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Nonlinear Modulation of Density Wave in Self-Gravitating Systems Nonlinear Modulation of Density Wave in Self-Gravitating Systems 池内, 了 IKEUCHI, Satoru NAKAMURA, Takashi metadata only access © 2014 The Physical Society of Japan Nonlinear modulation of density wave in self-gravitating systems is studied by the reductive perturbation method. Using gas dynamical equations, an infinitely long rotating cylinder and an infinitesimally thin disk are considered. In the former system, both one-dimensional and two-dimensional modulations, the so-called self-focusing, are found to be stable for all wave number when the carrier wave propagates perpendicularly to rotational axis. In the latter system, amplitude modulation becomes unstable for large wave number. This unstable amplitude modulation grows to the envelope solitary wave in a time of the order of rotational one even if the amplitude of carrier wave is as low as one hundredth of zeroth order quantity. It is concluded that nonlinear effects cannot be neglected in the density wave theory of spiral arms. The conjecture is proposed that this nonlinear modulationally unstable mode can be identified with the spiral arms of S-type galaxies. Oxford University Press 1976 eng journal article https://ir.soken.ac.jp/records/4254 https://doi.org/10.1143/PTP.55.1419 10.1143/PTP.55.1419 0033-068X Progress of Theoretical Physics Progress of Theoretical Physics 55 5 1419 1431