Dark Matter Lab Exercise.docx - Dark Matter I. Purpose...

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Dark MatterI. PurposeExamine the evidence for Dark Matter in NGC 2742.II. BackgroundDark Matter is simply matter that does not interact with light. We can detect it using gravitationalmethods (Newton’s and Kepler’s Laws), but not as light. Dark Matter neither absorbs norradiates light. We have absolutely no idea what this stuff is. Many tax dollars are being spent onphysics experiments that are trying to pin down some undiscovered particle that there could be alot of. Many astronomers are trying all sorts of clever observing schemes (if you can think ofany, let us know) to find observational evidence for this stuff. As yet, every experiment andobservation has come up empty.The matter that you and I think about as being matter (atoms, protons, neutrons, buildings,whales, etc.) is sometimes referred to as baryonic matter since it is made up of baryons (protonsand neutrons). Baryonic matter also contains leptons (electrons), but electrons do not contributemuch mass to the universe compared to the protons and neutrons. Many people believe that darkmatter is non-baryonic. Physicists have a lot of fun coming up with new, imaginary particles toexplain dark matter.In this lab, you will compare how much matter stars contribute to the total mass of the galaxyNGC 2742. You will see that the total mass of a galaxy can be found by measuring how fast thegalaxy rotates around its center. Then you will compare that total mass to the amount of light thatwe observe coming from the galaxy. This is called the Mass to Light Ratio.We want to see how much of the mass is the mass that is emitting light and how much is dark.II. Weighing the GalaxyFigure 2 is a graph of NGC 2742 (aka UGC 4779, galaxies get all sorts of names) thatastronomers call a “rotation curve”. It plots the radial velocity (how fast stuff is moving in theline of sight direction, as shown in Figure 1) versus the radius.Figure 1:Radial VelocityDeveloped by University of Washington Astronomy Department1

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