The Spiral Structure of the Galaxy: Something Old, Something New...
The Spiral Structure of the Galaxy: Something Old, Something New...
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发表时间：
2008-05
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通讯作者：
R. Benjamin
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作者：
R. Benjamin
I review some of the old difficulties in determining the spiral structure of the Galaxy using kinematic distances, and present some of the new results on the stellar spiral structure of the Galaxy which do not suffer from the same systematic difficulties. I conclude the discussion with something borrowed (spiral structure studies in other galaxies) and discuss how it relates to something blue (massive star formation). 1. Something Old: Kinematic Methods for Mapping the Galaxy From the very beginning, one of the chief goals of 21-cm mapping of neutral hydrogen was a global map of the spiral structure of the Galaxy. To get a sense of this history, I recommend starting with a meeting summary by Simonson (1970). Participants arrived filled with enthusiasm for converting HI surveys into maps, but left somewhat deflated due to lack of convergence in maps produced with the same data. Liszt (1985) describes the same state of affairs over a decade later. His Figure 7 collects several distinctly different maps, leading him to worry that ongoing CO surveys of the Galaxy would yield a similar impasse. The review of CO results by Combes (1991) shows that these concerns were not unfounded. Similarly, attempts to map the HII region distribution, most famously by Georgelin & Georgelin (1976), continue to suffer from significant ambiguities, e.g., Sewilo et al. (2004). Part of the problem is demonstrated in Fig. 1, which shows how, by assuming circular rotation and adopting a rotation curve, one can assign a radial velocity to every parcel of gas in the Galaxy. The Galaxy can be mapped, in principal, by converting the velocity back to position. The fundamental difficulties with this method are well known: (1) assuming circular rotation, the velocity-to-distance relation is double valued in the first (0 <l< 90◦) and fourth (270 <l< 360◦) quadrants. Additional information is needed to resolve the near-far distance ambiguity; (2) the distance accuracy depends upon the model dvrad/dr, which varies with longitude. Near l = 0 ◦ or l = 180◦, one loses all distance resolution; (3) gas has ”random” velocity (whose origin is poorly, or perhaps multiply, understood) of order 7-10 km s, producing distance uncertainties that are longitude dependent; and (4) significant non-circular motion is expected near spiral arms and near any non-axisymmetric structures in the inner galaxy, i.e., bars. In this case, the velocity-to-distance relation can be multiply valued, e.g., Gomez (2006). A thorough review of all of these issues is provided in Burton et al. (1992).
