Published in Proceedings of the IAU
Symposium 256, The Magellanic System: Stars, Gas, and Galaxies, Jacco
Th. van Loon & Joana M. Oliveira, eds. 2009, pp. 81-92.
astro-ph/0809.4268
For a PDF version of the article, click here.
KINEMATICAL STRUCTURE OF THE MAGELLANIC SYSTEM
Roeland P. van der Marel 1, Nitya
Kallivayalil 2 & Gurtina Besla
3
1 Space Telescope Science Institute, 3700 San Martin
Drive, Baltimore, MD 21218
2 MIT,
Kavli Inst. for
Astrophysics & Space Research,
70 Vassar Street, Cambridge, MA 02139
3 Harvard-Smithsonian Center for Astrophysics, 60
Garden Street, Cambridge, MA 02138
Abstract.
We review our understanding of the kinematics of the LMC and the SMC,
and their orbit around the Milky Way. The line-of-sight velocity
fields of both the LMC and SMC have been mapped with high accuracy
using thousands of discrete traces, as well as HI gas. The LMC is a
rotating disk for which the viewing angles have been well-established
using various methods. The disk is elliptical in its disk plane. The
disk thickness varies depending on the tracer population, with
V / 
ranging from
~ 2-10 from the oldest to the youngest
population. For the SMC, the old stellar population resides in a
spheroidal distribution with considerable line-of-sight depth and low
V / . Young stars
and HI gas reside in a more irregular rotating
disk. Mass estimates based on the kinematics indicate that each Cloud
is embedded in a dark halo. Proper motion measurements with HST show
that both galaxies move significantly more rapidly around the Milky
Way than previously believed. This indicates that for a canonical
1012
M
Milky
Way the Clouds are only passing by us for the
first time. Although a higher Milky Way mass yields a bound orbit,
this orbit is still very different from what has been previously
assumed in models of the Magellanic Stream. Hence, much of our
understanding of the history of the Magellanic System and the
formation of the Magellanic Stream may need to be revised. The
accuracy of the proper motion data is insufficient to say whether or
not the LMC and SMC are bound to each other, but bound
orbits do exist within the proper motion error ellipse.
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