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This feature is inspired by the questions we have received over the years from interested readers. If you have a question about an astronomical topic, please forward it to us. |
Thank you for your thoughtful questions. To answer your questions, let me start by describing the two methods of mass determination that we may use in cases such as the one I was discussing. We can measure the luminosity of the galaxy using a photometer and, supposing we know its distance, which is always uncertain, we can convert this to absolute units, such as units of the luminosity of the Sun. If all stars were similar to the Sun, then this number would also be the total mass of the stars. Actually, the average star in a galaxy like the Sombrero is less bright, for its mass, than is the Sun. So we have to multiply the total luminosity by some factor, usually 3 or more, depending upon the kind of stars which appear to be present, in order to estimate the total mass of all of the stars. If we want to include all of the visible material, we must add something to account for the gas and dust, which we can see as the dark band across the face of the galaxy, but this is generally a correction of only a few per cent. The second method of mass determination is to measure the rotational velocity. This is particularly easy when the galaxy is seen edge on because there is no projection factor to worry about. This measures the mass interior to a sphere whose radius is determined by the point at which the measurement is made. If the matter is not spherically distributed, then there is a small correction, but this can be dealt with. This will include all of the mass, because all of the mass contributes to the gravity, whether it be in the form of stars, of gas and dust, or in some other form which we have not yet identified but which we call, for lack of a better name, "dark matter."
When these two methods are applied to galaxies, we often find that the mass determined by the second method is several times as large as the visible mass determined by the first. Dynamical studies suggest that this extra mass is not in the disk of the galaxy where we find the bright stars and the gas and the dust, but is widely distributed in the surrounding halo, where we find relatively few stars. Supposing that it is always true that most of the matter in the halo is dark, then the halo of a galaxy like the Sombrero, where there are many stars, should be exceedingly massive. We were therefore surprised to find that the total mass of the halo of the Sombrero was quite average. It must be that something has happened in the halo which has caused the formation of an exceptionally large number of stars. This does not mean that there is no dark matter. We have seen only that the total mass is quite normal but that a larger than normal fraction of this mass is in the form of halo stars. With regard to the total dynamical mass, the number that we get is 0.6 trillion solar masses inside a 20 kiloparsec radius although what happens outside this radius is anybody's guess. The number which you quote (1.3 trillion) is somewhat larger, although not unreasonable provided it is a measure of the total, not just the visible mass. In all these calculations the distance remains an important source of uncertainty.
The most interesting part of our discussion is now quite an old story. By looking at the spiral structure, we were able to estimate how much of the mass was concentrated in the disk of the galaxy. The material in the disk moves under influence of the whole mass of the galaxy, including the halo. But the tendency of the matter to concentrate in spiral arms is governed by the self-gravity of the matter in the disk itself. With the help of some rather complicated mathematics, we can show that the tilt of the spiral arms (or their separation, which we were able to measure) is related to the amount of mass in the disk. We found that about 20 per cent of the total mass of the galaxy is located in the disk. This leads to the rather unexciting result that the Sombrero is really a very average sort of disk embedded in an average sort of halo. The only exceptional part is the large number of stars in the halo, and it is this that makes it such a striking object.
Also received was a $2,500 donation from The Catherine L. and Robert O. McMahan Foundation for the cost of a low-resolution area spectrograph.
The Pebble Beach Company generously donated $1,500 toward the installation of MIRA - Exploring the Universe from the Central Coast.
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