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Galaxies Astronomy: Horizons 10th edition Michael Seeds Galaxies • Less than a century ago, astronomers did not understand that there were galaxies. – Nineteenth-century telescopes revealed faint nebulae scattered among the stars, some of which were spiral. – Astronomers argued about the nature of these nebulae. – It was not until the 1920s, though, that they understood that some were other galaxies much like our own. Galaxies • It was not until recent decades that astronomical telescopes could reveal the tremendous beauty and intricacy of the galaxies. Galaxies • In this chapter, you will try to understand how galaxies form and evolve. – You will discover that the amount of gas and dust in a galaxy is a critical clue. • You will also discover that interactions between galaxies can influence their structure and evolution. The Shapes of Galaxies Galaxies • One, many galaxies have no disk, no spiral arms, and almost no gas and dust. – These elliptical galaxies range from huge giants to small dwarfs. The Shapes of Galaxies Galaxies • Disk-shaped galaxies usually have spiral arms and contain gas and dust. – Many spiral galaxies have a barred structure. – A small percentage of disk galaxies contain little gas and dust. The Shapes of Galaxies Galaxies • Finally, some galaxies are highly irregular in shape and tend to be rich in gas and dust. The Shapes of Galaxies Galaxies • You might also wonder what proportion of the galaxies are elliptical, spiral, and irregular. • That is a difficult question to answer. – In some catalogs of galaxies, about 70 percent are spiral. The Shapes of Galaxies Galaxies • Spiral galaxies contain hot, bright stars— and are thus very luminous and easy to see. • Most ellipticals are fainter and harder to notice. • Small galaxies, such as dwarf ellipticals and dwarf irregulars, may be very common—but they are hard to detect. The Shapes of Galaxies Galaxies • From careful studies, astronomers can conclude that ellipticals are more common than spirals and that irregulars make up only about 25 percent of all galaxies. – Among spiral galaxies, about two thirds are barred spirals. How Many Galaxies Are There? Galaxies • In 1995, astronomers picked a seemingly empty spot on the sky near the Big Dipper and used the Hubble Space Telescope to record a time exposure that lasted an astonishing 10 days. – This became known as a Hubble deep field. – It was deep in that it recorded very faint objects. – The image revealed that the ‘empty spot’ on the sky was filled with galaxies. How Many Galaxies Are There? Galaxies • There is no reason to believe that the two regions of the sky chosen for study are unusual. – So, it seems likely that the entire sky is carpeted with galaxies. • At least 100 billion would be visible with today’s telescopes. – Surely, there are other galaxies too distant or too faint to see. Building Scientific Arguments Galaxies • Different kinds of galaxies have different colors—depending mostly on how much gas and dust they contain. • If a galaxy contains large amounts of gas and dust, it probably contains lots of young stars. – A few of those young stars will be massive, hot, luminous O and B stars. – They will produce most of the light and give the galaxy a distinct blue tint. Building Scientific Arguments Galaxies • In contrast, a galaxy that contains little gas and dust will probably contain few young stars. – It will lack O and B stars. – The most luminous stars in such a galaxy will be red giants. – They will give the galaxy a red tint. Building Scientific Arguments Galaxies • As the light from a galaxy is a blend of the light from billions of stars, the colors are only tints. • Nevertheless, the most luminous stars in a galaxy determine the color. – You can conclude that elliptical galaxies tend to be red and the disks of spiral galaxies tend to be blue. Building Scientific Arguments Galaxies • Now, create a new scientific argument and analyze a different kind of observation. – Why are most galaxies in catalogs spiral in spite of the fact that the most common kind of galaxy is elliptical? Measuring the Properties of Galaxies Galaxies • Beyond the edge of Milky Way, astronomers find many billions of galaxies. • Great clusters—some containing thousands of galaxies—fill space as far as telescopes can see. Distance Galaxies • The distances to galaxies are so large that it is not convenient to express them in light-years, parsecs, or even kiloparsecs. • Instead, astronomers use the unit megaparsec (Mpc), or 1 million pc. – One Mpc equals 3.26 million ly, or approximately 2 x 1019 miles. Distance Galaxies • Most distance indicators are objects whose brightness is known. • Astronomers often refer to them as standard candles. – If you can find a standard candle in a galaxy, you can judge its distance. Distance Galaxies • As their period is related to their luminosity, Cepheid variable stars are reliable distance indicators. – If you know the period of the star’s variation, you can use the period–luminosity relation to learn its absolute magnitude. – By comparing its absolute and apparent magnitudes, you can find its distance. Distance Galaxies • Astronomers can use globular clusters in a different way. – Studies of nearby globular clusters with known distances show that they are about 25 pc in diameter. – If astronomers can detect globular clusters in a distant galaxy, they can assume the clusters are about 25 pc in diameter and use the small-angle formula to find the distance to the galaxy. Distance Galaxies • When a supernova explodes in a distant galaxy, astronomers rush to observe it. – Studies show that type Ia supernovae—caused by the collapse of a white dwarf—all reach about the same absolute magnitude at maximum. – By searching for Cepheids and other distance indicators in nearby galaxies where type Ia supernovae have occurred, astronomers have been able to calibrate these supernovae. Distance Galaxies • For example, a type Ia supernova was seen in 2002 exploding in the galaxy displayed in the image. – Astronomers were able to find Cepheid variables in the galaxy and so could find its distance. – Then, they could calculate the absolute magnitude of the supernova at its brightest. Distance Galaxies • When type Ia supernovae are seen in more distant galaxies, astronomers can measure the apparent magnitude at maximum and compare that with the known absolute magnitude of these supernovae to find the distance to the galaxy. Distance Galaxies • Astronomers like to refer to distance indicators such as Cepheids as standard candles. • However, an astronomer commented that type Ia supernovae are more like standard bombs. – When they explode, they reach the same maximum luminosity and can be calibrated as distance indicators. Distance Galaxies • At the greatest distances, astronomers must calibrate the total luminosity of the galaxies themselves. – For example, studies of nearby galaxies show that an average galaxy like Milky Way has a luminosity about 16 billion times the sun’s. – If astronomers see a similar galaxy far away, they can measure its apparent magnitude and calculate its distance. Distance Galaxies • The most distant visible galaxies are roughly 10 billion ly (3,000 Mpc) away. • At such distances, you see an effect akin to time travel. Distance Galaxies • When you look at a galaxy only a few million light-years away, you do not see it as it is now but as it was millions of years ago—when its light began the journey toward Earth. Distance Galaxies • When you look at a more distant galaxy, you look back into the past by an amount called the look-back time. – This is the time in years equal to the distance to the galaxy in light-years. Distance Galaxies • You may have experienced look-back time if you have ever made a longdistance phone call carried by satellite or watched a TV newscaster interview someone on the other side of the world via satellite. Distance Galaxies • A half-second delay occurs as a radio signal carries a question 23,000 miles out to a satellite, then back to Earth, and then carries the answer out to the satellite and again back to Earth. – That half-second look-back delay can make people hesitate on long-distance phone calls and produces a seemingly awkward delay in intercontinental TV interviews. Distance Galaxies • The Andromeda Galaxy has a look-back time of about 2 million years—a mere eye blink in the lifetime of a galaxy. • When you look at more distant galaxies, though, the look-back time becomes an appreciable part of the age of the universe. Distance Galaxies • When you look at the most distant visible galaxies, you are looking back over 10 billion years to a time when the universe may have been significantly different. – This effect will be important as you think about the origin and evolution of galaxies and the universe as a whole. The Hubble Law Galaxies • Although astronomers find it difficult to measure the distance to a galaxy, they often estimate such distances using a simple relationship. – Early in the 20th century, astronomers noticed that the lines in galaxy spectra were shifted slightly toward longer wavelengths—redshifts. – Interpreted as a consequence of the Doppler shift, these redshifts implied that the galaxies had large radial velocities and were receding from Earth. The Hubble Law Galaxies • In 1929, the American astronomer Edwin Hubble published a graph that plotted the apparent velocities of recession versus distance for a number of galaxies. – The points in the graph fell along a straight line. The Hubble Law Galaxies • This relation between apparent velocity of recession and distance is known as the Hubble law. – The slope of the line is known as the Hubble constant. The Hubble Law Galaxies • The Hubble law is important in astronomy for two reasons. – It is taken as evidence that the universe is expanding. – Astronomers use it to estimate the distance to galaxies. Diameter and Luminosity Galaxies • Elliptical galaxies cover a wide range of diameters and luminosities. – The largest, called giant ellipticals, are five times the size of Milky Way. – Many elliptical galaxies, though, are very small, dwarf ellipticals—only 1 percent the diameter of our galaxy. Diameter and Luminosity Galaxies • Clearly, the diameter and luminosity of a galaxy do not determine its type. – Some small galaxies are irregular and some are elliptical. – Some large galaxies are spiral and some are elliptical. • Other factors must influence the origin and evolution of galaxies. Mass Galaxies • This section will examine two fundamental ways to find the masses of galaxies. – One method involves the rotation of galaxies. – The other involves their motions. Mass Galaxies • To begin, you can eliminate a common misconception. • Some people think astronomers can see galaxies rotating. – Some galaxies definitely look like spinning pinwheels. – You know, though, that the orbital period of the sun around Milky Way is about 240 million years. – So, galaxies rotate very slowly. – No change is visible in a human lifetime. Mass Galaxies • Nevertheless, the rotation of galaxies can give you a clue to their masses. – You know the stars in the outer parts of the galaxy are in orbit. – So, you can use Kepler’s third law to find the mass from the size of the stellar orbits and their orbital period. Mass Galaxies • You can’t observe the orbital period, P, directly because humans don’t live long enough to see a galaxy rotate. • However, you can find the orbital velocity of the stars by measuring their Doppler shifts. – Then, you can find the orbital period by dividing the circumference of the orbit by the velocity. Mass Galaxies • As the galaxy rotates, one side moves away from Earth and one side moves toward Earth. – So, the emission lines would be redshifted on one side of the galaxy and blueshifted on the other side. Mass Galaxies • You could measure those changes in wavelength, use the Doppler formula to find the velocities, and plot a diagram showing the velocity of rotation at different distances from the center of the galaxy. – This diagram is called a rotation curve. Mass Galaxies • A related way of measuring a galaxy’s mass is the velocity dispersion method. – It is really a version of the cluster method. – Instead of observing the motions of galaxies in a cluster, astronomers observe the motions of matter within a galaxy. Mass Galaxies • In the spectra of some galaxies, broad spectral lines indicate that stars and gas are moving at high velocities. – If astronomers assume the galaxy is bound by its own gravity, they can ask how massive it must be to hold this moving matter within the galaxy. – This method, like the one before, assumes that the system is not coming apart. Mass Galaxies • The masses of galaxies cover a wide range. – The smallest contain about 10-6 as much mass as the Milky Way. – The largest contain as much as 50 times more than the Milky Way. Supermassive Black Holes in Galaxies Galaxies • Rotation curves show the motions of the outer parts of a galaxy. • It is also possible, though, to detect the Doppler shifts of stars orbiting close to the centers of galaxies. • Although these motions are not usually shown on rotation curves, they reveal something astonishing. – Stars near the centers of most galaxies are orbiting very rapidly. Supermassive Black Holes in Galaxies Galaxies • To hold stars in such small, short-period orbits, the centers of galaxies must contain masses of a million to a few billion solar masses. • Yet, no object is visible. – The evidence seems to require that the nuclei of galaxies contain supermassive black holes. – You have learned that Milky Way contains a supermassive black hole at its center. – Evidently, that is typical of galaxies. Supermassive Black Holes in Galaxies Galaxies • Such supermassive black holes cannot be the remains of a dead star. – That would produce a black hole of only a few solar masses. • Measurements show that the masses of these supermassive black holes are typically 0.5 percent the mass of the nuclear bulges. Supermassive Black Holes in Galaxies Galaxies • A galaxy with a large nuclear bulge has a supermassive black hole whose mass is greater than that in a galaxy with a small nuclear bulge. – This implies that the supermassive black holes formed long ago as the galaxies began forming. – Matter has continued to drain into the black holes. – However, they do not appear to have grown dramatically since they formed. Supermassive Black Holes in Galaxies Galaxies • A billion-solar-mass black hole sounds like a lot of mass. • It is, however, roughly 1 percent of the mass of a galaxy. – The 2.6-million-solar-mass black hole at the center of Milky Way contains only a thousandth of a percent of the mass of the galaxy. Dark Matter in Galaxies Galaxies • Given the size and luminosity of a galaxy, astronomers can make a rough guess as to the amount of matter it should contain. – They know how much light stars produce and how much matter there is between the stars. – So, it is quite possible to estimate very roughly the mass of a galaxy from its luminosity. Dark Matter in Galaxies Galaxies • When astronomers measure the masses of galaxies, however, they find that the measured masses are much larger than expected from the luminosities of the galaxies. • This seems to be true of most galaxies. – Measured masses of galaxies amount to 10 to 100 times more mass than you would expect from the appearance of galaxies. Dark Matter in Galaxies Galaxies • X-ray observations provide more evidence of dark matter. – X-ray images of galaxy clusters show that many of them are filled with very hot, low-density gas. Dark Matter in Galaxies Galaxies • The amount of gas present is much too small to account for the dark matter. – Rather, the gas is important because it is very hot and its rapidly moving atoms have not leaked away. – Evidently, the gas is held in the cluster by a very strong gravitational field. Dark Matter in Galaxies Galaxies • To provide enough gravity to hold such hot gas, the cluster must contain much more matter than is visible as galaxies. – For instance, the detectable galaxies in the Coma cluster amounts to only a small fraction of the total mass of the cluster. Gravitational Lensing and Dark Matter Galaxies • Albert Einstein described gravity as a curvature of space. – The presence of mass actually distorts space-time around it. – That is what you feel as gravity. – Einstein predicted that a light beam traveling through a gravitational field would be deflected by the curvature of space-time, much as a golf ball is deflected as it rolls over a curved putting green. – That effect has been observed and is a strong confirmation of Einstein’s theories. Gravitational Lensing and Dark Matter Galaxies • Gravitational lensing occurs when light from a distant object passes a nearby massive object and is deflected by its gravitational field. – The gravitational field of the nearby object is actually a region of curved space-time. – It acts as a lens to deflect the passing light. Gravitational Lensing and Dark Matter Galaxies • Astronomers have used gravitational lensing to detect dark matter. – When light from very distant galaxies passes through a cluster of galaxies on its way to Earth, it can be deflected by the strong gravitational field. Gravitational Lensing and Dark Matter Galaxies • That distorts the images of the distant galaxies into curving arcs. – The amount of the distortion depends on the mass of the cluster of galaxies. Gravitational Lensing and Dark Matter Galaxies • Observations of gravitational lensing made with very large telescopes reveal that clusters of galaxies contain far more matter than what we is seen. • That is, they contain large amounts of dark matter. – This confirmation of the existence of dark matter is independent of orbital motion and gives astronomers much greater confidence that dark matter is real. Gravitational Lensing and Dark Matter Galaxies • Dark matter is difficult to detect, and it is even harder to explain. – Some astronomers have suggested that dark matter consists of low-luminosity white dwarfs and brown dwarfs scattered through the halos of galaxies. Gravitational Lensing and Dark Matter Galaxies • Both observation and theory support the idea that galaxies have massive extended halos. • Also, searches for white dwarfs and brown dwarfs in the halo of Milky Way have been successful. – Nevertheless, the searches have not turned up enough of these low-luminosity objects to make up all the dark matter. Gravitational Lensing and Dark Matter Galaxies • The dark matter can’t be hidden in vast numbers of black holes and neutron stars. – Astronomers don’t see the X rays these objects would emit. – There is 10 to 100 times more dark matter than visible matter. – That many black holes would produce X rays that would be easy to detect. Gravitational Lensing and Dark Matter Galaxies • Until recently, neutrinos were thought to be massless. • Studies now suggest they have a very small mass. • Thus, they may represent part of the dark matter. – However, their masses are too low to make up all the dark matter. – There must be some other undiscovered form of matter in the universe that is detectable only by its gravitational field. Gravitational Lensing and Dark Matter Galaxies • Dark matter is not a small issue. – Observations show that 85 percent of the matter in the universe is dark matter. – The universe you see—the kind of matter that you and the stars are made of—has been compared to the foam on an invisible ocean. • Dark matter remains one of the fundamental unresolved problems of modern astronomy. The Evolution of Galaxies Galaxies • Why did some galaxies become spiral, some elliptical, and some irregular? – Clues to that mystery lie in the clustering of galaxies. Clusters of Galaxies Galaxies • The distribution of galaxies is not entirely random. – Galaxies tend to occur in clusters ranging from a few to thousands. – Deep photos made with the largest telescopes reveal clusters of galaxies scattered out to the limits of detectability. • This clustering of the galaxies can help you understand their evolution. Clusters of Galaxies Galaxies • For this discussion, you can sort clusters into two groups—rich and poor. • Rich galaxy clusters contain over a thousand galaxies, mostly elliptical, scattered through a spherical volume about 3 Mpc (107 ly) in diameter. Clusters of Galaxies Galaxies • A rich galaxy cluster is very crowded—with the galaxies more concentrated toward the center. • It often contains one or more giant elliptical galaxies at the center. Clusters of Galaxies Galaxies • The Coma cluster (located in the constellation Coma Berenices) is a rich cluster. – It lies over 100 Mpc from Earth and contains at least 1,000 galaxies—mostly E and S0 galaxies. – Its galaxies are highly crowded around a central giant elliptical galaxy and a large S0. Clusters of Galaxies Galaxies • One of the nearest clusters, the Virgo cluster, contains over 2,500 galaxies and is—by the discussed definition—a rich cluster. – It does contain a giant elliptical galaxy, M87, near its center. – However, it is not very crowded and contains mostly spiral galaxies. Clusters of Galaxies Galaxies • In contrast, poor galaxy clusters contain fewer than 1,000 galaxies, are irregularly shaped, and are less crowded toward the center. Clusters of Galaxies Galaxies • The Local Group, which contains Milky Way, is a good example of a poor cluster. – It contains a few dozen members scattered irregularly through a volume slightly over 1 Mpc in diameter. – Of the brighter galaxies, 14 are elliptical, 3 are spiral, and 4 are irregular. Clusters of Galaxies Galaxies • The total number of galaxies in the Local Group is uncertain because some lie in the plane of Milky Way and are difficult to detect. – For example, the Sagittarius Dwarf, a small dwarf galaxy, has been found on the far side of Milky Way—where it is almost totally hidden behind the star clouds of Sagittarius. Clusters of Galaxies Galaxies • Even closer to the center of Milky Way is the Canis Major Dwarf Galaxy. – The galaxy was found by mapping the distribution of red supergiants detected by the 2MASS infrared survey. – Other small galaxies in the Local Group have been found hidden behind the stars, gas, and dust of Milky Way. Clusters of Galaxies Galaxies • Classifying clusters as either rich or poor reveals a fascinating and suggestive clue to the evolution of galaxies. – In general, rich clusters tend to contain 80 to 90 percent E and S0 galaxies and few spirals. – Poor clusters contain a larger percentage of spirals. – Among isolated galaxies that are not in clusters, 80 to 90 percent are spirals. Clusters of Galaxies Galaxies • This suggests that a galaxy’s environment is important in determining its structure. – This has led astronomers to suspect that the secrets of galaxy evolution lie in collisions between galaxies. Colliding Galaxies Galaxies • There are several important points to note about interacting galaxies. Colliding Galaxies • One, interacting galaxies can distort each other with tides—producing tidal tails and shells of stars. – They may even trigger the formation of spiral arms. – Large galaxies can even absorb smaller galaxies. Galaxies Colliding Galaxies Galaxies • Also, the interactions can trigger star formation. Colliding Galaxies Galaxies • Evidence left inside galaxies in the form of motions and multiple nuclei reveals that they have suffered past interactions and mergers. Colliding Galaxies Galaxies • Finally, the beautiful ring galaxies are bull’s-eyes left behind by high-speed collisions. Colliding Galaxies Galaxies • Evidence of galaxy mergers is all around. – Milky Way is a cannibal galaxy—snacking on the Magellanic Clouds as they orbit around it. – Its tides are pulling the Sagittarius Dwarf Galaxy apart. – The Canis Major Dwarf galaxy has been almost completely digested as tides pulled stars away to form great streamers wrapped around Milky Way. – Almost certainly, our galaxy has dined on other small galaxies. The Origin and Evolution of Galaxies Galaxies • You can also argue that spiral and irregular galaxies cannot evolve into elliptical galaxies. – Spiral and irregular galaxies contain both young and old stars. – The old stars mean that spiral and irregular galaxies can’t be young. The Origin and Evolution of Galaxies Galaxies • The ellipticals appear to be the product of galaxy mergers—which triggered star formation and used up all the gas and dust. – Astronomers see star formation being stimulated to high levels in many colliding galaxies. The Origin and Evolution of Galaxies • The Antennae contain over 15 billion solar masses of hydrogen gas and will become a starburst galaxy as the merger triggers rapid star formation. Galaxies The Origin and Evolution of Galaxies Galaxies • Supernovae in a starburst galaxy may eventually blow away any remaining gas and dust that doesn’t get used up making stars. – A few collisions and mergers could leave a galaxy with no gas and dust from which to make new stars. – Astronomers now suspect that most ellipticals are formed by the merger of at least two or three galaxies. The Origin and Evolution of Galaxies Galaxies • In contrast, spirals seem never to have suffered major collisions. – Their thin disks are delicate and would be destroyed by tidal forces in a collision with a massive galaxy. • Also, they retain plenty of gas and dust and continue making stars. The Origin and Evolution of Galaxies Galaxies • Milky Way has, evidently, never merged with another large galaxy. • It has, however, clearly cannibalized smaller galaxies. – Astronomers have found streams of stars in the halo of the galaxy that are too metal rich for their location. – Another stream contains globular clusters with similar ages. – These streams are evidently the remains of smaller galaxies that were absorbed. The Origin and Evolution of Galaxies Galaxies • Barred spiral galaxies may be the product of tidal interactions. – Mathematical models show that the bars are not stable and eventually dissipate. – It may take tidal interactions with other galaxies to regenerate the bars. – As well over half of all spiral galaxies have bars, you can suspect that these tidal interactions are common. Building Scientific Arguments Galaxies • A growing body of evidence suggests that elliptical galaxies have been subject to collisions in their past and that spiral galaxies have not. – During collisions, a galaxy can be driven to use up its gas and dust in a burst of star formation. – The resulting supernova explosions can help drive gas and dust out of the galaxy. – This explains why elliptical galaxies now contain little star-making material. Building Scientific Arguments Galaxies • The beautiful disk typical of spiral galaxies is very orderly—with all the stars following similar orbits. • However, when galaxies collide, the stellar orbits get scrambled—and an orderly disk galaxy could be converted into a chaotic swarm of stars typical of elliptical galaxies. – It seems likely that elliptical galaxies have had much more complex histories than spiral galaxies have had.