标题： 气体对星系螺旋模式速度的动力学影响 显示英文标题

标题： Dynamical effect of gas on spiral pattern speed in galaxies

摘要： 在螺旋结构的密度波理论中，大尺度设计的双臂螺旋图案被认为以恒定且明确的模式速度在星系盘中刚性旋转。 尽管观测测量螺旋臂的模式速度非常困难，但在一些星系（如我们考虑的NGC 6946、NGC 2997和M 51）中已成功实现。 我们检验了对于观测到的旋转曲线和模式速度值，理论上是否允许对应稳定波的波数具有实数解。 我们发现，当盘中仅包含恒星时（这是文献中通常的做法），一般情况下不支持对于观测到的模式速度的稳定密度波。 相反，包含低速分散成分，即气体，是获得稳定密度波的关键。 此外，我们得到了一个理论允许的模式速度范围，该范围对应于某半径处的稳定密度波，并验证了对于所考虑的三个星系，观测到的模式速度落在各自的指定范围内。 即使在星系盘中加入少量（约15%）的质量分数的气体，也被证明对色散关系有显著的动力学影响，从而影响可能在真实富含气体的螺旋星系中看到的模式速度。 显示英文摘要

摘要： In the density wave theory of spiral structure, the grand-design two-armed spiral pattern is taken to rotate rigidly in a galactic disc with a constant, definite pattern speed. The observational measurement of the pattern speed of the spiral arms, though difficult, has been achieved in a few galaxies such as NGC 6946, NGC 2997, and M 51 which we consider here. We examine whether the theoretical dispersion relation permits a real solution for wavenumber corresponding to a stable wave, for the observed rotation curve and the pattern speed values. We find that the disc when treated to consist of stars alone, as is usually done in literature, does not generally support a stable density wave for the observed pattern speed. Instead the inclusion of the low velocity dispersion component, namely, gas, is essential to obtain a stable density wave. Further, we obtain a theoretical range of allowed pattern speeds that correspond to a stable density wave at a certain radius, and check that for the three galaxies considered, the observed pattern speeds fall in the respective prescribed range. The inclusion of even a small amount (~ 15 %) of gas by mass fraction in the galactic disc is shown to have a significant dynamical effect on the dispersion relation and hence on the pattern speed that is likely to be seen in a real, gas-rich spiral galaxy.