The Maintenance of the Spiral Arms in a Spiral Galaxy

A circulation hypothesis of spiral galaxies is proposed in this paper to explain their spiral structure: the spiral arm material is going to the galactic center along a spiral orbit and then ejects from the galactic center in the form of two-way jets in opposite directions and finally, the ejected jets return to the galaxy in the form of spiral arms again, thereby forming a closed circulatory system. According to this circulation hypothesis, (1)a spiral galaxy tends to form the two-armed spiral pattern, (2) there must be a warp in the spiral arms regarded as the backflows of the jets since the two-way jets locate in the opposite hemispheres respectively, (3)the spiral arm material and the bar material of a barred spiral galaxy should both have an inward flow. These inferences are in accordance with former observations. The flat rotation curve of the gas of the spiral arms could be explained in the prerequisite of the circulation hypothesis, The flat rotation curve may be caused by the following three elements:(1)magnetic fields in the spiral arms, (2) the magnetic Reynolds R M of the gas in the spiral arms far greater than 1, (3) the specific value k of the magnetic energy density of the magnetic field of the spiral arms to their thermal energy density far greater than 1. Therefore the magnetic field is certainly frozen in the spiral arm plasma and restricts the motion of spiral arm material, so that the magnetic field of the spiral arms and the spiral arm material controlled by the magnetic field and the self-gravitation of spiral arms could only be treated as a whole. Hence the magnitude of the going velocity of the spiral arms should be the same everywhere. The bar structure in barred spiral galaxies may also be explained in the prerequisite of the circulation hypothesis. The spiral arm material is acted upon in the normal direction by two forces: the normal component of the gravitation towards the galactic center F₁(r) and the outward normal centrifugal force F₂(r),the resultant force acting on the spiral arm in the outward normal direction is F(r)=F₂(r)-F₁(r). The direction of stress of one spiral arm is just opposite to that of the other spiral arm in the outer region F(r)0, thereby forming a pair of shearing forces. Being acted upon by this pair of shearing forces, the bar structure could form in some spiral galaxies. It is emphasizefd that there should be two-way jets at the center of every spiral galaxy, and there should be some link between the jet material and the spiral arms. It is considered that the cooling flows in some elliptical galaxies may also be the backflows of their observed two-way jets and the disk-like structure near young stars may also be the backflows of their observed two-way jets.