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# 16.9 Large Scale Structure

The Hubble Deep Field. Click here for original source URL.

Galaxies are so dim and distant that we cannot see their detailed properties. Instead, we can use them to learn more about the large-scale structure of the universe — in other words, the spatial distribution of galaxies on the largest scales. The distances between most galaxies are far greater than their sizes. As a result, astronomers can treat the entire effect of the gravity of billions of stars within a galaxy as being equivalent to a single large mass in space. Galaxies are tracers of expanding space, just as corks thrown on a river would be tracers of the motions of the water. To help explain the largest structures in the universe, astronomers imagine galaxies as "particles" in a grand physics experiment that has been going on for billions of years.

Panoramic view of the entire near-infrared sky reveals the distribution of galaxies beyond theÂ Milky Way. Click here for original source URL.

The Hubble Ultra Deep Field. Click here for original source URL.

Galaxies are not scattered randomly throughout the universe. Galaxies are clustered in space, and the clustering gives us important information about how the universe evolved. We can see this if we think about how gravity acts of galaxies. In a random distribution of galaxies, there will some that are closer to each other, and they will exert slightly stronger gravity on each other. As they move closer the mutual gravity increases and they can start to pull other neighbors closer. So the situation is not stable, and it will tend towards galaxies concentrating into particular regions, with large gaps or voids in between. Galaxies from an all-sky survey can be found in every direction except toward the plane of the Milky Way; galaxies exist in these directions but they are difficult to identify due the crowding of stars and obscuration in the disk of our galaxy. It is obvious from a 2-dimensional map that the galaxies are not uniformly or randomly distributed. There are regions with relatively few galaxies and regions with a high concentration of galaxies.

A simulated view of the entire observable universe, approximately 93 billion light years (or 28 billion parsecs) in diameter. Click here for original source URL.

The clustering of galaxies is analogous to the clustering of stars, but it occurs on a much larger scale. Just as we find binary stars in the Milky Way, we can find binary galaxies in the vast depths of space beyond the Milky Way. Binary galaxies are two stellar systems that are bound by gravity in a majestic waltz. It may take many millions of years for them to orbit each other. Just as we find groups of stars in the Milky Way, we can find galaxy groups that include anywhere from a few to a few dozen galaxies. In fact, the typical situation for any galaxy is a small group ranging from a few to a few dozen. Few galaxies are truly isolated and few are in rich and dense clusters.

Grand design spiral galaxy M81. Click here for original source URL.

M101, the Pinwheel galaxy, as seen by the Hubble Space Telescope. Click here for original source URL.

The Virgo cluster of galaxies. Click here for original source URL.

We live in the Local Group, which includes three moderate-sized spiral galaxies, each with about a dozen small companions, plus a few other dwarf galaxies. These galaxies are loosely sprinkled over a region about 1 Mpc across. Beyond the Local Group, we encounter the Sculptor and M 81 groups at 2 to 3 Mpc. Each is like the Local Group, with two or three large galaxies and a number of smaller ones. At a distance of 5 to 6 Mpc, we find a loose cluster centered on the giant spiral galaxy M 101, the Pinwheel Galaxy. At the edge of our cosmic neighborhood, about 15 Mpc away in the direction of the Virgo constellation, we encounter the first truly impressive concentration of galaxies. The Virgo Cluster has thousands of galaxies, with many more dwarfs than giants. Between these structures, there are large regions with essentially no galaxies; such regions are called voids.