Poster paper presented at the 48th Dutch Astronomers Conference, May 6-8 1993, De Haan aan Zee, Belgium.

Extinction by dust in highly inclined galaxies

Rolf Jansen


On this poster we present the results of a surface photometry study of three highly inclined galaxies with well defined dust lanes. The purpose of this study is to shed light on the properties of dust outside our own Galaxy by looking at the dust extinction law in the dust lanes of these galaxies.

Reflection method

We obtained high-quality CCD images of the galaxies in the optical U, B, V, R and I bands and in the Near-Infrared J and K' bands. The symmetry of the surface brightness profiles along the minor axis and along cuts parallel to the minor axis at NIR wavelengths and the geometry of the dust lanes in these galaxies allowed us to use the ``reflection method'' as described in Knapen et al.(1991) for the Sombrero galaxy M 104 (NGC 4594).
We reflect the unobscured part of a surface brightness profile about the major axis (as determined in K') and assume that this would be the light profile were there no dust. We then find the extinction in the obscured part of the profile (where the dust lane runs) by subtracting the true light profile from the reflected profile.

Extinction ratios and extinction law

We plot the extinction values found for each cut in each band against the extinction values found in the V band. We compare our data with several models in the literature. Only for the ``uniform model'' -- dust mixed uniformly with the stars in a galaxy -- we find good general agreement between the values for the extinction ratios A/AV in all three galaxies and those for our Galaxy, as well as M 104. If this model gives a good representation of the extinction in galaxies, our data are consistent with a universal extinction law in which the Galactic numbers can be used.
The uniform model, however, does not fit very well the data for the minor axis profiles at high extinctions. Scattering of light from high z into the line of sight and parasitic light of stars in front of the dust alone cannot explain this, as these effects would increase instead of diminish the deviation from a straight line for higher extinctions. A combination of parasitic light and the dust layer still being in front of the center of the distribution of star light might result in the observed lack of curvature in the A versus AV plots, while yielding apparent extinction ratios that differ from the intrinsic ones.


Knapen, J.H., Hes, R., Beckman, J.E., Peletier, R.F., 1991, A & A 241, 42
Rieke, G.H., Lebofsky, M.J., 1985, Ap.J. 288, 618 and references therein

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