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3.5. Dark Matter

The question whether the Universe contains an appreciable amount of `dark', unseen matter of unknown nature is one of the most urgent problems of present-day cosmology. It seems that the larger the system studied, the larger the fraction of dark matter needed to keep the system in gravitational equilibrium.

One of the reasons most commonly quoted for the belief that galaxies contain a substantial amount of dark matter is that the rotation curves remain high out to large distances from the centre in a way that cannot be explained by gravitational forces from the observed visible matter.

As mentioned in Sections 3.2 and 3.4, Jörsäter and van Moorsel (1995) determined the rotation curve of NGC 1365 from a rather complete set of H I measurements with the VLA. They found that the rotation curve is clearly dropping. In addition, they claim the outer part of the disk to be warped which causes the adopted line of nodes to continously change position angle with the radius. Correcting for this warp enhances the drop of the rotation curve which now becomes well represented by a Keplerian curve from R = 240" to the limit of the measurements at 400", i.e. from about 20 up to 35 kpc from the nucleus. From the Keplerian fall-off Jörsäter and van Moorsel find a total mass for the galaxy of Mtot = 3.6 x 1011 Msun (corrected to a distance of 18.6 Mpc). With a total absolute magnitude MB = -21m.5 and assuming MBsun = 5m.48 we get a mass-luminosity ratio Mtot / LB = 5.8 in solar units. This is somewhat smaller than the average value of 6.8 derived by Broeils and van Woerden for galaxies with log Mtot > 10 (Broeils 1992 p. 76). Comparison of optical and H I photometry also indicated to Jörsäter and van Moorsel that NGC 1365 has an unusually small H I halo. From all these facts they concluded that NGC 1365 contains unusually little dark matter, if any at all.

Jörsäter and van Moorsel note that the rotation curve becomes approximately Keplerian already at the same radius where the warp sets in. This seems to leave very little mass for the outer regions of the disk containing the outer parts of the spiral arms. P.A.B. Lindblad et al. (1996) raised the rotation curve in the region 220" < R < 340" by less than 10 km s-1, thus lowering the effect of the warp, in order to reproduce the radial extent of the outer spiral arms. This adjustment of the rotation curve will raise the estimate of the total mass by less than 10%. Kristen (1998) in an investigation of rotation curves for a number of barred galaxies found that NGC 1365, regardless of the warp, seems to be fairly unique as concerns the drop of its rotation curve.

Thus, NGC 1365 may lack a very massive dark matter halo. As the presence of a massive dark halo has been considered to suppress the formation of a bar (see Section 3.3), the question is whether the lack of a massive halo is a condition for the strong bar in NGC 1365.

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