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 affected 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. We find in general a considerable
amount of extinction in spiral galaxies in the central regions, all the way out
to their effective radii. In the outer parts, at DK,21​, or at 3 times the
typical exponential scale lengths of the stellar distribution , we find a
maximum optical depth 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
limit would go down to 0.1.Comment: 4 pages, postscript, gzip-compressed, uuencoded, includes 2 figures.
Accepted for publication in Astronomy & Astrophysics, Letter