J.H. Knapen1, R.A.Jansen2, J.E. Beckman1, R.F. Peletier123 and R. Hes2
1 Instituto de Astrofísica de Canarias, E-38200 La
Laguna, Tenerife, Spain
2 Kapteyn Astronomical Institute, Postbus~800, NL-9700~AV
Groningen, The Netherlands
3 Observatorio del Roque de los Muchachos, Apartado 321,
E-38700 Santa Cruz de la Palma, Tenerife, Spain
The issue of whether or not spiral galaxies possess significant
quantities of dust in their discs is an important one, and has given
rise to an interesting debate over the last few years. If, as some
authors claim, the major parts of spiral discs are in general opaque,
that would have important effects on their overall observational
properties. It would also significantly change our ideas on the mass
distribution in and around galaxies (the ``dark matter'' issue), since it
implies that by measuring the optical emission from galaxies we greatly
underestimate the amount of luminous matter in the universe. |
An important question is whether the extinction, absorption and
scattering-properties of the dust in external galaxies differ widely, or
even at all, from those in our own Galaxy. Not many studies have been
published on this specific subject, and it is not obvious that one can
assume that the Galactic extinction law is in fact universal when
discussing dust in external spirals. |
In order to compare the minor axis profiles of each galaxy, all the
different frames available have to be precisely aligned. This is
achieved by making Gaussian fits to foreground stars in the images. In
the case of MCG 02-10-009, no stars were available in the frame, the
frames could thus not be aligned, and the galaxy could not be used in
the subsequent analysis. Note that the centres of the galaxies cannot
be used for the alignment, because dust extinction due to the outer
parts of the dust lanes may obscure the central region slightly, and
thus change the central position in especially the bluer bands. |
In fact, when plotting the measured values of A against
AV for these three galaxies, we see that the points
lie along lines that significantly deviate from the ``Galactic'' line in
some cases. To explain these deviations, we have invoked a simple
``uniform'' model, where we assume that the dust and the stars in the
dust lane are uniformly mixed. The results of fitting the data points
with such a model are shown graphically in Figure 4,
where we plotted the fitted values of the extinction ratios
A /
AV as a function of the logarithm of the wavelength,
indicating along the abscissa the location of the photometric bands used
in our imaging. We have also included in the plot the Galactic
extinction law, and the results found in our earlier work for the
Sombrero galaxy. Within the uncertainties, all galaxies follow the
Galactic extinction law. There are some deviations, notably in the
blue, which could well be due to scattering of light within the
galaxies. In the case of UGC 11552, the slight deviations from the
Galactic law visible are possibly a result of a less-than-perfect
determination of the position of the centre. In that galaxy, the dust
lane shows its effects on the light distribution up to quite small
distances from the centre. But in general the results confirm that we
can use the Galactic reddening law in external galaxies, so that it is
almost justifiable, and certainly tempting, to talk of a ``universal
dust extinction law'', at least in the visible and NIR ranges. |
Although the images in visible light of these dust lane galaxies show
that there is a significant amount of dust in the discs of these
galaxies, this cannot be simply taken as proof that these galaxies would
be optically thick in dust if seen face-on. In the central positions we
find maximum values for the extinction of about 2 V-mag in the three
galaxies studied here. The peaks in the extinction quickly decrease at
increasing radial distances from the centre. Considering typical ratios
(of ~10) between scale lengths and heights of dust and stars, we
can conclude that these galaxies would be optically thin in dust over
most off their discs when seen face-on. |