRACT We report a mid-infrared color and surface brightness analysis of IC 10, NGC 1313, and NGC 6946, three of the nearby galaxies studied under the Infrared Space Observatory Key Project on Normal Galaxies. Images with less than 9" (170 pc) resolution of these nearly face-on, late-type galaxies were obtained using the LW2 (6.75 micron) and LW3 (15 micron) ISOCAM filters. Although their global flux ratios are similar and typical of normal galaxies, they show distinct trends of this color ratio with mid- infrared surface brightness. We find that I_{nu}_(6.75 micron)/I_{nu}_(15 micron) <~ 1 only occurs for regions of intense heating activity where the continuum rises at 15 micron and where polycyclic aromatic hydrocarbon destruction can play an important role. The shape of the color-surface brightness trend also appears to depend, to the second order, on the hardness of the ionizing radiation. We discuss these findings in the context of a two-component model for the phases of the interstellar medium and suggest that star formation intensity is largely responsible for the mid-infrared surface brightness and colors within normal galaxies, whereas differences in dust column density are the primary drivers of variations in the mid-infrared surface brightness between the disks of normal galaxies. Key words: galaxies: individual (IC 10, NGC 1313, NGC 6946)-galaxies: ISM
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