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Session 67 - ISM in Galaxies.
Display session, Wednesday, January 15
Metropolitan Ballroom,

[67.05] The UV-Infrared Energy Budget in Spiral Galaxies: Is There a Single Dominant Grain-Heating Population?

L. V. Jones (U. Florida), W. C. Keel (U. Alabama)

We consider the energy balance between absorption of radiation from various stellar populations by dust grains and far-infrared (FIR) emission from heated grains. We use HIRES processing of the IRAS data for improved spatial resolution along with optical, H\alpha, and archival UV imagery to trace stellar populations of different characteristic ages testing the assertion that OB stars dominate the grain heating in typical spiral galaxies. We convolve the images with point spread functions derived from the 100\mum beam images in order to blur them to the same resolution achieved in the 100\mum HIRES images. The relative luminosities for three basic populations were derived for comparison to the FIR luminosity in those regions: very young (OB stars) from the H\alpha flux, (2) young (through about main-sequence turnoff A) from the direct UV starlight (near 2250 Å\ ), and (3) an old bulge-like population from BVRI imagery. Our sample, defined jointly by angular size and availability of UV images (UIT or rocket), includes M33, M51, M74, M77, M81, M83, and M101.

Based on energy-budget arguments, no single population is always dominant in grain heating within the disks of spiral galaxies. The ratio of emerging starlight to FIR usually increases with distance from the center. This is most likely linked to a decrease of typical dust obscuration (or decrease in amount of dust) with radius in the disk. There is no correlation between the dominant stellar population at each point and the FIR color temperature. This is an indication that the dominant heating source cannot be determined solely from gobal FIR flux ratios.

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