34th Meeting of the AAS Division on Dynamical Astronomy, May 2003
3 Galactic Dynamics
Oral, Monday, May 5, 2003, 1:15-3:55pm,

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[3.05] NGC 4365: A Truly Triaxial Galaxy

T.S. Statler (Ohio University), SAURON Team

NGC 4365 is a photometrically ordinary, but kinematically intriguing, E2-3 galaxy. The map of the stellar mean velocity field obtained by the SAURON integral field spectrograph on the William Herschel Telescope shows that the apparent rotation axis, or zero-velocity contour (ZVC), is unaligned with the photometric axes, and that the velocity field (VF) is not symmetric about the ZVC. These traits are both expected hallmarks of non-axisymmetry. Moreover, the galaxy has a pronounced counterrotating core. We have modeled the VF and surface photometry of NGC 4365 using an approach in which the equation of continuity is solved in ellipsoidal coordinates and the resulting projected models are compared with the data by Bayesian methods to constrain the shape profile and orientation of the galaxy. We find that NGC 4365 is indeed highly triaxial, with mean axis ratios of roughly 1 : 0.8 : 0.6. Axisymmetric shapes are strongly ruled out. There is no indication of a significant change of intrinsic shape anywhere in the galaxy, including the counterrotating core. Since absorption line index measurements and the lack of photometric fine structure both indicate that NGC 4365 is at least 10 Gyr old, the dynamical results demonstrate that triaxiality in stellar systems can be long lived, and is not necessarily destroyed by orbital chaos.

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Bulletin of the American Astronomical Society, 35 #4
© 2003. The American Astronomical Soceity.