The Parker Instability in Three Dimensions: Corrugations and Superclouds along the Carina-Sagittarius Arm

Here we present three-dimensional MHD models for the Parker instability in a thick magnetized disk, including the presence of a spiral arm. The B field is assumed parallel to the arm, and the model results are applied to the optical segment of the Carina-Sagittarius arm. The characteristic features of the undular and interchange modes are clearly apparent in the simulations. The interchange mode appears first and generates small interstellar structures in the interarm regions, but its development inside the arm is hampered by the acceleration of the spiral wave. In contrast, the undular mode follows its normal evolution inside the spiral wave, creating large gas concentrations distributed along the arm. This results in a clear arm/interarm difference; the instability triggers the formation of large interstellar clouds (with masses in the range of 106-107 M☉) inside the arms but generates only small structures with slight density enhancements in the interarm regions. The resulting clouds are distributed in an antisymmetric way with respect to the midplane, and their masses are similar to those inferred for H I superclouds in our Galaxy. Such a cloud distribution results in an azimuthal corrugation along the arm, and for conditions similar to those of the optical segment of the Carina-Sagittarius arm, it has a wavelength of about 2.4 kpc. This structuring, then, can explain the origin of both H I superclouds and the azimuthal corrugations in spiral arms. In particular, the wavelength of the fastest growing undular mode matches the corrugation length derived with the young stellar groups located in the optical segment of the Carina-Sagittarius arm.