The following contribution appeared in:
``The Opacity of Spiral Disks''
eds.: J.I. Davies & D. Burstein (Dordrecht: Kluwer), p. 185

Dust extinction in highly inclined spirals

J.H. Knapen1, J.E. Beckman1, R.A. Jansen2, 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, Spain


We recently studied a small number of highly inclined spiral galaxies, and used their special geometry to determine absolute extinction values of the dust in their dust lanes. We apply a simple model of a uniform mixture of stars and dust to the data, and find that the extinction values in these galaxies are the same as those making up the Galactic extinction law. This Galactic law seems to be universally applicable.

An important question for the subject of how the opacity of spiral discs can influence the interpretation of observations of galaxies is whether the properties of the dust in external galaxies differ widely, or even at all, from those in our own Galaxy. It is not obvious that one can assume that the Galactic extinction law is in fact universal when discussing dust in external spirals.
In a previous study of the Sombrero galaxy (Knapen et al. 1991) we looked at the dust properties in the dust lane that made the galaxy so well-known. The high degree of symmetry and the special geometry of this galaxy, highly inclined, but not edge-on so that the dust lane does not obscure the centre of the galaxy, allowed us to obtain absolute dust extinction values in the dust lane by simply subtracting the unobscured half of the minor axis light profile from the half that is obscured by the dust lane, assuming intrinsic underlying symmetry in the light distribution. By doing this in optical and near-infrared (NIR) bands, and assuming a simple model where the dust is uniformly mixed with the stars (see below), we could conclude that the dust properties were in fact very similar to those of the dust in the Galaxy.

Figure 1 Figure 2

Figure 1. (left): Extinction profiles perpendicular to the major axis, over the dust lane, in UGC 3065, as a function of distance from the centre r. From top to bottom: optical B,V,R and I and NIR J and K' bands.
Figure 2. (right): Extinction ratios A\lambda / AV as functions of the logarithm of the wavelength for the observed data of UGC 3065, UGC 3214 and UGC 11552, fitted with a model where stars and dust are uniformly mixed in the dust lane. Results are compared to the extinction law for our Galaxy (solid line) and for the Sombrero galaxy.

Recently we studied a few more highly inclined spiral galaxies with a geometry very much like that of the Sombrero galaxy: a dust lane along the major axis, but slightly offset from the centre of the galaxy (Jansen et al. 1994). We obtained images in the optical B, V, Rand I bands, and in the NIR J and K' bands of the four galaxies, UGC 3065, UGC 3214, UGC 11552 and MCG 02-10-009. In all the galaxies it is clearly seen that although the dust lane cuts out significant emission from one half of the galaxy in the bluer optical bands, the effects of dust extinction gradually become less important toward the red, and have almost completely disappeared in the K band.

We aligned the frames using positions of foreground stars, and after determining the correct major axis position, subtracted the unobscured part of the minor axis light profile from the dust-obscured part, thus obtaining dust extinction profiles across the dust lanes, for each photometric band studied. In Figure 1 we show these differential profiles for one of the galaxies, UGC 3065. The extinction is given in absolute magnitudes, which are derived as an intrinsic product of the differential method used to produce the extinction profiles. The extinction is strongest, as expected, in the B band, with a peak value of AB = 2.3 mag, and gradually weakens while moving towards longer wavelengths. From these dust extinction profiles we then derived, for each galaxy, ratios A\lambda / AV at different distances from the centre. In a schematic picture, if the dust had the exact properties of Galactic dust, the ratios A\lambda / AV would be exactly those as determined in our Galaxy (extinction law) at all points.

In fact, measured values of A\lambda / AV against AV for these three galaxies are along lines that significantly deviate from the ``Galactic'' line in some cases. To explain these deviations, we have invoked a simple ``uniform'' model, following Walterbos & Kennicutt (1988) and Disney et al. (1989), 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 2, where we plot the fitted values of the extinction ratios A\lambda / 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. The main conclusion is that, 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, an effect we have not studied in detail in the present work. In the case of UGC 11552, the slight deviations from the Galactic law 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.


Disney, M., Davies, J.I. and Phillips, S., 1989, Mon. Not. R. Astr. Soc. 239, 939
Jansen, R.A., Knapen, J.H., Beckman, J.E., Peletier, R.F. and Hes, R., 1994, Mon. Not.R. Astr. Soc. 270, 373
Knapen, J. H., Hes, R., Beckman, J. E. and Peletier, R. F., 1991, Astr. Astrophys., 241, 42
Walterbos, R.A.M. and Kennicutt, R.C., 1988, Astr. Astrophys. 198, 61

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