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. 1781 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 %). 3 4 1
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
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 < -17 9 10 Fig. 23.9a 11 3
Irregular Galaxies LMC Blue Strong emission lines Very dusty Highly asymmetrical Few globular clusters Low surface brightness Rotating Rarely found in clusters -18 < M < -10 13 14 1920 s Classification based on appearance 15 16 4
Fig. 23.14 The Hubble Ultra Deep Field The edge of the universe - Hubble's Deepest View Ever Unveils Earliest Galaxies 18 The Earliest Galaxies 19 5
The Very Large Telescope, Mt Fig.06.26 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
The revolution of the Sun around the Galactic center P 2 3 π d = Gm ( + M) 2 4 2 4π 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. 23.21 Rotation curves for spiral galaxies Black holes in the centers of galaxies M = 2 vd G 7
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 = 10 0.2(m-M+5) M, m, distance 8
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 74.05 1912: 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
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 ~ 14-15 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
Pre-galactic Blobs Our picture of galaxy evolution First galaxies - 13.6 billion years ago 0.5-1 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
Evolution of Galaxies Different for different types 45 12