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A1 21 Galaxies


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Miller's Astronomy 1 lecture notes on Galaxies

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A1 21 Galaxies

  1. 1. Galaxies LACC: §25.2, 3, 5 • Types of Galaxies: Ellipticals, Spirals, Irregular; The Hubble Tuning Fork Diagram; what kinds of stars make up galaxies (mass to light ratio) • Galaxy Clusters; Galaxy Superclusters • Measuring the Distances to Galaxies An attempt to answer the “big questions”: where are we? what is the universe made of? Monday, November 30, 2009 1
  2. 2. Galaxies like colorful pieces of candy fill the Hubble Deep Field - humanity's most distant yet optical view of the Universe. The dimmest, some as faint as 30th magnitude (about four billion times fainter than stars visible to the unaided eye), are the most distant galaxies and represent what the Universe looked like in the extreme past, perhaps less than one billion years after the Big Bang. To make the Deep Field image, astronomers selected an uncluttered area of the sky [about 2.5 arcmin across] in the constellation Ursa Major (the Big Bear) and pointed the Hubble Space Telescope at a single spot for 10 days accumulating and combining many separate exposures. With each additional exposure, fainter objects were revealed. The final result can be used to explore the mysteries of galaxy evolution and the infant Universe. Monday, November 30, 2009 2
  3. 3. Hubble Tuning-Fork Diagram Monday, November 30, 2009 3
  4. 4. Types of Galaxies All bright galaxies fall into one of three broad classes according to their shape: • Spiral Galaxies (~75%) • Elliptical Galaxies (20%) • Irregular Galaxies (5%) Monday, November 30, 2009 4
  5. 5. Types of Galaxies Despite the fact that the Hubble Sequence is based only on the appearance of galaxies (morphology of galaxies), several physical properties of galaxies vary smoothly along the sequence. We have, little gas and dust <----------------------> lots of gas and dust mainly Pop II stars <----------------------> Pop I & II stars Reddish <----------------------------------> Bluish little ongoing star formation <------------> star formation large bulge <------------------------------> small bulge tight,smooth arms <---------> open, loose arms Mass: 108-1013 MSun (Ellipticals) <--> 1012-109 MSun (Spirals) Monday, November 30, 2009 5
  6. 6. Types of Galaxies Spiral Galaxies Elliptical Galaxies Irregular Galaxies Properties: Properties: Properties: • Mass: 109 - 1012 Msun • Mass: 105 - 1013 Msun • Mass: 106 - 1011 Msun • Diameter: 5 - 50 kpc • Diameter: 1 - 200 kpc • Diameter: 1 - 10 kpc • Luminosity: 108 - 1011 • Luminosity: 106 - 1012 • Luminosity: 106 - few Lsun Lsun x 109 Lsun Structure & Dynamics: Structure & Dynamics: Structure & Dynamics: • Disk + Spheroid • Spheroid of old stars • Chaotic structure, lots • Supported by with little gas or dust of young blue stars relatively rapid rotation, • Supported by random • Moderate rotation in but spheroid is puffed motions of stars with Irregulars, but very up by random motions. some very slow rotation chaotic motions as well. Monday, November 30, 2009 6
  7. 7. Spiral Galaxies Classification Description Sa Bright core, tightly wound spiral arms Core dimmer than Sa, spiral arms more Sb loose Core dimmer than Sb, open spiral Sc structure, more dust and gas Dim core, loosely wrapped spiral structure, Sd lots of dust and gas and new star growth Monday, November 30, 2009 7
  8. 8. Barred Spiral Galaxies Classification Description SBa Bright core, tightly wound spiral arms Core dimmer than SBa, spiral arms more SBb loose Core dimmer than SBb, open spiral SBc structure, more dust and gas Dim core, loosely wrapped spiral structure, SBd lots of dust and gas and new star growth Monday, November 30, 2009 8
  9. 9. Spiral Galaxies Some other important properties of spiral galaxies include (Sparke and Gallagher, page 172 – 214): • Spiral galaxies produce most of the luminous light in the Universe due to new star birth in the spiral arms • Majority of galaxies in the Universe are spiral galaxies • Half of all spiral galaxies are in the bared spiral class • Spiral galaxies obey the Tully-Fisher relation – brighter galaxies rotate faster • Spiral galaxy rotation curves are dominated by Dark Matter • Rotation of stars in the spiral arms are organized while the rotation of stars in the bulge are not (random rotation orbits about the nucleus) Monday, November 30, 2009 9
  10. 10. Spiral Galaxies: Supermassive Black Holes The results ... show a close relationship between the black hole mass and the stars that comprise an elliptical galaxy or the central bulge stars of a spiral galaxy. Monday, November 30, 2009 10
  11. 11. Elliptical Galaxies Type E: Ellipticals Show little internal structure: • Elliptical in shape • No disks, spiral arms, or dust lanes • Brightest stars are red Classified by the degree of apparent flatness: • E0 is circular • E7 is flattest (~3:1 aspect ratio) Monday, November 30, 2009 11
  12. 12. Elliptical Galaxies: Supermassive Black Hole Though much more analysis remains, an initial look at Hubble evidence favors the idea that titanic black holes did not precede a galaxy's birth but instead co-evolved with the galaxy by trapping a surprisingly exact percentage of the mass of the bulbous hub of stars and gas in a galaxy. Monday, November 30, 2009 12
  13. 13. Irregular Galaxies Monday, November 30, 2009 13
  14. 14. Irregular Galaxies Type I: Irregulars Show an irregular, often chaotic structure. Little evidence of systematic rotation. Catch-all class: • Proposed systematic subclasses, but many irregulars defy classification. Significant dwarf irregular population, classified as "dI" Monday, November 30, 2009 14
  15. 15. Galaxies Relative Stellar & Gas Content [mass to light ratio] Spirals: • Range is ~10-20% gas • On-going star formation in the disks • Mix of Pop I and Pop II stars Ellipticals: • Very little or no gas or dust • Star formation ended billions of years ago • See only old Pop II stars Irregulars: • Can range up to 90% gas • Often a great deal of on-going star formation • Dominated by young Pop I stars Dwarf Irregulars: • Very metal poor (<1% solar) • Forming stars for the first time only now. Monday, November 30, 2009 15
  16. 16. Dwarf Galaxies Dwarf Galaxies Low-luminosity Ellipticals & Irregulars. • Significant number of dwarfs • Most common type of galaxy by number There are no (convincing) Dwarf Spirals. Possibilities: • Small versions of their larger cousins • Different population of objects despite their superficial similarities to larger E's and Irr's Monday, November 30, 2009 16
  17. 17. Galaxy Formation Monday, November 30, 2009 17
  18. 18. Galaxy Mergers The Milky Way and the Andromeda galaxy will likely fall together and merge within a few billion years. In this speculative simulation, the two galaxies flyby one another, exciting tidal tails and bridges and collide on a second pass finally merging after several convulsions. The last remnants of the smashed spirals show up as shells and ripples surrounding a newborn elliptical galaxy. (6 min) (1 min) Monday, November 30, 2009 18
  19. 19. Galaxy Collisions collision-course-or-uniting-as-an-intergalactic-super- system/2008/04/24/1208743153745.html Monday, November 30, 2009 19
  20. 20. The Local Group local_group_0305_diagram_800.jpg Monday, November 30, 2009 20
  21. 21. Galaxy Clusters LecturesBennett/DistanceScale/expansion.html Monday, November 30, 2009 21
  22. 22. CL0024+17 Galaxy Cluster How do we know that dark matter isn't just normal matter exhibiting strange gravity? A new observation of gravitationally magnified faint galaxies far in the distance behind a massive cluster of galaxies is shedding new dark on the subject. The above detailed image from the Hubble Space Telescope indicates that a huge ring of dark matter likely exists surrounding the center of CL0024+17 that has no normal matter counterpart. What is visible in the above image, first and foremost, are many spectacular galaxies that are part of CL0024+17 itself, typically appearing tan in color. Next, a close inspection of the cluster center shows several unusual and repeated galaxy shapes, typically more blue. These are multiple images of a few distant galaxies, showing that the cluster is a strong gravitational lens. It is the relatively weak distortions of the many distant faint blue galaxies all over the image, however, that indicates the existence of the dark matter ring. The computationally modeled dark matter ring spans about five million light years and been digitally superimposed to the image in diffuse blue. A hypothesis for the formation of the huge dark matter ring holds that it is a transient feature formed when galaxy cluster CL0024+17 collided with another cluster of galaxies about one billion years ago, leaving a ring similar to when a rock is thrown in a pond. Monday, November 30, 2009 22
  23. 23. The Local Supercluster The Local The Local Group Supercluster Monday, November 30, 2009 23
  24. 24. The Local Supercluster This (somewhat blurred) map identifies We jump now to a map that carries out to galaxies and galaxy clusters across a 1,000,000,000 light years which includes the field of view 400 million light years supercluster the Milky Way lies within: across Monday, November 30, 2009 24
  25. 25. Distant Ladder Monday, November 30, 2009 25
  26. 26. The Tully-Fisher Relation Monday, November 30, 2009 26
  27. 27. The Tully-Fisher Relation An observed relation between the luminosity of spiral galaxies and their maximum rotation velocity. The Tully- Fisher relation is used as a way of estimating distances to spirals. The form is a linear relation between the absolute magnitude of a galaxy and the logarithm of the velocity at the flat part of the rotation curve, although the slopes and intercepts of these relations are different for Sa, Sb, and Sc type galaxies. Approximations made in deriving the relation are that the mass-to-light ratios are constant for all galaxies and that the average surface brightness of all galaxies is also equal. Monday, November 30, 2009 27
  28. 28. Hubble’s Law Monday, November 30, 2009 28
  29. 29. LACC HW: Franknoi, Morrison, and Wolff, Voyages Through the Universe, 3rd ed. • Ch. 25, pp. 577-578: 3. • Ch 26: Tutorial Quizzes accessible from: http:// fid=M20b&product_isbn_issn=9780495017899&discipline_number=19 Due beginning of next class period. Be working on you Distance Ladders. Monday, November 30, 2009 29