The value we propose, has been obtained by choosing the most appropriate conditions for this task. This value is similar to typical values for other galaxies (de Grijs and van der Kruit, 1996; Peletier et al., 1994; de Jong, 1996). This value seems quite low when compared with the obtained values in the optical (see for instance van der Kruit, 1986; and Barteldress and Dettmar, 1994) but it is well known than the exponential scale lengths decrease with increasing wavelength (de Grijs and van der Kruit, 1996; Peletier et al., 1994; de Jong 1996, Tully et al., 1996). Peletier et al. (1994) found that the ratio is in the range 1.2-2 in a sample of 37 Sb's and Sc's edge-on galaxies, with this ratio increasing with axis ratio, showing that it is an effect that is mostly due to extinction. In this case our determination of the radial scale length of the Milky Way is compatible with values in optical bands as large as 4 kpc. Our value is virtually free of extinction effects, so it is a proper determination of the true scale length of the mass distribution of the Milky Way disc. A similar value ( kpc)has been obtained by Ruphy et al. (1996). Fux and Martinet used indirect methods based on the asymmetric drift equation, and also obtained a similar value ( kpc). We therefore conclude that with respect to the radial scale length, our galaxy is a typical one.
© European Southern Observatory (ESO) 1998
Online publication: January 8, 1998