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Dark Matter & Galaxies
Measuring Our Home Galaxy
How big is the Milky Way galaxy? How fast does it rotate? Where, in its vast expanses, is our Sun located?
When you're inside something, it's hard to get the big picture. Such is the case with our own Milky Way, which astronomers can only observe from within. Based on radio astronomical investigations of the distribution of hydrogen clouds, we do know that the Milky Way is a rotating disc-shaped gathering of stars, gas, and dust about 100,000 light-years across and 2,500 light-years thick. Our Sun is located along one of the Milky Way's spiral arms, but its exact distance from the center of the galaxy is unknown.
SIM Lite will use optical interferometry to measure the distances of stars throughout the galaxy with unprecedented accuracy.
With its precise measuring ability, SIM Lite will allow astronomers to create a new, more accurate roadmap of our home galaxy and answer many fundamental questions about its size, shape and structure.
Dark Matter
Gravity is the force that keeps things in orbit in the Universe. Much as the Earth orbits the Sun because of the Sun’s gravity, the billions of stars in a galaxy orbit around the galaxy because of the total mass of the galaxy. Yet observations have shown that stars orbit galaxies much faster than would be expected based on the total mass of stars, gas, and dust that can be seen at all wavelengths of light. This has necessitated the idea of dark matter. Matter which does not interact with light and hence is ‘dark,’ but still has mass and can provide the extra gravitational pull necessary to explain the faster-than-expected motions of stars in all galaxies.
What is the dark matter made of? How is it distributed in galaxies and groups of galaxies and how did it get that way? How does this distribution differ from that of the luminous matter and why? These important questions lie at the heart of modern cosmology and galaxy formation and SIM Lite will play an essential role in providing answers to them.
Some of the most definitive tests of dark matter in local galaxies require measurements of tiny (microarcseconds per year) motions of stars in nearby galaxies moving under the influence of gravity from both luminous and dark matter - measurements that are uniquely the domain of SIM Lite.
Hypervelocity stars are another probe of dark matter, but on the scale of the gravitational field of our Milky Way galaxy. These stars are moving so fast that they are escaping from the Milky Way. A handful of these stars have been found recently, and with galactocentric velocities much greater than 600 km/s they must have been ejected from close to the supermassive black hole at the center of our galaxy. Their 3-D motions provide a means of measuring the shape of the Milky Way’s mass distribution out to large distances. For these individual stars, SIM Lite will provide the precision distances and motions needed for this determination.
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