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AS1001:Extra-Galactic Astronomy

Explore the fundamental aspects of galaxies, including the number of stars, formation processes, the number and clustering of galaxies, and the mass density of the Universe.

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AS1001:Extra-Galactic Astronomy

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  1. AS1001:Extra-Galactic Astronomy Lecture 3: Galaxy Fundamentals Kenneth Wood 316 kw25@st-andrews.ac.uk http://www-star.st-and.ac.uk/~kw25

  2. Galaxy Fundamentals • How many stars are in a galaxy ? • How did galaxies form ? • How many galaxies are there ? • How far apart are they ? • How are they clustered ? • What is the mass of a typical galaxy ? • What is the mass density of the Universe ?

  3. Extra-galactic Distances • In Extragalactic astronomy we work in • 6 • 1 Mpc = 10 pcs • NB: M still defined to be at 10pc d in pc d in Mpc

  4. How many stars in a Galaxy ? • e.g., Andromeda which is at 0.9Mpc and has an apparent magnitude mv=3.5 • 1) • 2) Assume M*=5.48 (i.e., Solar) • 3) • 4) n*=50 billion stars

  5. Hierarchical Merging Initial Collapse How did Galaxies Form ? TWO COMPETING SCENARIOES

  6. Hierarchical merging For Mergers seen Ellipticals in high density environments Irrs isolated Against Ellipticals seen at early epochs Irregulars forming today Initial Collapse For Ellipticals are old Ellipticals seen at high z Spirals/Irrs rotating Irregulars forming today Against Mergers seen How did Galaxies Form? PROBABLY SOME OF EACH OCCURRING

  7. The Antennae Galaxy: mid-merger

  8. Cluster Formation Simulation John Dubinski: www.cita.utoronto.ca/~dubinski

  9. Cluster Formation Simulation John Dubinski: www.cita.utoronto.ca/~dubinski

  10. How many galaxies are there ? • STEP1: Take all sky photos • STEP2: Count galaxies brighter than some magnitude • STEP3: Assume most galaxies are like the MW* • STEP4: Calculate depth and volume of sky • STEP5: Calculate the SPACE-DENSITY of galaxies • [* This is a bit of a fudge but works because the most easily detected galaxies are like the Milky Way, i.e., big bright spirals. This does not mean they’re the most numerous just the most visible !]

  11. The Space Density of Galaxies • For example the MW has MB = -20 and there are ~10,000 MW-like galaxies known, brighter than 14th mag over the whole sky. How many galaxies are there per Mpc cubed ? • i.e., There is ~1 MW-like galaxy every 100Mpc cubed Use magnitude equation to get the distance => Use geometry to get the volume => n = number density

  12. 3 100Mpc 4.6 Mpc How far apart are they ? 1/3 • The mean separation of galaxies is therefore ~(100) = 4.6 Mpc • In reality though we know that galaxies are strongly clustered

  13. Large Scale Structure in the Universe The Stickman: 1980 150 Mpc Geller & Huchra

  14. Large Scale Structure in the Universe The Great Wall: 1988

  15. Large Scale Structure in the Universe

  16. Mass-to-Light Ratios • Let us assume that the amount of light a galaxy emits relates to its mass • i.e., there exists a mass-to-light ratio [we will explore the validity of this later] • Typically this is expressed in solar units: • X=1 For our sun • X~10 For a galaxy Solar Mass Mass-to-light ratio Solar Luminosity

  17. The Mass of M31 • Given that the average mass-to-light ratio is about 10 what mass does this imply for M31 which has MV = -20.5 mags ?

  18. The Density of the Universe • By multiplying together the space density of galaxies and the mass of the typical galaxy we can get an approximate value for the density of the visible Universe: • This is the matter that we can see. However…

  19. More accurate observations incorporating dynamics of galaxies and clusters, yields the total density (includes luminous and dark matter): • Most of matter in Universe we cannot see! • Mass of hydrogen atom: mH=1.7 10-27 kg • If the Universe was smoothly spread out there would be a couple of hydrogen atoms per cubic metre. But the air we breathe contains about 1025 atoms per cubic metre. • The Universe is a very empty place!

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