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The Cosmological Principle

Explore the concept of isotropy and homogeneity in the universe, illustrated through analogies, 2-dimensional examples, and galactic classifications. Discover how the Cosmological Principle plays a role in shaping the cosmos.

christopherrobinson
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The Cosmological Principle

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  1. The Cosmological Principle On a large enough scale, the universe is both isotropic and homogeneous ISOTROPY: There is no preferred direction in space. (All directions are alike.) HOMOGENEITY: One randomly-chosen large volume of the universe will have the same physical properties (and identical physical laws) as another randomly-chosen large volume of the universe. (All places are alike.)

  2. 2-Dimensional Examples of Isotropy and Homogeneity • Surface of plain white “cue” ball used for playing pool (billiards) • Infinite forest of identical trees

  3. Isotropic Forest

  4. Anisotropic Forest (trails = “preferred direction”)

  5. Four 2-Dimensional Universes • Which are isotropic on a large scale (same in all directions)? • Which are anisotropic on a large scale (have preferred directions)? • Which are homogeneous on a large scale (all places are alike)? • Which are inhomogeneous on a large scale? • Which satisfy the Cosmological Principle?

  6. Obviously isotropic and homogeneous Satisfies the Cosmological Principle

  7. Homogeneous on a large scale (the pattern repeats over and over), but is obviously anisotropic Does not satisfy Cosmological Principle

  8. Homogeneous on a large scale, but is actually anisotropic (look at the diagonals!) Does not satisfy the Cosmological Principle

  9. On a large scale, this is both homogeneous and isotropic Satisfies the Cosmological Principle This is actually a computer simulation of how galaxies may have formed in the universe after the Big Bang. It matches the real, observed universe remarkably well. Yellow = many galaxies present, black = very few galaxies present.

  10. Morphological Classification of Normal Galaxies • Based on OPTICAL morphology (V-band; around 550 nm); this is the light EMITTED in the optical part of the spectrum • Morphology correlates strongly with physical properties (stellar content, gas content, dynamics) • Morphology correlateswith local number density of galaxies to a large extent • Primary system due to Edwin Hubble (ellipticals, spirals, irregulars, S0/lenticular) – how do you “label” the galaxies? • Reference: “Hubble Atlas of Galaxies” by Alan Sandage • Note: here we will look only at “modern” galaxies (z ~ 0)

  11. Transmission Curves for “Johnson-Cousins” Filters

  12. No striking features (“ball of light”, no spiral arms, no dark dust lanes) Light is almost exclusively from stars Only “old” stars, almost no gas or dust Optical color is yellowish-red “Red and dead”(TODAY - but not in the past) Widest range of mass of all types of galaxies (105 Msun to 1013 Msun) Widest range of luminosities of all types of galaxies (3x105 Lsun to 1011 Lsun) Shape on the sky is a 2-d projection; ellipticals that look round are probably not spherical (except maybe the giant ellipticals) Elliptical Galaxies

  13. Elliptical Galaxies - Classification • Smooth distribution of light (no bright clumps) • Smooth gradient in surface brightness (no sharp discontinuities) • Absence of dust (no “dust lanes”) • Not resolved into bright blue stars or knots • Images have complete rotational symmetry • Denoted by En where n = 10(1-b/a) • Range from E0 to E7

  14. NGC 4278 (E1)

  15. NGC 4406/M86 (E3)

  16. NGC 3377 (E6)

  17. Spiral Galaxies • Like MW, all spirals have bulge, disk, and a faint, luminous “halo” of stars • Masses range from 109 Msun to 4x1011 Msun • Luminosities range from 108 Lsun to 2x1010 Lsun • Old stars in the halo, mix of old and young stars in the disk and bulge • Gas and dust in the disk and bulge = active star formation in most spiral galaxies • The bluer are the arms, the more star formation (and the more gas the galaxy has)

  18. Normal (unbarred) Spirals (S) - Classification • Denoted by “S” and subcategory “a”, “b”, or “c” based on: (1) openness of spiral arms, (2) degree of “resolution” of arms into stars and knots, (3) bulge-to-disk ratio • Sa have tightly wound spiral arms, bulge is large compared to disk, and arm are poorly-resolved (continuous lines) • Sc have loosely wound arms, bulge is small compared to disk, well-resolved (“ratty”) arms that sometimes branch off new arms • Sb are the middle ground between Sa and Sc • Sd galaxies have tiny bulges and weak but detectable spiral structure (look like Sc’s having a bad hair day)

  19. NGC 4274 (Sa)

  20. NGC 3623/M65 (Sa)

  21. NGC 5194/M51/Whirlpool (Sc) Note how the arms dominate the image!

  22. NGC 5457/M101 (Sc) Note the branching into smaller arms!

  23. NGC 3031/M81 (Sb)

  24. NGC 4565 (Sb) Note the strong dust lane!

  25. Spiral Galaxies with Bars About 43% of spirals (including the MW!) have bars

  26. Barred Spirals (SB) - Classification • Subcategories “SBa”, “SBb”, “SBc” as in normal spirals • Often specified as having arms that emerge tangent to a ring ( r ), arms that “spring from the ends of a bar” ( s ), or both ( sr ) • Roughly 43% of all spirals are barred (including the Milky Way!) • Bar fraction increases for bluer galaxies (bars and star formation are linked)

  27. NGC 1300; SBb(s)

  28. NGC 1398; SBb( r )

  29. Lenticular/S0 Galaxies(“failed” spirals?) Bulges and disks are present (and even bars), but no spiral arms. These galaxies have little gas and no active star formation.

  30. S0/Lenticular Galaxies - Classification • Unknown to Hubble • Intermediate class between E & S • Bulge and disk but NO spiral arms (often VERY hard to distinguish from E) • Symmetrical, but can be flatter than E7 • Both barred (SB0) and unbarred (S0) • S0 subclasses based on amount of dust in the disk: S01 (no absorption), S02 (some absorption), S03 (prominent, complete band of absorption) • SB0 subclasses based on prominence of bar: SB01 (bar not very prominent), SB02 (bar has “intermediate” prominence), SB03 (bar is narrow and well-defined)

  31. NGC 3115 (S01)

  32. NGC 4710 (S03)

  33. NGC 4262 (SB02,3)

  34. NGC 5101 (SB03)

  35. Irregular Galaxies Everything else gets lumped into this category Many have bright blue knots, lots of star formation, H-II regions, and irregular patterns of dust Masses between 108 Msun and 3x1010 Msun Luminosities between 107 Lsun and 109 Lsun

  36. Irregular Galaxies (Irr) - Classification • Irr I: highly-resolved into 0 and B stars and HII regions, PROMINENT spiral structure is missing (but can look like a badly disturbed Sc) • Irr II: smooth light distribution without rotational symmetry, not resolved into bright stars, often have irregular patterns of dust lanes Large Magellanic Cloud M82

  37. Locally, How Many of Each Type?(depends on how you count!) Count up the observed bright galaxies in the local universe: ~77% spirals ~20% ellipticals ~3% irregulars Use a “volume limited” sample: ~33% spirals ~13% ellipticals ~54% irregulars due to “Malmquist bias”

  38. Tuning Fork Diagram (modified from Hubble to include S0s) Irr I “Early” “Late” Irr II

  39. Fundamental Characteristics of Galaxies: Ellipticals • Primarily Pop II stars (old, metal-poor) and old Pop I stars • Virtually no cold gas or dust, no active star formation • Some large ellipticals have significant HOT (X-ray) gas component; frequently found in cluster environments • “red and dead”

  40. Fundamental Characteristics of Galaxies: Spirals • 3 principle components: bulge, disk, luminous halo • Halo: Pop II stars, little gas or dust, no star formation • Bulge: Pop I and Pop II stars, cold gas, dust, active star formation • Disk: Pop I in arms, cold gas, dust, active star formation, hot gas in the form of HII regions, planetary nebulae and some X-ray sources • “Lateness” correlates with cold gas content and star formation rate

  41. Fundamental Characteristics of Galaxies: Irregulars • Pop I and Pop II • Active star formation (abundant cold gas and dust)

  42. Fundamental Characteristics of Galaxies Note: masses and diameters below refer to visible regions only Ellipticals Spirals Irregulars M (Msun) 105 to 1013 109 to 4x1011 108 to 3x1010 L (Lsun) 3x105 to 1011 108 to 2x1010 107 to 109 Diam. (kpc) 1 to 200 5 to 50 1 to 10 Dust? ~none yes (disk & bulge) yes Spectral Type K K ; F-K; A-F A-F MHI/Mtot ~zero ~2%; ~5%; ~10% ~22% Sa; Sb; Sc Spectra of normal ellipticals do not contain emission lines; Spectra of late-type spirals and irregulars do contain emission lines

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