Physical Properties Of GalaxIES

The observable universe presents an estimated minimum of 100 billion galaxies, and many of these have billions of stars in them. Most of these galaxies are located far from Earth and the Milky Way Galaxy, and thus are difficult to observe closely. To measure distances to and sizes of these distant galaxies one must use some objects, such as planetary nebulae or certain kinds of supernovae that have known brightnesses, and then use their apparent brightness to measure their distance from Earth. Another relationship that has been exploited to measure the distance to faraway galaxies is to measure their rotational speeds, and then correlate these with a known relationship between the rotational speed of a galaxy, its mass, and its luminosity. The rotational speed can be measured at great distances, and the absolute brightness calculated; then when compared with the apparent brightness, the distance to the galaxy can be calculated. Using these methods to measure distances to galaxies, scientists have found that most lie at vast distances from the Earth, most much greater than 20 megaparsecs away. Furthermore, there is some order to the large-scale structure in the arrangement of galaxies, with many residing in galaxy clusters, superclusters, and other even larger structures.

Determining the masses of distant galaxies can be difficult. For spiral galaxies within about 50 kilo-parsecs of Earth, the rotational speed of the different spiral arms can be determined from the Doppler shifts of each arm, and if the distance from the galactic center is known, then Newton's laws of motion can be used to calculate the mass of a galaxy. For more distant galaxies one must depend on less reliable methods to estimate their masses. One way is to search for binary galaxy systems, then measurements of their orbital size and their orbital period enable the calculation of their mass using Kepler's third law. These different methods reveal that most spiral galaxies and large elliptical galaxies have about 1011-1012 solar masses in them, while the irregular galaxies tend to be less massive, with 108-1010 solar masses. Dwarf ellipticals are the least massive, typically containing 106-107 solar masses.

The rotational properties of most spiral galaxies and many elliptical galaxies indicates that they have excess mass surrounding them, but this mass is not luminous and is thought to be dark matter. The amount of dark matter in many cases is estimated to be 3-10 times the mass of the luminous matter in the galaxies. Galaxy clusters also appear to be associated with massive amounts of dark matter, with calcula tions showing that there must be between 10 and 100 times the masses of individual galaxy clusters. These calculations lead to the shocking conclusion that about 90 percent of the universe must be made up of invisible dark matter not detectable at any electromagnetic wavelength but can be observed only by its gravitational effects.

X-ray observations of galaxy clusters have demonstrated that some clusters are associated with strong emissions of X-ray radiation, and these are interpreted to be coming from hot gases that exist as intergalactic gas within the clusters. The mass of this gas is estimated, in some cases, to be about the same as or even more than the mass of the visible matter, but still substantially less than the mass needed to explain the gravitational observations by a factor of 10 to 100.

The motions of galaxies show interesting patterns on different scales of observation. The motion of individual galaxies within clusters of galaxies appears random, but the clusters show very ordered patterns to their motions at some of the largest scales of observation in the universe. Some of these motions have been partly understood for nearly a century. In 1912 Vesto Slipher, an American astronomer working with Percival Lowell (1855-1916), the American astronomer who founded Lowell Observatory and was president of Harvard University, discovered that every spiral galaxy he observed had a redshifted spectrum; Slipher concluded they were all moving away from the Earth. This observation has since been extended to include all known galaxies, which are moving away from the Earth in all directions. Individual galaxies not in clusters are moving away, as are the groups of galaxies in clusters, even though they have some random motions within the clusters. Furthermore, as observations improved, it became clear that the farther away the galaxy is from the Earth, the greater the redshift, and the faster it is receding.

In the 1920s the astronomer Edwin Hubble made a series of plots of the redshifts of galaxies and their calculated recessional velocities with distance from the Earth. He found that there is a straight-line relationship such that velocity increases steadily with distance. This proportionality is known as Hubble's law, and the general picture of all of the galaxies moving apart from one another is known as Hubble flow. Hubble's flow provides clear evidence that the universe is expanding.

Hubble's law can be written as follows:

where V is the recessional velocity, D is the distance, and H0 is the proportionality constant (as Hubble's


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Plot of recessional velocity versus distance for many galaxies within about 1 billion parsecs of Earth, illustrating Hubble's law, that recessional velocity is proportional to distance (modified from Chaisson and MacMillan)

constant) between the recessional velocity and distance. The slope of the straight line on a distance/ recessional velocity diagram is equal to Hubble's constant, which turns out to be approximately 75 km/sec per megaparsec (46.5 miles/sec per 3.3 million light years). There is, however, uncertainty in the exact value of the Hubble constant, with nearly all estimates falling in the range of 37-56 miles per second (60-90 km/sec) per megaparsec. Hubble's constant represents the best estimate of the rate of expansion of the universe.

Hubble's law is also extremely useful for measuring distances to faraway objects. Since the recessional velocity is proportional to the distance of the object, it is simple to measure the recessional velocity (from the redshift of the spectrum), then use Hubble's law to estimate the distance directly. This method works well even for very distant objects and is used to calculate the distance to the most distant objects yet known in the universe—object Q051-279, which has a redshift showing a recessional velocity of 93 percent the speed of light, and a distance of 4,000 megaparsecs. The electromagnetic radiation now observable on Earth from Q051-279 was generated about 13 billion years ago, close to the time of the big bang (presently estimated to be 13.73 billion +/- 120 million years ago). Another extremely distant object was discovered in 2004 using the Hubble space Telescope and an effect of general relativity called gravitational lensing, where massive objects in the foreground of a distant object can bend and magnify the light from the distant object, making it more observable. In 2004 a team of scientists discovered an object magnified by a gravitational lens in a galactic cluster (Abell 2218), and that object is a small, compact system of stars approximately 2,000 light years across and about 13 billion light years away. Using the present estimate of the age of the universe, scientists estimate the light from this object now reaching Earth was generated when the universe was only 750 million years old.

using powerful telescopes and Hubble's law, scientists can now map the large-scale structure of the universe. It turns out that the universe is not a random collection of star and galaxy systems but rather a patterned distribution of galaxies and clusters of galaxies, which are arranged in a network of string-or filament-like groups, separated by largely empty space known as voids. Astronomers have mapped these stringlike features to be on the surfaces of bubblelike voids, as if the universe were made of a system of empty bubbles with galaxy clusters forming chains along the surfaces of the bubbles. The areas where several bubbles intersect tend to be where the densest galaxy clusters and superclusters are located. The origin of these bubblelike structures is debated but must be related to density fluctuations or ripples in the earliest stages of the formation of the universe that grew during time and the expansion of the universe.

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