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
- Kapteyn Astronomical Institute, Postbus 800, NL-9700 AV Groningen
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.
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
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
Knapen, J.H., Hes, R., Beckman, J.E., Peletier, R.F., 1991, A & A
Rieke, G.H., Lebofsky, M.J., 1985, Ap.J. 288, 618 and
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