Universe Tenth Edition Chapter 23 Galaxies Roger Freedman Robert Geller William Kaufmann III 23-1 How astronomers first observed other galaxies 23-2 How astronomers determined the distances to other galaxies 23-3 The basic types of galaxies 23-4 What techniques astronomers use to determine distances to remote galaxies 23-5 How the spectra of remote galaxies tell us that the universe is expanding 23-6 How galaxies are grouped into clusters and larger structures 23-7 What happens when galaxies collide 23-8 What observations indicate the presence of dark matter in other galaxies and clusters 23-9 How galaxies formed and evolved By reading this chapter, you will learn A Pioneering View of Another Galaxy 23-1: When galaxies were first discovered, it was not clear that they lie far beyond the Milky Way A Modern View of the Spiral Galaxy M51 The Andromeda Galaxy 23-2: Hubble proved that the spiral nebulae are far beyond the Milky Way Measuring Galaxy Distances with Cepheid Variables Spiral Galaxies 23-3: Galaxies are classified according to their appearance Barred Spiral Galaxies Elliptical Galaxies Giant Elliptical Galaxies A Dwarf Elliptical Galaxy A Lenticular Galaxy Hubbles Tuning Fork Diagram The Large Magellanic Cloud (LMC) A Supernova in a Spiral Galaxy 23-4: Astronomers use various techniques to determine the distances to remote galaxies A Supernova in a Spiral Galaxy The Distance Ladder 23-4: Astronomers use various techniques to determine the distances to remote galaxies Measuring the Distance to a Galaxy Using Masers Relating the Distances and Redshifts of Galaxies The Hubble Law 23-5: The Hubble law relates the redshifts of remote galaxies to their distances from Earth The Hubble Law The Hercules Cluster 23-6: Galaxies are grouped into clusters and superclusters The Local Group The Canis Major Dwarf The Coma Cluster Nearby Clusters of Galaxies Structure in the Nearby Universe The Large-Scale Distribution of Galaxies Foamy Structure of the Universe X-ray Emission from a Cluster of Galaxies A Starburst Galaxy 23-7: Colliding galaxies produce starbursts, spiral arms, and other spectacular phenomena The M81 Group A Simulated Collision Between Two Galaxies Collision Between Andromeda and the Milky Way When Galaxies Collide The Rotation Curves of Four Spiral Galaxies 23-8: Most of the matter in the universe is mysterious dark matter Gravitational Lensing by a Cluster of Galaxies Isolated Dark Matter in a Cluster of Galaxies The Building Blocks of Galaxies 23-9: Galaxies formed from the merger of smaller objects The Small Magellanic Cloud (SMC) NGC 5128 in Visible and Infrared Wavelengths Key Ideas The Hubble Classification: Galaxies can be grouped into four major categories: spirals, barred spirals, ellipticals, and irregulars. The disks of spiral and barred spiral galaxies are sites of active star formation. Elliptical galaxies are nearly devoid of interstellar gas and dust, and so star formation is severely inhibited. Lenticular galaxies are intermediate between spiral and elliptical galaxies. Irregular galaxies have ill-defined, asymmetrical shapes. They are often found associated with other galaxies. Key Ideas Distance to Galaxies: Standard candles, such as Cepheid variables and the most luminous supergiants, globular clusters, H II regions, and supernovae in a galaxy, are used in estimating intergalactic distances. The Tully-Fisher relation, which correlates the width of the 21-cm line of hydrogen in a spiral galaxy with its luminosity, can also be used for determining distance. A method that can be used for elliptical galaxies is the fundamental plane, which relates the galaxy s size to its surface brightness distribution and to the motions of its stars. Key Ideas The Hubble Law: There is a simple linear relationship between the distance from the Earth to a remote galaxy and the redshift of that galaxy (which is a measure of the speed with which it is receding from us). This relationship is the Hubble law, v = H 0 d. The value of the Hubble constant, H 0, is not known with certainty but is close to 73 km/s/Mpc. Key Ideas Clusters and Superclusters: Galaxies are grouped into clusters rather than being scattered randomly throughout the universe. A rich cluster contains hundreds or even thousands of galaxies; a poor cluster, often called a group, may contain only a few dozen. A regular cluster has a nearly spherical shape with a central concentration of galaxies; in an irregular cluster, galaxies are distributed asymmetrically. Key Ideas Our Galaxy is a member of a poor, irregular cluster called the Local Group. Rich, regular clusters contain mostly elliptical and lenticular galaxies; irregular clusters contain spiral, barred spiral, and irregular galaxies along with ellipticals. Giant elliptical galaxies are often found near the centers of rich clusters. Key Ideas Galactic Collisions and Mergers: When two galaxies collide, their stars pass each other, but their interstellar media collide violently, either stripping the gas and dust from the galaxies or triggering prolific star formation. The gravitational effects during a galactic collision can throw stars out of their galaxies into intergalactic space. Galactic mergers may occur; a large galaxy in a rich cluster may tend to grow steadily through galactic cannibalism, perhaps producing in the process a giant elliptical galaxy. Key Ideas The Dark-Matter Problem: The luminous mass of a cluster of galaxies is not large enough to account for the observed motions of the galaxies; a large amount of unobserved mass must also be present. This situation is called the dark-matter problem. Hot intergalactic gases in rich clusters account for a small part of the unobserved mass. These gases are detected by their X-ray emission. The remaining unobserved mass is probably in the form of dark-matter halos that surround the galaxies in these clusters. Particles called WIMPs are the favored hypothesis for dark matter. Gravitational lensing of remote galaxies by a foreground cluster enables astronomers to glean information about the distribution of dark matter in the foreground cluster. Key Ideas Formation and Evolution of Galaxies: Observations indicate that galaxies arose from mergers of several smaller gas clouds. Whether a protogalaxy evolves into a spiral galaxy or an elliptical galaxy depends on its initial rate of star formation.