MadSci Network: Astronomy

Re: Are there any known objects that orbit perpendicular to the galactic plane?

Date: Tue Aug 12 22:11:24 2008
Posted By: Joseph Lazio, Radio Astronomer
Area of science: Astronomy
ID: 1217516654.As

The answer to the first part of your question is, Yes, many, many stars are now known whose orbits are well out of the Galactic plane, if not exactly perpendicular to the Galactic plane. One of the major discoveries of the past decade has been that of stellar streams. It has long been thought that major galaxies, like the Milky Way, must absorb their smaller satellites, like the Magellanic Clouds, over time. What has been realized recently is just how important this process is in the growth of major galaxies as well as observational evidence in support of this idea.

Astronomers now find that there are multiple, long trails of stars across the sky. The stars in these trails often have similar colors and velocities, so that the trail itself can be recognized fairly easily in large surveys of stars. (A researcher using data from the Sloan Digital Sky Survey compares it to using meteor streams to find the orbits of long-dead comets within the solar system.) Many such stellar streams have been within the SDSS, including one area of the sky dubbed the "Field of Streams". Similar streams are seen in other galaxies, including the Andromeda Galaxy.

There are so many such trails because when two galaxies "collide," they don't behave in ways to which we are accustomed. For instance, if one were to take a cup of water colored blue and a cup of water colored red and mixed them together, one quickly ends up with purple (or brown) water. That's because, when mixed, the water molecules from the two cups actually end up "bumping into" or colliding with each other. The distances between stars are so great that when galaxies collide the stars almost never actually collide. Their orbits can be affected by the gravitational interaction between the two groups of stars, but this process is very slow compared to actual collisions.

The second part of your question is more difficult to answer, for two reasons. First, one has to ask the question precisely. Consider sitting in a car travelling down an expressway. Most of the cars around you will be travelling in the same direction, at approximately the same speed. Relative to you, they will appear to have low velocities. Now consider a car travelling perpendicular to your car, perhaps it is on an overpass. It will appear to have a high speed, relative to you, because part of its velocity is directed perpendicular to you.

The same is true for stars. Stars travelling in the Galaxy disk, in orbits similar to that of the Sun, tend to have velocities fairly similar to that of the Sun. Stars travelling perpendicular to the Sun's path appear to have high velocities.

The second reason why it is difficult to answer this part of the question is that there are so little data. Obtaining the full space velocity of a star is difficult. Astronomers can use the Doppler effect to measure a star's velocity toward or away from us. However, only through proper motion, can we measure the velocity of a star across our line of sight. Coupled to this measurement difficulty is that obtaining even the radial velocity of a star is difficult, and often tedious. One needs to collect the spectrum of a star; in the case of a stellar stream, one needs to collect the spectra for a large number of stars in the stellar stream. Obtaining a stellar spectrum often requires a lot of telescope time, and then the analysis to interpret a stellar spectrum is even more time. Thus, obtaining large numbers of stellar spectra is time consuming and has been done for only a relatively small number of stars.

One can make some simplifying assumptions, and determine if these make sense. Let me give the answer first, then justify it. From the little data I've found, the velocity of stars in streams is consistent with them having similar velocities to stars in the Galactic disk. In other words, this gravitational lensing hypothesis is not supported.

The space velocity of a star is often described in terms of three components, V2 = VU2 + VV2 + VW2, where VU,V represent the motion of the star in the plane of the Galaxy and VW represents its motion perpendicular to the plane. Now suppose that VU = VV = VW. Further suppose that the star has a velocity comparable to the Sun, V ~ 200 km/s. Some simple algebra allows us to solve for VW. I find that we might expect stellar stream stars to have velocities of order 115 km/s. The little data I've been able to find suggest that stellar stream stars do have apparent velocities higher than that of stars near the Sun (which have velocities around 10 km/s) and that their velocities can approach 100 km/s.

Thus, I conclude that the limited data are consistent with stellar stream stars having space velocities similar to those of the Sun and stars in the Galactic disk.

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