TRIGGERING THE BISYMMETRIC (M=1) MAGNETIC-FIELDS IN SPIRAL GALAXIES

Observations have shown that the global magnetic field of a spiral galaxy is either mostly axisymmetric (m = 0) or mostly bisymmetric (m = 1) in shape, as one travels around a circular gas orbit around the galactic center. An axisymmetric magnetic field points in the same direction (inward for 360° or vice versa). A bisymmetric magnetic field points in one direction for half the circular orbit and then in the opposite direction for the other half (inward for 180° and then outward for 180°, or vice versa). A search is made to find the observational parameter(s) whose value(s) can be used to predict the global magnetic field shape (m = 0 or m = 1) with reasonable success. For the six spiral galaxies with a definitely known magnetism and the six others with a probable best estimate for their magnetism, I find here that all three spiral galaxies with a H I mass in excess of 8.5 × 109 M⊙ (using H0 = 75 km s-1 Mpc-1) have an axisymmetric (m = 0) magnetic field, while all nine spiral galaxies with a H I gas mass less than that value have a bisymmetric (m = 1) magnetic field or a more complex one. Using the theory of small statistics, the Student's t probability of getting this result by chance is less than 1%, for 12 galaxies. This finding confirms the relation first detected by Vallée (1986) between H I gas mass and magnetic field shape, from a smaller amount of data. In some dynamo theories, the presence of a large disk thickness generates the m = 0 mode and suppresses the m = 1 mode, in accordance with the observations. Other observational parameters are assessed as possible triggers of the m = 1 mode in spiral galaxies, such as tidal effects from a companion galaxy, macroscopic shocks from density wave arms, rotational shears from a differential rotation, and single warps in galaxies. In most dynamo theories, the presence of a small disk thickness generates several modes, with the predominating m = 0 mode followed by the m = 1 mode and then by the weak m = 2 mode, contradicting the observed absence of the m = 0 mode in galaxies with a small H I gas mass and the observed predominance of the m = 1 mode.