Transcript Document
Modern Astronomy Stars & Galaxies Lecture 8 Galaxies: From normal Galaxies to quasars Geraint F. Lewis University of Sydney 2007 Outline The Milky Way: A reminder The family of galaxies Colliding galaxies AGN: The eccentric cousins Cosmological Spaghetti The Milky Way: A reminder Scientific notation & units Scientific numbers; 103=1000 and 4.1£ 102 = 410 Solar mass: M¯ = 1.99£1030 kg Solar luminosity: L¯ = 3.90£1026 W Parsec: pc = 3.09£1016 m The family of galaxies Spiral Elliptical Irregular Spiral Galaxies Mass: 109!4£1011 M¯ 8 10 L¯ Luminosity:10 !2£10 Diameter: 5!50 kpc Stars: All ages Gas & dust: Some Rotation: Yes Classifying spirals: Sa Large bulge-to-disk ratio! Classifying spirals: Sb Lower bulge-todisk ratio ESO Classifying spirals: Sc Small bulge-to-disk ratio HST Classifying spirals: bars Sa Sb Sc Lenticular (S0) Elliptical Galaxies Mass: 105!1013 M¯ 5 10 L¯ Luminosity:3£10 !10 Diameter: 1!200 kpc Stars: Intermediate + Old Gas & dust: Very little Rotation: Very little Classifying Elliptical Ellipticals are classified on the basis of their shape and are assigned a number where a is the longest length and b is the shortest length (i.e. circular is equal to zero) E1 E5 Hubble tuning fork http://www.uni-sw.gwdg.de/~bziegler/images/galaxies/tuningfork_Frei.gif Classification…. … is art!! Irregular Galaxies: The dustbin Mass: 108!3£1010 M¯ 7 9 Luminosity:10 !10 L¯ Diameter: 1!10 kpc Stars: Young & Intermediate Gas & dust: Lots Rotation: Yes & No Irregular galaxies Large Magellanic Cloud Irregular galaxies Image credit: Westmoquette (UCL), WIYN/NASA/HST The big and the small Big galaxies are relatively easy to see Small galaxies are hard to see Whenever we look hard, we see many small galaxies for every large galaxy! This is true in our very own backyard The Local Group Where do galaxies live? Galaxies rarely live alone The Milky Way is part of the Local Group with Andromeda and many smaller galaxies Most galaxies in the Universe are seen to live in groups similar to our own! Galaxy groups Stephan’s Quintet Hickson Group Gemini images Galaxy clusters While rarer, galaxy clusters represent the largest bound objects in the Universe The can contain thousands of galaxies Galaxy clusters can be grouped together to make superclusters of galaxies! The Coma Cluster The not-so-local Universe The little black dot is out Local Group of galaxies. The side length of the box is ~200Mpc (more than 200x the distance between us and Andromeda). The Local Group is pulled by the gravitational attraction of the clusters and we are falling into the Virgo Cluster!!! Mike Hudson (U Waterloo) Abell 1689 (HST) Galaxy clusters Usually at the centre of galaxy clusters we mind cD galaxies, the most massive galaxies we know (100x Milky Way) Galaxies whip around clusters at thousands of km/s (evidence for dark matter) The immense gravitational field squeezes gas in the cluster, making it hot and glow in X-rays! X-ray clusters The Centaurus Cluster Large scale structure www.sdss.org Numbers The observable Universe contains Around 100 billion galaxies Containing ~1022 stars Galaxies sit on a cosmological foam Mainly ellipticals in clusters Mainly spirals “in the field” Galaxy collisions With so many galaxies in a small volume, collisions occur. What happens? Material is thrown over a large region. Stars: rush passed one another and do not collide Gas: clouds collide and collapse, resulting in a burst of star formation Galaxy collisions John Dubinski http://www.cita.utoronto.ca/~dubinski/nbody/ Galaxy collisions Painting by Adolf Schaller Sometimes gas pools into the centre of the colliding system, resulting in a massive burst of star formation (more than 1000 new stars per year!) This burst produces masses of dust in supernovae, making the galaxy glow brightly in the infrared. Feeding the monster! Remember that the heart of the Milky Way houses a supermassive black hole Detailed observations of stars in the centres of other nearby galaxies reveal that they too have black holes What happens when gas sinks into the centre of a galaxy during a collision? AGN: Eccentric cousins The light we receive from a galaxy is simply the sum of the light of each star Sometimes galaxies have a bright core, but the radiation is not starlight Often this bright core outshines the entire starlight of a galaxy Such bright cores are the signatures of Active Galactic Nuclei (AGN) Active galaxies Active galaxy Starlight • Non-thermal emission: High speed electrons, strong magnetic fields, extreme environments •Broad emission line: High speed gas •~10% have strong radio emission Radio jets Jets can cover several hundred kiloparsecs to a couple of megaparsecs (remember the Milky Way has a diameter of several 10s of kiloparsecs). Cygnus A (6cm Carilli NRAO/AUI) Active galaxies There are many different kinds of AGN Quasars, Seyferts, Blazars, Liners, BL Lac, FR I, FR II etc. Classification depends upon energy output, how they were discovered… The idea is, however, that all AGN are variants of the same theme, a power source which consists of a supermassive black hole. The unified model Supermassive black hole Accretion disk of hot gas Jets High velocity clouds Thick torus of gas, dust & stars Low velocity clouds All of this is packed in a volume not much larger than the solar system, but can output as much energy as a 100,000 billion Suns. The unified model What you see depends upon which angle you are looking at the central black hole! Cosmological spaghetti Where do galaxies come from? • Were they born fully formed? • Did they grow over time? Such questions are not easy to answer, and require building a universe inside a computer. We will look at this in more detail next week, but for now.. A numerical universe Little things form first Little things merger to become bigger Ultimately a few large things dominate and continually feed on the smaller objects This suggests that an object like the Milky Way has not finished feeding. As we saw last week, there are several dwarf galaxies, including the Sagittarius dwarf and Canis Major dwarf which are being consumed at the moment! Elliptical vs spiral Ellipticals: Violent formation, all gas used up quickly, rotation destroyed Spirals: Formed more sedately, slow recycling of gas, rotation maintained What does this mean for Andromeda and the Milky Way who meet in 3 billion years? See you next week! http://www.cita.utoronto.ca/~dubinski/tflops/