Modelling `Warped' Galaxies without Warps

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Session 22 -- Galaxy Structure and Kinematics
Oral presentation, Monday, 30, 1994, 2:00-3:30

[22.01] Modelling `Warped' Galaxies without Warps

G. R. Roelofs (University of Chicago)

Velocity maps of disk galaxies never look like purely planar, circular flows; other velocity components are invariably present and add asymmetries to the flow maps. The traditional interpretation of a large class of these distortions has been in terms of warped disks, and there is good motivation for this: many edge-on galaxies {\sl are} visibly warped, either in the optical or, especially, in radio observations of neutral hydrogen. In many more galaxies, however, the presumed warp is seen only in velocity maps. Recent numerical experiments by R.~H.~Miller and B.~F.~Smith have found that galaxies can oscillate in radial normal modes with relatively large amplitude (10--20 km/s for a Milky-Way-sized galaxy), and it has long been known that such radial motions produce velocity signatures similar to those in warped disks. Given this, it is reasonable to investigate the extent to which kinematic observations of disk galaxies can be interpreted in terms of planar disks with a radial flow component, rather than warped disks with purely circular flows. We have done this for several galaxies and find that in at least some cases the radial-flow models fit the observations as well as or better than the tilted-rings models do. This implies more generally that, while warps may still be necessary to explain observed kinematic features in many galaxies, less extreme warps (in combination with radial motions) may now suffice. That in turn eases the burden on models attempting to explain the longevity of warps.

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