THE EXTINCTION BY DUST IN THE OUTER PARTS OF SPIRAL GALAXIES

To investigate the distribution of dust in Sb and Sc galaxies we have analyzed near-infrared and optical surface photometry for an unbiased sample of 37 galaxies. Since light in the K-band is very little affect ed by extinction by dust, the B - K colour is a good indicator of the amount of extinction, and using the colour-inclination relation we can statistically determine the extinction for an average Sb/Sc galaxy. W e find in general a considerable amount of extinction in spiral galaxi es in the central regions, all the way out to their effective radii. I n the outer parts, at D-K,D-21, or at 3 times the typical exponential scale lengths of the stellar distribution, we find a maximum optical d epth of 0.5 in B for a face-on galaxy. If we impose the condition that the dust is distributed in the same way as the stars, this upper limi t would go down to 0.1.