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Some indirect arguments against and favouring the magnetic hypothesis

Apart from the objection by Cuddeford and Binney (1993) to the one-dimension model above mentioned, which was solved in the two-dimension model, Persic and Salucci (1993) considered the magnetic hypothesis as "neither necessary nor sufficient". Setting aside the question of how a theory can be "not necessary", they argued that galaxy pairs need dark matter, but we have seen that galaxy pairs admit other interpretations, in particular that of common halos.

Sánchez-Salcedo (1996) has considered the possibility that a relation found by Bosma (1978, 1981, 1993) between HI and dark matter density could be explained under the magnetic hypothesis. On qualitative grounds this would be reasonable, because the higher the gas density, the higher the magnetic strength that could be amplified.

There is a possible connection between the truncation of stellar disks and the magnetic hypothesis for the rotation curves. Once stars are born, the centripetal magnetic force, previously acting on the progenitor gas cloud, is suddenly interrupted and stars move to larger orbit radii or escape. This escape would be responsible for the truncation of stellar disks, which is a common feature in spirals.

Vallée (1994, 1997) also addresses this point. He considered that newly formed stars would acquire ballistic velocities of the order of the rotation velocity of the parent gaseous cloud. Stars with this velocity have not been observed. If stars were rotating in Keplerian orbits - Vallée argues - they should decelerate. The answer to this problem raised by Vallée (1997) lies in the fact that many new-born stars could escape. Others would simply migrate to more energetic orbits. A numerical model of high velocity new-born stars, escape in the radial direction and truncation of the stellar disk is currently being constructed.

Pfenniger, Combes and Martinet (1994) and Jopikii and Levy (1993) argued that, following the Virial theorem, magnetic fields should have an expansive effect, in contrast with the magnetic centripetal force deduced by Battaner et al. (1992). This is a peculiar argument, as the Virial theorem is deduced from the equation of motion, which was the equation integrated by Battaner et al. (1992). This could constitute a paradox in the one-dimension model. The solution should be found in the two-dimension model (Battaner and Florido, 1995), where it was shown that there is an escape of gas in the vertical direction. In other words, magnetic fields have a contracting effect in the radial direction, but an expansive one in the direction perpendicular to the disk; hence, the net effect of magnetic fields could be expansive. We have also suggested that a large fraction of new-born stars could escape in the radial direction, which is also an expansive dynamical effect. It should be borne in mind that the two-dimension model (1995) was published after the study of Pfenniger, Combes and Martinet (1994).

The inclusion of magnetic fields by Nelson (1988), Battaner et al. (1992) and others have -in the opinion of Pfenniger, Combes and Martinet- "the implicit hope that by complicating the physics new alternatives can emerge". However, magnetic fields were introduced in Physics several centuries ago, while the inward force due to the magnetic tension in a magnetized disk is a conclusion of really elementary physics.

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