2010MNRAS.405.1531E

We study the relation between the occurrence of a regular chain of star complexes and superclouds in a spiral arm, and other properties of the spiral arm. A regular string of star complexes is located in the north-western arm of M31; these have about the same size, 0.6 kpc with spacing of 1.1 kpc. Within the same arm segment, a regular magnetic field with a wavelength of 2.3 kpc was found by Beck et al. We note that this wavelength is twice as large as the spacing between complexes and suggest that these were formed as a result of the magneto-gravitational instability developed along the arm. In this north-western arm, star complexes are located inside the gas-dust lane, whilst in the south-western arm of M31 the gas-dust lane is upstream of the bright and uniform stellar arm. Earlier, evidence for the age gradient was found in the south-western arm. All these are signatures of a spiral shock, which may be associated with an unusually large (for M31) pitch angle of this south-western arm segment. Such a shock may prevent the formation of a regular magnetic field, which might explain the absence of star complexes there. Anticorrelation between shock wave signatures and the presence of star complexes is observed in the spiral arms of a few other galaxies. Regular chains of star complexes/superclouds in spiral arms are rare, which may imply that a specific mechanism is involved in their formation; the most probable is the Parker-Jeans instability. The spiral pattern of our Galaxy is briefly discussed; it may be of M101 type in the outer parts. The regular bi-modal spacing of Hi superclouds is found in the Carina and Cygnus (Outer) arms, which may be indirect evidence for the regular magnetic field along these arms.