GalaxiesTypes of GalaxiesSlide 3Galaxy CollisionsSlide 5Active Galactic Nuclei (AGN)QuasarsMeasuring the Diameter of Astronomical Objects by Using Their Light VariabilitySlide 9Active Galactic Nuclei (AGN)Slide 11Distances to the GalaxiesRecessional Velocity of GalaxiesThe Expansion of the UniverseGalaxy Mass and Dark MatterGalaxy ClustersGalaxy Clusters and Dark Matter: Galaxy VelocityGalaxy Clusters and Dark Matter: Hot Gas in ClustersGravitational LensesGalaxy Clusters and Gravitational LensesGalaxy Clusters, Gravitational Lenses and Dark MatterDark Matter in ClustersDark Matter CandidatesLarge Scale Structure of the UniverseLarge Scale Structure and Dark Matter1GalaxiesThe Andromeda Galaxy - nearest galaxy similar to our own.Only 2 million light years away!•Galaxies are clouds of millions to hundreds of billions of stars held together by their mutual gravity. •Often galaxies also contain enormous clouds of gas and dust from which new stars can form.•Galaxies can have many different shapes and sizes.•The distribution of galaxies across the Universe indicates that they generally appear in clusters with very large voids separating clusters from one another.2Types of Galaxies•Galaxies are classified based on general characteristics and then further subdivided based on more specific characteristics. •Spiral galaxies–disk-shaped with spiral arms winding out from the center.–usually have clouds of gas and dust–usually have both young (Pop. I) and old (Pop. II) stars–some spirals have a rectangular-shaped bar through the central bulge and are referred to as Barred SpiralsA Hubble Space Telescope view of the Whirlpool Galaxy M513Types of Galaxies•Elliptical galaxies are always smooth in appearance.–can be spherical, egg-shaped or flattened in shape–usually have little or no gas and dust–usually only contain old stars (Pop. II)•Irregular galaxies have a random distribution of stars.–often have large clouds of gas and dust–often contain young stars (Pop. I)4Galaxy CollisionsA Hubble Space Telescope imageof the Antennae Galaxies. Large streamsof stars and gas are trailing off the galaxieswhile new stars are being formed near the center.•galaxies were smaller in the past•collisions between galaxies appear to have been common in the early Universe•collisions can cause bursts of star formation as clouds of gas and dust collapse•galaxies may eventually merge together forming large elliptical galaxies56•Spiral (or disk) galaxies with nuclei that are more luminous than the rest of the stars in galaxy.–Spectrum of the nucleus is non-stellar–Luminosity of the nucleus may change over short (hours-months)•Some elliptical galaxies show radio-wave emitting jets on scales much larger than the visible light size of the galaxyActive Galactic Nuclei (AGN)7Quasars•Originally detected in images as point-like (quasi-stellar) objects, although spectra are non-stellar •Underlying faint host galaxies recently detected in some quasars by the Hubble Space Telescope•Largest redshifts of any astronomical object–Hubble law implies they are at great distances (as much as 10 billion light-years away)–To be visible at those distances, they must be about 1000× more luminous than the Milky Way•Based on output fluctuations, quasars resemble the AGNs of radio galaxies and Seyfert galaxies in that they are small (fractions of a light-year in some cases)8Measuring the Diameter of Astronomical Objects by Using Their Light Variability•Technique makes three assumptions–The rate at which light is emitted from an active region is the same everywhere in that region–The emitting region completely defines the object of interest (there are no “dead” areas of significance)–The speed of light is finite (a safe bet)•The light variation then is just a measure of the time it takes light to travel across the active surface•Multiplying this time by the speed of light gives the size of the emitting object9Measuring the Diameter of Astronomical Objects by Using Their Light Variability10•Large luminosity •Variable on short timescales–Implies small physical size•Can not fit enough luminous stars (or supernovae) into such a small region•A supermassive black hole is thought to be the source of energy for these AGNs.•The black hole likely formed initially as the remnant of a massive star supernova at the center of a galaxy•The black hole fed on the vast reservoir of gas in the galaxy nucleus•Eventually grows large enough to capture stars•Galaxy-galaxy interactions may cause gas infall within galaxy, supplying more fuel•The variable, luminous source is the accretion disk and associated gas clouds surrounding the Black HoleActive Galactic Nuclei (AGN)11106 < M● < 109 solar-masses “Super-Massive!”Black Holes and Galaxy Formation•Black Holes may play a role in galaxy formation•Nearly all galaxy nuclei have a Black Hole (active, or inactive)•Black Hole mass is correlated with galaxy bulge mass12Distances to the Galaxies•Determining accurate distances to galaxies requires knowledge of the properties of stars.•From the luminosity of a star and its apparent brightness the star’s distance can be found.•Certain stars (called Cepheid variables) show regular patterns of variation in brightness. •The period of these variations are directly related to the stars luminosity.•So by measuring the time it takes these stars to vary in brightness and their apparent brightness their distance can be found.Cepheid stars are very brightand can be observed in nearbygalaxies. Other methods must be used for more distant galaxies.13Recessional Velocity of Galaxies•In 1911, it was discovered that all galaxies (with but a few exceptions) were moving away from the Milky Way•Edwin Hubble found that these radial speeds, calculated by a Doppler shift analysis and called a recessional velocity, increased with a galaxy’s distanceRPUInsert Figure 17.18a here14The Expansion of the Universe•Astronomers in the early 20th century found that Doppler shifts seen in the spectra of galaxies indicated that almost all galaxies are moving rapidly away from us. •Edwin Hubble, using careful determinations of galactic distances, showed that the farther a galaxy is from us the faster it appears to be moving away. •He showed that the velocity of the galaxy was simply equal to its distance times a constant.•This is known as Hubble’s Law and the constant