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  1. The History of Light: How Stars Formed in Galaxies Kai Noeske European Space Agency/ Space Telescope Science Institute Hubble Science Briefing, 1 Mar 2012

  2. What is a Galaxy? 2

  3. The Milky Way 100 Billion Stars like our sun 3

  4. The Milky Way 4 www.atlasoftheuniverse.com

  5. Meet the Neighbors. M31 (“Andromeda Galaxy”), our close neighbor and similar to the Milky Way M51 (“Whirlpool Galaxy”) M104 (“Sombrero Galaxy”) 5

  6. 6

  7. Stars are not evenly distributed in the universe. Stars are born and live in galaxies. Most galaxies have billions of stars. There are billions of galaxies in the known universe. Did they always look the same? 7

  8. A long time ago in galaxies far, far away: The HST Ultra Deep Field 8

  9. A long time ago in galaxies far, far away: The HST Ultra Deep Field 9

  10. A long time ago in galaxies far, far away: The HST Ultra Deep Field 10

  11. Two immediate results: I. Galaxies formed at some point in the distant past II. Galaxies evolved with time Where do the Stars and Galaxies come from? 11

  12. Timeline • (very rough) • Most galaxies have • very old stars • Most galaxies • started forming their • stars some 10-13 • Billion years ago, • shortly after the • beginning of the • Universe 12

  13. 22% 74% 3.2% 13

  14. The Cosmic Microwave Background: a baby photo of the Universe when it was just 300,000 years old It reveals tiny irregularities; the density of matter varied by parts in a million 14

  15. Dark Matter is more abundant, and dominates gravity. To understand how gravity created structure (galaxies) from the early homogeneous Universe, we need to simulate Dark Matter. Outcome depends strongly on the structure/geometry of the Universe and the content of Dark Matter 15

  16. Supercomputer simulations of Dark Matter: gravity grows the initial density perturbations, structure forms From http://cosmicweb.uchicago.edu/filaments.html choose a “rotating box” version such as http://cosmicweb.uchicago.edu/images/mov/s02_0.gif 16

  17. Gravity grows a “Cosmic Web” of Dark Matter - voids, filaments, clusters of clumps that host galaxies 17 Simulation: A.Kravtsov

  18. Galaxies form from overdense regions Gravity grows a “Cosmic Web” of Dark Matter - voids, filaments, clusters of clumps that host galaxies 18 Simulation: A.Kravtsov

  19. Luminous matter, formation of gas disk and stars: Luminous matter (gas!) is viscous, and heated as it falls into dark matter halos; then heat is radiated away -gas cools - contracts angular momentum is conserved ->spin-up of rotation (“figure skater”) - fast rotating disk energy in turbulent/random motions (perpendicular to disk) is dissipated (viscosity->friction->heating ->heat is radiated away) -> motions perpendicular to ordered rotation disappear ->cold, dense gas disk -> STARS 19

  20. Recap: From Dark Matter to Stars • The Universe contains mostly • Dark Matter • 2) Tiny irregularities in the Dark • Matter density in the early • Universe grew rapidly through • gravity • 3) Gas fell into the resulting Dark • Matter clumps/”halos” (galaxies) • and formed cold, dense gas disks • 4) Stars are born and live in galaxies • because they need cold, dense • gas to form 20

  21. Hierarchical galaxy formation; disks merge to disk bulges and Ellipticals In a “hierarchical” scenario, smaller structures form first, and later merge into bigger ones: -Galaxies merge to form larger ones -Mergers of roughly equal-sized galaxies often (not always) turn Spirals into Ellipticals Blue: Dark matter Halo;yellow: gas;red: stars Bertola et al. 21

  22. Galaxy interactions/mergers: Observations and Numerical simulations http://www.youtube.com/watch?v=agqLEbOFT2A&feature=youtu.be Credits: Patrik Jonsson, Greg Novak & Joel Primack, University of California, Santa Cruz 22

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  27. II. How did we learn about galaxy formation? 27

  28. New Sky Surveys at many Wavelengths 28

  29. Multiwavelength surveys: combined efforts to get the whole picture. A new era of astronomy: big collaborations, huge databases 29

  30. SPITZER (infrared) GALEX (UV) XMM (X-ray) Chandra (X-ray) HST (visual, near infrared) DEEP2 (KECK,DEIMOS) VLA (radio) (gas, mass, black holes, star formation) Redshift, dynamics, ... Dust, star form., black holes... star formation Multiwavelength surveys: combined efforts to get the whole picture. A new era of astronomy: big collaborations, huge databases 30

  31. Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back even more distant galaxy distant galaxy nearby galaxy Text time light travels to reach us Long (billions of years) Short (millions of years) 31

  32. Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back even more distant galaxy distant galaxy nearby galaxy Text time light travels to reach us Long (billions of years) Short (millions of years) 32

  33. Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back even more distant galaxy distant galaxy nearby galaxy Text time light travels to reach us Long (billions of years) Short (millions of years) 33

  34. Astronomers can look back in time: light from very distant galaxies took billions of years to reach us. Looking far is looking back even more distant galaxy distant galaxy nearby galaxy Text time light travels to reach us Long (billions of years) Short (millions of years) 34

  35. Large telescopes on the ground: Spectroscopy gives each galaxy a “time stamp” 35

  36. DEIMOS spectrograph on the Keck II telescope Built by Sandra Faber & team, UC Santa Cruz Can observe spectra of hundreds of distant galaxies at a time 36

  37. Overlapping slitmask layout 37

  38. 120 spectra of distant galaxies emission lines of ionized gas wavelength The emission lines are at longer wavelengths than measured in the lab: They are “redshifted”. This is because distant galaxies move away from us (“Doppler effect”, expansion of the Universe). The redshift (=velocity) measures the distance and how far we look back in time 38 wavelength

  39. For galaxies in the early universe, the infrared matters: 39

  40. older stars older stars young stars (starbirth) young stars (starbirth) Visible Light UV Infrared Nearby Galaxy (not redshifted) spectral flux Distant Galaxy (redshifted) wavelength For distant galaxies, light from young stars (UV) and older stars (visible) is redshifted to long wavelengths (Infrared) 40

  41. The Spitzer Space Telescope provided infrared data: pierce through the dust, measure star formation rates Spitzer Extended Deep Survey Reduction: M. Ashby 41

  42. Hubble & JWST Probe the Early Universe HST: currently the most sensitive telescope in the short-wavelength infrared (near-infrared): Can observe redshifted UV (star formation) from the most distant galaxies JWST (launch: 2018) will be more sensitive, and reach longer infrared wavelengths: will reach even further back in time, and observe redshifted visible & infrared light in earliest galaxies 42

  43. JWST will have much improved sensitivity to faint distant galaxies: First Stars & Galaxies Small galaxies across cosmic time ... JWST Ultra Deep Field Simulation HST Ultra Deep Field 43

  44. Star formation in galaxies over the last 10 billion years now 10 Billion yrs ago now 10 Billion yrs ago Big Galaxies Space Density of Star Formation Space Density of Star Formation Small Galaxies Heavens et al. 2004 Hopkins & Beacom 2006 44

  45. Star formation in galaxies over the last 10 billion years now 10 Billion yrs ago now 10 Billion yrs ago Big Galaxies Space Density of Star Formation Space Density of Star Formation Small Galaxies Heavens et al. 2004 Hopkins & Beacom 2006 Co-moving star formation rate (SFR) density declined by ~x10 Galaxy star formation histories are mass-dependent: massive galaxies formed bulk of stars quickly and early, less massive galaxies formed on longer timescales (“Downsizing”) 45

  46. Star formation in galaxies over the last 10 billion years now 10 Billion yrs ago now 10 Billion yrs ago Big Galaxies Space Density of Star Formation Space Density of Star Formation Small Galaxies Heavens et al. 2004 Hopkins & Beacom 2006 Co-moving star formation rate (SFR) density declined by ~x10 Galaxy star formation histories are mass-dependent: massive galaxies formed bulk of stars quickly and early, less massive galaxies formed on longer timescales (“Downsizing”) Reason for declining star formation: Galaxies run out of gas! 46

  47. today rapid star birth & gas consumption billions of years ago 47 big galaxies small galaxies (image: Driver 1998)

  48. today rapid star birth & gas consumption slow star birth & gas consumption billions of years ago 48 big galaxies small galaxies (image: Driver 1998)

  49. today rapid star birth & gas consumption slow star birth & gas consumption billions of years ago 49 big galaxies small galaxies (image: Driver 1998)

  50. today rapid star birth & gas consumption slow star birth & gas consumption billions of years ago 50 big galaxies small galaxies (image: Driver 1998)