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Galaxies Galaxy Classification Distances to Galaxies Galaxy Mass Galaxy Clusters Interacting Galaxies Large Scale Structure of the Universe Galaxies Star systems, some like our Milky Way They contain a few thousand to tens of billions of stars They contain varying amounts of gas and dust They come in a large variety of shapes and sizes The Family of Galaxies Even seemingly empty regions of the sky contain thousands of very faint, very distant galaxies Galaxy morphologies: Spirals Ellipticals Irregular (some interacting) Galaxy Classification Elliptical Galaxies E0, …, E7 Spiral Galaxies Sa E0 = Spherical E1 Large nucleus; tightly wound arms Sb Sc E7 = Highly elliptical E6 Small nucleus; loosely wound arms Gas and Dust in Galaxies Spirals are rich in gas and dust Ellipticals are almost devoid of gas and dust Galaxies with disk and bulge, but no dust are termed S0 Grand-Design Spiral Galaxies Grand-design spirals have two dominant spiral arms. M 100 Flocculent (woolly) galaxies also have spiral patterns, but no dominant pair of spiral arms. NGC 300 The Whirlpool Galaxy Grand-design galaxy M 51 (Whirlpool Galaxy): Self-sustaining star forming regions along spiral arm patterns are clearly visible. Barred Spirals Some spirals show a pronounced bar structure in the center. They are termed barred spirals: Sequence: SBa, …, SBc, analogous to regular spirals. Is the Milky Way a Barred Spiral? Distribution of stars and neutral hydrogen Distribution of dust Sun Bar Ring Irregular Galaxies The Cocoon Galaxy NGC 4038/4039 Often: result of galaxy collisions or mergers Often: Very active star formation (―Starburst galaxies‖) Some: Small (―Dwarf galaxies‖) satellites of larger galaxies (e.g., Magellanic Clouds) Large Magellanic Cloud A summary of galaxy properties by type Distance Measurements to Other Galaxies a) Cepheid method: Using period – luminosity relation for classical Cepheids: 1. Measure Cepheid’s period 2. Find its luminosity from graph 3. Measure apparent magnitude 4. Calculate its distance Cepheid variables allow measurement of galaxies to about 25 Mpc away. However, some galaxies have no Cepheids and most galaxies are farther away than 25 Mpc. Distance Measurements to Other Galaxies b) Type Ia supernovae (collapse of an accreting white dwarf in a binary system): 1. Type 1a supernovae have well known standard luminosities 2. Measure apparent magnitude 3. Calculate its distance c) Tully–Fisher relation: correlates a galaxy’s rotation speed (which can be measured using the Doppler effect) to its luminosity. All are ―Standard-candle‖ methods: 1. Determine the absolute magnitude (luminosity) 2. Measure apparent magnitude 3. Calculate its distance Measuring Distances in Space With these additions, the cosmic distance ladder has been extended to about 1 Gpc. Distance Measurements to Other Galaxies: Hubble’s Law Edwin Hubble (1913) found that distant galaxies are moving away from our Milky Way, with a recession velocity, vr, proportional to their distance d: vr = H0 d H0 ≈ 70 km/s/Mpc is the Hubble constant. 1. Measure vr using the Doppler effect 2. Calculate the distance. Measuring Distances in Space This puts the final step on our cosmic distance ladder The last step cannot be used to justify Hubble’s Law and its cosmological implications. We will look at that later. Type Ia Supernovae Tully-Fisher Method Cepheid Variables Spectroscopic Parallax Trigonometric Parallax Radar The Extragalactic Distance Scale Many galaxies are typically millions or billions of parsecs from our galaxy. Typical distance units: Mpc = megaparsec = 1 million parsecs Gpc = gigaparsec = 1 billion parsecs Distances (d) of Mpc or even Gpc The light we see left the galaxy millions or billions of years ago!! ―Look-back times‖ (t = d/c)of millions or billions of years Galaxy Sizes and Luminosities Vastly different sizes and luminosities: From small, lowluminosity irregular galaxies (much smaller and less luminous than the Milky Way) to giant ellipticals and large spirals, a few times the Milky Way’s size and luminosity Rotation Curves of Galaxies From blue/red shift of spectral lines across the galaxy infer rotational velocity Observe frequency of spectral lines across a galaxy. Plot of rotational velocity vs. distance from the center of the galaxy: rotation curve Determining the Masses of Galaxies Based on rotation curves, use Kepler’s 3rd law to infer masses of galaxies Adding ―visible‖ mass in stars, interstellar gas, dust, etc., we find that most of the mass is ―invisible,‖ just as we did for the Milky Way. Dark Matter Determining the Masses of Galaxies Another way to measure the average mass of galaxies in a cluster is to calculate how much mass is required to keep the cluster gravitationally bound. Dark Matter in the Universe Galaxy mass measurements show that galaxies need between 3 and 10 times more mass than can be observed to explain their rotation curves. The discrepancy is even larger in galaxy clusters, which need 10 to 100 times more mass. The total needed is more than the sum of the dark matter associated with each galaxy. Dark Matter in the Universe There is evidence for intracluster superhot gas (about 106 K) throughout clusters, densest in the center This head–tail radio galaxy’s lobes are being swept back, probably because of collisions with intracluster gas It is believed this gas is primordial—dating from the very early days of the Universe. There is not nearly enough of it to account for most of the matter in galaxy clusters. Dark Matter in the Universe This map of dark matter in and near a small galaxy cluster was created by measuring distortions in the images of background objects Supermassive Black Holes From the measurement of stellar velocities near the center of a galaxy: Infer mass in the very center central black holes! Several million, up to more than a billion solar masses! Super massive black holes like we found for the Milky Way The Origin of Supermassive Black Holes Most galaxies seem to harbor supermassive black holes in their centers. They are fed and fueled by stars and gas from the nearcentral environment Galaxy interactions may enhance the flow of matter onto central black holes Clusters of Galaxies Galaxies do not generally exist in isolation, but form larger clusters of galaxies. Rich clusters: Poor clusters: 1,000 or more galaxies, diameter of ~3 Mpc, condensed around a large, central galaxy Less than 1,000 galaxies (often just a few), diameter of a few Mpc, generally not condensed towards the center Gravitational Lensing The huge mass of gas in a cluster of galaxies can bend the light from a more distant galaxy. This is an effect of the General Theory of Relativity. Image of the galaxy is strongly distorted into arcs. Gravitational Lensing This is what appeared at first to be a double quasar, but on closer inspection the two quasars turned out to be not just similar, but identical—down to their luminosity variations. This is not two quasars at all—it is two images of the same quasar. Gravitational Lensing This could happen by gravitational lensing. From this we can learn about the quasar itself, as there is usually a time difference between the two paths. We can also learn about the lensing galaxy by analyzing the bending of the light. Gravitational Lensing Here, an intervening galaxy has made four images of a distant quasar. Gravitational Lensing Here are two spectacular images of gravitational lensing: Distant galaxies being imaged by a whole cluster A cluster with images of what is probably a single galaxy. Our Galaxy Cluster: The Local Group The Local Group Some galaxies of our local group are difficult to observe because they are located behind the center of the Milky Way, which obscures our view. Dwingaloo 1 Interacting Galaxies Cartwheel Galaxy Particularly in rich clusters, galaxies can collide and interact. Galaxy collisions can produce ring galaxies and NGC 4038/4039 tidal tails. Often triggering active star formation: Starburst galaxies Starburst Galaxies Starburst galaxies are often very rich in gas and dust; bright in infrared Ultraluminous infrared galaxies Simulations of Galaxy Interactions Numerical simulations of galaxy interactions have been very successful in reproducing tidal interactions like bridges, tidal tails, and rings. Tidal Tails Example for galaxy interaction with tidal tails: The Mice Computer simulations produce similar structures. Mergers of Galaxies NGC 7252 is probably the result of the merger of two galaxies, ~109 years ago Small galaxy remnant in the center is rotating backwards! Radio image of M64: Central regions rotating backwards! Multiple nuclei in giant elliptical galaxies Interactions of Galaxies with Intergalactic Matter Galaxies may not only interact with each other directly, but also with the gas between them. Gas within a galaxy is stripped off the galaxy by such an interaction. The Furthest Galaxies The most distant galaxies visible by HST are seen at a time when the universe was only ~109 years old. The Universe on Large Scales Galaxy clusters join in larger groupings, called superclusters. This is a 3-D map of the Local Supercluster, of which our Local Group is a part. It contains tens of thousands of galaxies. The Universe on Large Scales This slice of a larger galactic survey shows that, on the scale of 100–200 Mpc, there is structure in the universe – walls and voids. The Universe on Large Scales This survey, extending out even farther, shows structure on the scale of 100–200 Mpc, but no sign of structure on a larger scale than that. The decreasing density of galaxies at the farthest distance comes from the difficulty of observing them. Large-Scale Structure A large survey of distant galaxies shows the largest structures in the universe Filaments and walls of galaxy superclusters and voids (basically empty space).