M51中螺旋臂上的几何偏移：气体和恒星形成示踪物的性质
Geometric Offsets Across Spiral Arms in M51: Nature of Gas and Star Formation Tracers
摘要：
我们报告的气体螺旋臂和相关的恒星形成区域的大设计的螺旋星系M51之间的几何偏移的测量。这些偏移量是一个建议的恒星形成时间尺度后，在螺旋臂进入压缩气体的措施。在最近对附近旋涡星系的偏移测量中，有一个令人惊讶的差异，相差了一个数量级。使用CO和H-α发射的测量发现M51中存在大而有序的偏移。相反，使用HI 21 cm和24 μ m的发射发现了小的或无序的偏移，这可能是反对气流通过螺旋臂的证据，因此反对具有固定螺旋图案的传统密度波理论。本文的目的是了解这种差异的原因。我们通过使用等效数据、方法和参数重复先前的测量来调查潜在原因。我们发现偏移量与以前的测量结果一致，并得出结论，气体示踪剂的差异，即，HI和CO是主要原因。HI辐射被最近形成的恒星光解离的气体严重污染，不一定能追踪到恒星形成的前体压缩气体。HI气体和恒星形成区在空间上一致，并倾向于显示小的偏移。我们发现CO和H-α之间的大部分正偏移量具有很大的分散性，这表明气体流过螺旋臂（即，密度波），尽管螺旋图案不一定是静止的。
英文摘要：
We report measurements of geometric offsets between gas spiral arms and associated star forming regions in the grand-design spiral galaxy M51. These offsets are a suggested measure of the star formation timescale after the compression of gas at spiral arm entry. A surprising discrepancy, by an order of magnitude, has been reported in recent offset measurements in nearby spiral galaxies. Measurements using CO and H-alpha emission find large and ordered offsets in M51. On the contrary, small or non-ordered offsets have been found using the HI 21cm and 24 micron emissions, possible evidence against gas flow through spiral arms, and thus against the conventional density-wave theory with a stationary spiral pattern. The goal of this paper is to understand the cause of this discrepancy. We investigate potential causes by repeating those previous measurements using equivalent data, methods, and parameters. We find offsets consistent with the previous measurements and conclude that the difference of gas tracers, i.e., HI versus CO, is the primary cause. The HI emission is contaminated significantly by the gas photo-dissociated by recently-formed stars and does not necessarily trace the compressed gas, the precursor of star formation. The HI gas and star forming regions coincide spatially and tend to show small offsets. We find mostly positive offsets with substantial scatter between CO and H-alpha, suggesting gas flow through spiral arms (i.e., density-wave) though the spiral pattern may not necessarily be stationary.
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