M31星系的旋臂结构
Spiral arm structures revealed in the M31 galaxy
摘要：
在M31星系的西北臂中发现了引人注目的彗星。位于这个臂中的恒星复合体都间隔1.2 kpc，大小相似，约为0.6 kpc。Beck等（1989）在同一臂区内探测到一个规则的磁场，我们发现它的波长就是复合物之间的间距。在这条臂中，HII区域位于恒星复合体内部，而恒星复合体又位于气体和尘埃带内部。相反，M31的西南旋臂在上游分裂成一个气体和尘埃带，在下游分裂成一个致密的恒星旋臂，HII区主要位于旋臂的这两部分之间的边界。西南旋臂的恒星密度远高于西北旋臂，并且西南旋臂没有分裂成恒星复合体。在早期的观测中已经发现了这支手臂的年龄梯度。根据经典的SDW理论，这些巨大的差异可能是由于它们不同的俯仰角：臂的西北部约为0度，西南部约为30度。对M31、M51、M74和其他一些星系的数据表明，恒星复合体大多位于没有尘埃带的臂段，即没有螺旋激波。复合体沿密度波旋臂分布的不均匀性很可能是由于帕克-金斯不稳定性的发展，如果旋臂中的初始SFR较低，并且磁场沿旋臂是规则的，那么帕克-金斯不稳定性就会形成星型复合体。
英文摘要：
Striking regularities are found in the northwestern arm of the M31 galaxy. Star complexes located in this arm are all spaced 1.2 kpc apart and have similar sizes of about 0.6 kpc. Within the same arm region Beck et al. (1989) detected a regular magnetic field, and we found that its wavelength is the spacing between the complexes. In this arm, groups of HII regions lie inside star complexes, which, in turn, are located inside the gas and dust lane. In contrast, the southwestern arm of M31 splits into a gas and dust lane upstream and a dense stellar arm downstream, with HII regions located mostly along the boundary between these components of the arm. The stellar density in the southwestern arm is much higher than in the northwestern arm, and the former is not fragmented into star complexes. The age gradient across this arm have been found in earlier observations. According the classical SDW theory, these drastic differences may be due due to their different pitch angles: about 0 degree for northwestern part of the arm and about 30 degree for the southwestern segment. Data on M31, M51, M74, and some other galaxies suggest that star complexes are mostly located in the arm segments that are not accompanied by a dust lane upstream, i.e. do not host spiral shock wave. The regularities in the distribution of complexes along a density wave spiral arm are most probably due to the development of the Parker-Jeans instability, which builds up star complexes if the initial SFR in the arm is low and, as a sequence, magnetic field is regular along the arm.
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