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Lab 9 Dark Matter.pdf - Dark Matter Name: Tyler Jordan _ I....

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Dark MatterName:Tyler Jordan_______________________I. PurposeExamine the evidence for Dark Matter in galaxy NGC 2742 by determining itsmass in two waysfrom the revolution of its stars around the center of massand from its total luminosity.II. Background and TheoryHow do you weigh a galaxy? Astronomers use two methods. The first measuresthe gravitational mass from the speed of rotation of a galaxy as a function ofdistance from the center, taking advantage of Newton's Laws to determine themass around which the stars orbit. The second measures the total lightproduced by a galaxy; since we know how much light individual stars produce,we can determine how many stars are in the galaxy, and, hence, its luminousmass. The discrepancy between measurements of a galaxy's luminous mass andits gravitational mass provided the first evidence for the presence of DarkMatter.Dark Matter is simply matter that does not interact with lightit neitherabsorbs nor emits any type of electromagnetic radiation. We are only aware ofits presence due to the gravitational effect it has on matter we can see. The stuffthat we think about as being matter (trees, people, mountains, planets, stars,etc.) is sometimes referred to as baryonic matter since it is made up of baryons(protons and neutrons). Many scientists think that dark matter is made up ofnon-baryonic particles, and there are many experiments underway around theworld that are attempting to detect these hypothetical particles.Our investigation will compare the total mass to the luminous mass of galaxyNGC 2742. This is a spiral galaxy similar to the Milky Way, but at a distanceof about 65 million light years. NGC 2742 is oriented neither face-on nor edge-on, but has an intermediate inclination. We can determine the inclination of thegalaxy by comparing its largest and smallest diameters and assuming thegalaxy would be circular if we could see it face-on. The intermediateinclination of NGC 2742 allows us to measure both the luminous mass and thetotal mass of the galaxy, and then compare them.
III. Gravitational massFigure 1 below is a graph of radial velocities within NGC 2742 (also known asUGC 4779) that astronomers call a “rotation curve.” The graph plots the radialvelocity of the galaxy as a function of distance from the center of the galaxy.Note that radial velocity is the relative speed of the galaxy's stars directlytoward or away from the observer, and is measured in kilometers per second.The intersection of the dashed lines represents the galactic center, and distancefrom the center is measured in kiloparsecs (thousands of parsecs). We canmeasure the radial velocity by taking a spectrum at points across the entirewidth of the galaxy. The light from the galaxy is placed on a slit (the dark linein the negative image below) that allows light from the region of the galaxy we

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Term
Summer
Professor
JOHNWHITMER
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