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For refcode 1990ApJ...361..419V:
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1990ApJ...361..419V TRIGGERING THE BISYMMETRIC (m = 1) MAGNETIC FIELDS IN SPIRAL GALAXIES J. P. VALLEE Radio Astronomy Section, Hezberg Institute of Astrophysics, National Research Council of Canada; and JCMT Unit, Royal Observatory, Blackford Hill, Edinburgh Received 1989 November 15; accepted 1990 April 2 ABSTRACT 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^deg^ 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^deg^ and then outward for 180^deg^, 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 gas mass in excess of 8.5 x 10^9^ M_sun_ (using H_0_ = 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 Vallee (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. Subject headings: galaxies: interstellar matter - interstellar: magnetic fields - polarization
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