Galaxies. The majority of known galaxies fall into one of three major classes: spirals (78 %), ellipticals (18 %) and irregulars (4 %).

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1 Galaxies Collection of stars, gas and dust bound together by their common gravitational pull. Galaxies range from 10,000 to 200,000 light-years in size Charles Messier 1923 Edwin Hubble The distribution of galaxies in the sky 1 The determination of an external galaxy s morphology is generally based on its appearance in optical images. M 31 The Andromeda galaxy The majority of known galaxies fall into one of three major classes: spirals (78 %), ellipticals (18 %) and irregulars (4 %)

2 The Milky Way MW Number of Stars Mass Shape Size Age Sun s location 1920 s Classification based on appearance 5 Red color Smooth Profile High Surface Brightness Egg shaped Many globular clusters Little or no dust line Absorption lines only No rotation Found in clusters -22 < M < -18 Elliptical Galaxies 7 8 2

3 Spiral Galaxies Red Bulge, Bluish Arms/Disk Moderate Surface Brightness Dusty Absorption + Emission lines Rotating Disk Numerous Globular Clusters Found in small or large clusters -21 < M < Fig. 23.9a 11 3

4 Irregular Galaxies LMC Blue Strong emission lines Very dusty Highly asymmetrical Few globular clusters Low surface brightness Rotating Rarely found in clusters -18 < M < s Classification based on appearance

5 Fig The Hubble Ultra Deep Field The edge of the universe - Hubble's Deepest View Ever Unveils Earliest Galaxies 18 The Earliest Galaxies 19 5

6 The Very Large Telescope, Mt Fig Paranal, Chile The most distant galaxy known 13,230 million lightyears away; seen when the Universe was 470 million years young 21 Galaxy Collisions 22 Galaxy Collisions 6

7 The revolution of the Sun around the Galactic center P 2 3 π d = Gm ( + M) π d P = GM 2π d P = v M = 2 vd G 2 3 Mass of material inside Sun s orbit: 100 billion Solar masses 25 The predicted and observed rotation curve of a typical spiral galaxy Structure of galactic disk and halo Density of visible matter Density of non-luminous matter Fig Rotation curves for spiral galaxies Black holes in the centers of galaxies M = 2 vd G 7

8 The cosmic distance scale: MW Stellar parallax The cosmic distance scale: MW Cluster HR diagram fitting ( pc) d = p 1 ( arcsec) Cepheid variable stars, RR Lyrae stars The cosmic distance scale: MW Individual stars photometry calibrations in terms of absolute magnitude d = (m-M+5) M, m, distance 8

9 The cosmic distance scale: nearby galaxies and very distant galaxies Cepheid variables (20 Mpc) Angular size of HII regions (25 Mpc) Average brightness of globular clusters (25 Mpc) Tully Fisher relation (100 Mpc) - tight correlation between the rotation rates (how wide is the 21 cm line) and absolute luminosities of galaxies Supernovae (3000 Mpc) Luminosity vs. Hubble type - distances to clusters of galaxies Hubble s Law Figure : Vesto Slipher begins taking spectra of galaxies Star-like (absorption lines) Very faint - 20/40 hour exposures! 20 years = 40 galaxies The first astronomer to show that most are redshifted (all but closest) 1920 s: Edwin Hubble begins measuring distances to galaxies (mostly using brightest star method) 9

10 Hubble s Law Universal recessional motion v = H x d d = v/h H = 70 km/s/mpc Redshift in almost all galactic spectra The Universe is expanding (the expansion does not have a center) Hubble flow Nearby galaxies Distant galaxies Hubble law Redshift as a distance indicator Hubble constant Current value, Meaning, Significance Evolution of Galaxies Current Theory 1. Groups, Clusters, Superclusters of galaxies 2. All galaxies have formed at the same time; they are equally old ~ x 10 9 yrs 3. Different galactic types are at different evolutionary stages Elliptical Galaxies all stars born long ago during giant single star-forming process, exhausting all interstellar star-forming material Spiral Galaxies ongoing star-forming processes 4. Reason for this difference different physical parameters of different galactic types Compactness Rotation Environment 39 Hidden matter 40 10

11 Pre-galactic Blobs Our picture of galaxy evolution First galaxies billion years ago billion years later larger clumps grow from merging of smaller once 42 Our picture of galaxy evolution Our picture of galaxy evolution 1 billion years later - after growing to a fraction of the size of our own galaxy, the clumps are large enough for the Hubble Space Telescope to see them 2-4 billion years later larger irregular looking objects form through collisions and mergers between these subgalactic sized 43 clumps 44 11

12 Evolution of Galaxies Different for different types 45 12

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