How did it go with the galaxies in the previous section, did you manage to classify them all? Do you think that it is enough to observe their velocity fields?
Well, actually most of the times it is not that easy, and now we will see why.
In the section How do galaxies move? We mentioned that in spiral galaxies rotation motions dominate. However there are also some spiral galaxies in which dispersion motions are just as dominant as the rotational ones. On the other hand we saw that in most of the elliptical galaxies it is the dispersion movements that dominate, although there are some in which the rotation is also very noticeable. In these cases, by looking at the velocity field diagrams it is not so easy to identify whether if it is a spiral or an elliptical galaxy. For example let us look at the velocity fields of the galaxy 9889-9102.
In the stellar velocity field we can see that the right part is moving in one direction, while the left part is moving in the opposite direction, then we could tell that it is a rotating spiral galaxy; but if we look at the gas velocity field it does not seem to be rotating, in fact most of it is blue and there are red parts that seem to be random. This is what we would expect in an elliptical galaxy.
So, how can we know if a galaxy is dominated by dispersion or rotational motions?
Well, astronomers calculate an average of both, the velocity dispersion and the rotation velocity for the whole galaxy. Now let us see how this is done.
Once the rotation velocity and velocity dispersion in each spaxel have been measured, the next step is to determine an elliptical region within the galaxy that encloses its brightest part (just like in figure 15). After that the velocity dispersion of all the spaxels within the ellipse are then averaged and the result is the average velocity dispersion of the galaxy. For the rotation velocity, the procedure is more complex since not only statistical calculations have to be made, but also corrections for the geometry of the galaxy are needed, and also corrections for the speed with which they move within the universe.
Once the average velocity dispersion and mean rotation velocity in the galaxy have been calculated, we can compare them and see which one is higher. This will help us to know if the galaxy is dominated by dispersion or rotational motions and then we can classify our galaxy with more reliability.
Let us see the average velocities of the galaxy 9889-9102 in our example of figures 15 and 16:
Mean rotation velocity: 84.77 km/s
Average velocity dispersion: 224.18 km/s
It turns out that the velocity dispersion is much greater than the rotation velocity, this means that 9889-9102 is an elliptical galaxy:
