Rotating Halos and Spirals in Low Surface Brightness Galaxies

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Abstract
As the angular momentum of visible matter in a spiral galaxy is derived from the host cloud, the dark matter halo left behind in the cloud is likely to be in rotation also. In the merging model of galaxy formation, it is shown that the conservation of mass and angular momentum yields a spin parameter within the halo increasing toward the core as r-1/3 for an isothermal halo and as r-1/2 for a halo with the universal density profile. Hence, almost corotation between the halo and disk can become possible in the inner galaxy for a relatively high angular momentum halo. Much like normal galaxies, the high angular momentum, low surface brightness galaxies have been observed to exhibit a wide variety of spiral patterns. In this work we explore a mechanism by which disk density waves are excited by corotation resonance between disk and halo, even in disks with a Toomre Q exceeding unity by a sizable margin. Apart from the resonant condition, the effectiveness of disk-halo coupling is found to be determined also by the halo scale height, and enhanced excitation of disk density waves occurs only when the halo scale height is sufficiently small. The spiral-arm pitch angles depend on the halo scale height as well, with the more loosely wound arms associated with smaller halo scale height.

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