What is the significance of peculiar motion in galactic rotation data analysis?

[Bill McKeeman]

Does anyone know of galactic rotation data (any galaxy) of the following form:

For simplicity assume an [x y z] coordinate system with the origin in the center of the galaxy and [x y] representing the plane of rotation. At any point [x y z] there is a corresponding velocity vector [v_x v_y v_z] representing the motion of matter at that point. For objects in a galactic plane (that is: z near 0), the expectation is that v_z is near 0 and the vector [v_x v_y] is at right angles to the radius vector to [x y]. A difference from the expectation is said to be peculiar motion.

I imagine a display of this data as a galactic-shaped 3D graph with an arrow at each sample point. I imagine the data base describing this data as a table with six columns [x y x v_x v_y v_z] or perhaps something similar using spherical coordinates. A science librarian and a cosmologist have said they do not know of any such dataset. Please note the considerable elaboration of this request as contrasted to merely speed as a function of radial distance from the center.

 

[phyzguy]

You might find this paper interesting, as it discusses the state of the data for the Milky Way.

Think about what would be required to obtain the data you seek. In order to have the velocity vector in 3 dimensions, you need to measure not just the radial velocity, which can be measured with spectroscopy, but also the tangential velocity. Measuring the tangential velocity requires knowledge of the precise distance and proper motion, which I think is virtually impossible for the foreseeable future for any galaxy except the Milky Way. The Gaia satellite is in the process of measuring the parallax and proper motion for a large number (millions - perhaps as many as 1 billion) of stars in the Milky Way. Even after this data is available, this will only be a small portion of the galaxy, and it will be biased toward the stars near the sun. Getting this data for a full galaxy outside the Milky Way seems nearly impossible to me. This is why people have focused on measuring the radial velocities as a function of radius. If you take the average radial velocity of a large number of stars, the tangential velocities average out and you can draw some conclusions. I think hope for the data you are looking for would be N-body simulations, which already exist in the form you are looking for.

 

[Bill McKeeman]

Phyzguy, thanks for the detailed reply. Perhaps my query was/seemed overambitious, but I was never expecting anything more than a sample of entries distributed over a galaxy. 100 points? Maybe 1000 points? That does not seem out of the question to me. What I am sure is that it takes a lot of time consuming work to collect the physical data and I am told cosmologists do not do that kind of work without an objective in mind, and could probably not get instrument time otherwise. As for reducing the raw data to a form I would find usable, MacEvoy's website
http://www.handprint.com/ASTRO/galaxy.html
gives an approximation approach. Assume all objects have circular orbits about the galactic center. Use the galactic [x y] position of a object to determine the direction of the velocity vector, and use the doppler data to determine its magnitude. I will read the paper on simulations you cited.
In the meantime, my shallow reason for wanting something other than radial distance is that the speeds seem to show a large range of values at each given r, and perhaps might show less variation if they were separated out by galactic longitude.

 

[phyzguy]

If you look at N-body simulations of galaxies or galaxy clusters, the assumption that "all objects have circular orbits about the galactic center" is a bad one. Most of the "orbits" are not even closed curves. I suggest you take an N-body simulation tool like Gadget and build or download an N-body simulation of a galaxy to become more familiar with what the stellar motions look like.