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.
Highlights
Recent progress in the study of galaxy formation has centered around the origin of the Hubble sequence
An emerging opinion derived from these recent studies is that there should exist a nonnegligible population of low surface brightness (LSB) galaxies associated with large disk angular momentum (OÏNell, Bothun, & Cornell 1997 ; Schneider & Schombert 2000 ; Bouwens & Silk 2000)
Disk galaxies of high Toomre Q often result from their low surface densities and high angular momentum
Summary
Recent progress in the study of galaxy formation has centered around the origin of the Hubble sequence. Despite the Ðnding that spiral arms in LSB galaxies may be due to selection e†ects, the arm pitch angles do vary over a sizable range, much like those high surface brightness galaxies in di†erent phases of the Hubble sequence This suggests that some factors other than the disk Q-value are perhaps responsible for the excitation of spiral arms in these high angular momentum galaxies. We wish to point out a di†erent mechanism for producing the spiral density waves : excitation by the corotation resonance between halo and disk in high angular momentum galaxies. A rapidly rotating halo tends to be of oblate shape, having a relatively small scale height Such a tendency favors the high angular momentum halo exciting disk density waves through the corotation resonance. The Appendix gives the condition under which the adiabatic halo assumption adopted to derive our main results is valid
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