Galaxies (normal and active)
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Galaxies (normal and active)
- 1. The Universe of Galaxies Prepared by: Judy-ann P. Jardinan III-BEED
- 2. Chapter 18: Galaxies: Normal and Active
- 3. The Family of Galaxies Overview Measuring the Properties of Galaxies Active Galactic Nuclei Supermassive Black Holes The Evolution of Galaxies
- 4. Milky Way Galaxy is only one of the many billions of galaxies visible in the sky
- 6. The Shapes of Galaxies 1) Many galaxies have no disk, no spiral arms, and almost no gas and dust. These elliptical galaxies range from huge giants to small dwarfs.
- 7. 2) Disk-shaped galaxies usually have spiral arms and contain gas and dust. Many of these spiral galaxies have a central region shaped like an elongated bar and are called barred spiral galaxies. A few disk galaxies contain little gas and dust. The Shapes of Galaxies
- 8. 3) Irregular galaxies are generally shapeless and tend to be rich in gas and dust. The Shapes of Galaxies
- 10. Distance • The distances to galaxies are so large that it is not convenient to measure them in light-years, parsecs, or even kilo parsecs. Instead, astronomers use the unit megaparsec (Mpc),or 1 million pc. One Mpc = 3.26 million ly, or approximately 3x1019km pr(2x1019miles)
- 11. • Such objects are called distance indicators because they can be used to find the distance to a galaxy. •Astronomers often refer to them as standard candles. If you can find a standard candle in a galaxy, you can judge its distance Distance
- 12. astronomers Edwin Hubble and Milton Humason were able to measure the distances to a number of galaxies using Cepheid variable stars. The Hubble Law
- 13. The Hubble Law 1929, they published a graph that plotted the apparent velocity of recession versus distance for their galaxies. The points in the graph fell along a straight line Vr = Hd
- 14. Diameter and Luminosity Irregular galaxies tend to be small, 1 to 25 percent the size of our galaxy, and of low luminosity. Our Milky Way Galaxy is large and luminous compared with most spiral galaxies, though astronomers know of a few spiral galaxies that are even larger and more luminous. The results of such observations show that galaxies differ dramatically in size and luminosity Elliptical galaxies cover a wide range of diameters and luminosities The largest, called giant ellipticals, are five times the size of our Milky Way
- 15. Mass •Using such a graph to find the mass of the galaxy is called the rotation curve method. •It is the most accurate way to find the mass, but it works only for the nearer galaxies, whose rotation curves can be observed.
- 16. Supermassive Black Holes in Galaxies To hold stars in such small, short period orbits, the centers of galaxies must contain millions or even billions of solar masses in a very small region. • The evidence shows that the nuclei of many galaxies contain supermassive black holes. The Milky Way contains a supermassive black hole at its center, and evidently that is typical of galaxies.
- 17. Dark Matter in Galaxies • X-ray images of galaxy clusters show that many of them are filled with very hot, low-density gas. 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 Gravitational lensing occurs when light from a distant object passes a nearby massive object and is deflected by the gravitational field.
- 18. Theorists conclude that ……. Dark matter must be made up of some as yet undiscovered subatomic particles that do not interact with normal matter, with each other, or with light. Dark matter is detectable only through its gravitational field. Dark matter is not an insignificant cant issue. Dark Matter in Galaxies Dark matter is independent of Newton’s laws and gives astronomers great confidence that dark matter is real
- 20. Cluster of Galaxies •Rich clusters contain a thousand or more galaxies, mostly ellipticals, scattered through a volume roughly 3 Mpc (107ly) in diameter.
- 21. •Poor clusters contain fewer than a thousand (and often only a few) galaxies spread through a region that can be as large as a rich cluster. That means the galaxies are more widely separated. Cluster of Galaxies
- 22. Our Milky Way Galaxy is a member of a poor cluster known by the unimaginative name of the Local Group Cluster of Galaxies The total number of galaxies in the Local Group is uncertain, but it probably contains about 40 galaxies scattered irregularly through a volume roughly 1 Mpc in diameter
- 23. Colliding Galaxies Galaxies should collide fairly often. The average separation between galaxies is only about 20 times their diameter
- 24. Interacting Galaxies Interacting galaxies can distort each other with tides producing tidal tail sand shells of stars. They may even trigger the formation of spiral arms. In fact, large galaxies can even absorb smaller galaxies, a process called galactic cannibalism. Interactions between galaxies can trigger rapid star formation Evidence left inside galaxies in the form of motions and multiple nuclei reveals that they have suffered past interactions and mergers. the beautiful ring galaxies are understood to be bull’s-eyes left behind by high- speed collisions Colliding Galaxies
- 25. Assembling Galaxies This process of galaxy assembly is still occurring today - we see many examples of galaxies colliding and merging to form new galaxies. Many billions of years from now! Scientists today know that galaxies existed about one billion years after the Big Bang. While most of these early galaxies were smaller and more irregular than present-day galaxies, some are very similar to those seen nearby today.
- 27. What is Active Galactic Nuclei ?
- 28. #History101 In 1950s, astronomers discovered that some galaxies, dubbed radio galaxies , were bright at radio wavelengths. Later, when telescopes went into orbit, these galaxies were found to be emitting energy at other wavelengths as well, and they became known as active galaxies . Modern observations show that the energy comes from the nuclei of the galaxies, which are now known as active galactic nuclei (AGN)
- 29. Seyfert Galaxies Observing at visual wavelengths, Seyfert found that some spiral galaxies have small, highly luminous nuclei with peculiar spectra .These galaxies are now known as Seyfert galaxies
- 30. Double-Lobed Radio Sources • When optical telescopes studied the locations of these radio sources, they revealed galaxies located between the two regions emitting radio energy, and the galaxies were dubbed double- lobed radio galaxies.
- 31. Quasars First called quasistellar objects, they were soon referred to as quasars because the signals came from one place, like a star Most quasars have been found billions of light-years away
- 33. Disks and Jets Matter flowing inward toward a black hole spins very fast and becomes very hot. It spins because it must conserve angular momentum as it sinks inward where it forms a flattened disk around The inner part of the accretion disk around a supermassive black hole can reach temperatures of millions of degrees and emit X-rays.
- 34. The Search for a Unified Model Astronomers studying active galaxies have developed a unified model of active galaxy cores that is well supported by evidence. A monster black hole is the centerpiece
- 35. What could trigger a supermassive black hole to erupt?
- 36. Triggering Eruptions A sudden flood of matter flowing into the accretion disk around a supermassive black hole would trigger it into eruption
- 37. Supermassive Black Holes Through Time •supermassive black holes always make up the same small percentage (0.5 percent) of the mass of their central bulge, astronomers conclude that the supermassive black holes formed at the same time •Most of the quasars and active galaxies that astronomers see with today’s telescopes have been triggered into eruption by the interaction and collision of galaxies, a process that throws matter into the central black holes
- 38. In a Nutshell Astronomy is changing you. As you learn more about stars and galaxies and quasars, you are learning more about yourself and your connection with nature. “Perspective” can mean a view of things in their true relationships. As you study astronomy, you are gaining perspective. Our galaxy, our sun, our planet, and the local shopping mall take on new meaning when you think astronomically.
- 39. References • Universe (Solar systems, Stars, and Galaxies) 7th Edition by Seeds and Backman • http://www.astro.cornell.edu/academics/courses/astro201/hubbles_l aw.htm • https://jwst.nasa.gov/galaxies.html • http://www.space.com/17262-quasar-definition.html
- 40. Mabalos!
Hinweis der Redaktion
- This chapter will expand your horizon to discuss the different kinds of galaxies, their complex histories, and violent eruptions.
- (intro)Astronomers classify galaxies according to their shapes in photographs made at visual wavelengths using a system developed by Edwin Hubble in the 1920s. >Elliptical galaxies are round or elliptical, contain no visible gas and dust, and lack hot, bright stars. They are classified with a numerical index ranging from 1 to 7; E0s are round, and E7s are highly elliptical. The index is calculated from the largest and smallest diameter of the galaxy used in the following formula and rounded to the nearest integer.
- >Spiral galaxies contain a disk and spiral arms. Their halo stars are not visible, but presumably all spiral galaxies have halos. Spirals contain gas and dust and hot, bright O and B stars, as shown at right and below. The presence of short-lived O and B stars alerts us that star formation is occurring in these galaxies. Sa galaxies have larger nuclei, less gas and dust, and fewer hot, bright stars. Sc galaxies have small nuclei, lots of gas and dust, and many hot, bright stars. Sb galaxies are Intermediate >Roughly 2/3 of all spiral galaxies are barred spiral galaxies classified SBa, SBb, and SBc. They have an elongated nucleus with spiral arms springing from the ends of the bar, as shown at left. Our own galaxy is a barred spiral.
- >Irregular galaxies(classified Irr) are a chaotic mix of gas, dust, and stars with no obvious nuclear bulge or spiral arms. The Large and Small Magellanic Clouds are visible to the unaided eye as hazy patches in the southern hemisphere sky. Telescopic images show that they are irregular galaxies that are interacting gravitationally with our own much larger galaxy. Star formation is dramatic in the Magellanic Clouds. The bright pink regions are emission nebulae excited by newborn O and B stars. The brightest nebula in the Large Magellanic Cloud is called the Tarantula Nebula
- the first step in your study of galaxies is to find out how far away they are. Once you know a galaxy’s distance, its size and luminosity are relatively easy to find. Later in this section, you will see that, just as for stars, finding the masses of galaxies is more difficult
- To find the distance to a galaxy, astronomers must search among its stars, nebulae, and star clusters for familiar objects whose luminosity they know. >When you look at a distant galaxy, you look back into the past by an amount called the look-back time, a time in years equal to the distance in light-years the light from the galaxy traveled to reach Earth.
- Although astronomers must work carefully to measure the distance to a galaxy, they often estimate such distances using a simple relationship that was first noticed at about the same time astronomers were beginning to understand the nature of galaxies. >In 1929, Hubble estimated the value of the expansion factor, now called the Hubble constant, to be about 500 km/sec/Mpc. Today the value is still rather uncertain, but is generally believed to be in the range of 45-90 km/sec/Mpc.
- Once you find the distance to a galaxy from distance indicators or the Hubble law, you can calculate its diameter and its luminosity. With a good telescope and the right equipment, you could easily photograph a galaxy and measure its angular diameter in arc seconds. If you knew the distance, you could use the small-angle formula to find its linear diameter.
- Although the mass of a galaxy is difficult to determine, it is an important quantity. It tells you how much matter the galaxy contains, which in turn provides clues to the galaxy’s origin and evolution. >If astronomers can measure the radius of a galaxy and the velocity with which it rotates, they can easily find the orbital periods of the stars as they orbit the galaxy. Then they can use Newton’s version of >More distant galaxies appear so small astronomers cannot measure the radial velocity at different points across the galaxy. Kepler’s third law
- Rotation curves show the motions of the outer parts of a galaxy, but it is also possible to detect the Doppler shifts of stars orbiting very close to the centers. Although these motions are not usually shown on rotation curves, they reveal something astonishing. A billion-solar-mass black hole sounds like a lot of mass, but note that it is roughly 1 percent of the mass of a galaxy. The 4-million-solar-mass black hole at the enter of the Milky Way Galaxy contains only one thousandth of 1 percent of the mass of our galaxy. Later in this chapter you will discover that these supermassive black holes can produce titanic eruptions, but they still represent only a small fraction of the mass of a galaxy.
- 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 they know about how much matter there is between the stars, so it is quite possible to estimate the mass of a galaxy from its luminosity. >Gravitational lensing occurs when light from a distant object passes a nearby massive object and is deflected by the gravitational field. The gravitational field of the nearby object is actually a region of curved space-time that acts as a lens and deflects the passing legitimate the mass of a galaxy from its luminosity
- Single, isolated galaxies are rare. Instead, most galaxies occur in clusters containing a few to a few thousand galaxies in volumes 1 to 10 Mpc across. Our Milky Way Galaxy is a member of a small cluster, and surveys have cataloged thousands of other cluster >Such a cluster is nearly always condensed; that is, the galaxies are more crowded near the cluster center. At their centers, such clusters often contain one or more giant elliptical galaxies s.
- Th e total number of galaxies in the Local Group is uncertain because some galaxies lie in the plane of the Milky Way Galaxy and are diffi cult to detect.
- The galaxy classes tell you something important, but a single galaxy does not change from one class to another any more than a cat can change into a dog.Another old idea was that each galaxy formed from a single cloud of gas that contracted and formed stars. Astronomers call such a proposal a top-down theory.
- In spite of their small size, the cores of Seyfert galaxies produce tremendous amounts of energy. The brightest emit a hundred times more energy than the entire Milky Way Galaxy
- Unlike Seyfert galaxies, which emit intense radiation from their cores, these radio galaxies were producing energy from two external radio lobes >Many radio sources consist of two bright lobes — double-lobed radio sources — with a galaxy, often a peculiar or distorted galaxy, located between them. Evidence suggests these active galaxies are emitting jets of high-speed gas that inflate the lobes as cavities in the intergalactic medium. This has been called the double-exhaust model. Where the jets impact the far side of the cavities, they create hot spots. >Radio galaxy NGC 5128 lies between two radio lobes, and, like many active galaxies, is strangely distorted. The dust ring rotates about an axis perpendicular to the ring, but the spherical cloud of stars rotates about an axis that lies in the plane of the ring. NGC 5128 appears to be two galaxies, a giant elliptical and a spiral, passing through each other. This has triggered multiple eruptions. An earlier eruption has produced a large outer pair of lobes, and a more recent eruption has produced an inner pair.
- Through the 1950s, radio astronomers became familiar with celestial radio sources that were either huge clouds of gas or distant radio galaxies, so they were surprised in the early 1960s when some radio sources turned out to look like stars in visual wavelength photographs >Quasars are part of a class of objects known as active galactic nuclei (AGN). Other classes include Seyfert galaxies and blazars. All three require supermassive black holes to power them.
- No one knows exactly how the supermassive black hole and its disk produce jets of gas and radiation, but magnetic fi elds are a factor. Because the disk is at least partially ionized, magnetic fields are trapped in the gas of the disk, drawn inward, and wound up. Th eorists suggest that this creates powerful magnetic tubes extending along the axis of rotation, channeling hot gas outward in opposite directions. Th e jets seem to originate very close to the supermassive black hole and are then focused and confined by the enclosing magnetic tubes
- When a field of research is young, scientists find many seemingly different phenomena, such as Seyfert galaxies, double lobed radio galaxies, quasars, cosmic jets, and so on. >According to the unifi ed model, what you see when you view the core of an active galaxy depends on how the black hole’s accretion disk is tipped with respect to your line of sight
- Th is explains why active galaxies are often distorted; they have been twisted by tidal forces as they interacted or merged with another galaxy. Some active galaxies have nearby companions, and you can suspect that the companions are guilty of tidally distorting the other galaxy and triggering an eruption. Images of some quasars reveal that they are embedded in galaxies that are distorted and lie near other distorted galaxie
- Some of the most distant quasars could be caused by the formation of supermassive black holes, but these extremely distant objects are difficult to image with existing telescope