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Galaxy collisions & galaxy formation

Galaxy collisions & galaxy formation. Collisions of galaxies Formation of galaxies Dark Matter. NGC4622. Collisions of galaxies. Galaxy collisions are comparatively common (and spectacular!) Major collision collision of 2 big galaxies Quite rare Minor collision

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Galaxy collisions & galaxy formation

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  1. Galaxy collisions & galaxy formation • Collisions of galaxies • Formation of galaxies • Dark Matter

  2. NGC4622

  3. Collisions of galaxies • Galaxy collisions are comparatively common (and spectacular!) • Major collision • collision of 2 big galaxies • Quite rare • Minor collision • Collision of a large galaxy with a small “dwarf” galaxy • Very common!

  4. M51 Credit : Tony and Daphne Hallas

  5. The Antennae Galaxy

  6. Merger of two Spiral Galaxies Chris Mihos & Sean Maxwell

  7. Merger of a Spiral and an Elliptical Galaxy

  8. Merger of two Elliptical Galaxies

  9. When Spirals Collide

  10. The Antennae Galaxy

  11. Collisions of galaxies • Galaxy collisions are comparatively common (and spectacular!) • Major collision • collision of 2 big galaxies • Quite rare • Minor collision • Collision of a large galaxy with a small “dwarf” galaxy • Very common!

  12. Big Galaxies Tear up Small Ones

  13. Spiral Galaxy dining on a Dwarf Spheroidal (side view)

  14. Spiral Galaxy dining on a Dwarf Spheroidal (top view)

  15. The Cartwheel Galaxy

  16. Simulation of the Cartwheel Galaxy

  17. Internal evolution • Galaxy collision can drive “internal evolution” of galaxies… • Rapid star formation • Galactic collisions makes gas clouds collapse and turn into stars • Makes galaxy look blue (since there can be many young, hot stars) • Quasar activity • Galactic collision drives gas into center of galaxy • Gas can rain onto central massive black hole and produce tremendous amounts of energy… • More about this possibility in next class

  18. III : Galaxy formation • How did galaxies form? • Believed that universe started off very uniform/smooth… just small ripples • Gravity caused ripples to grow… • These eventually collapsed to become galaxies and clusters of galaxies! • Nowadays, can study this process using computer simulations

  19. Zoom in on a forming galaxy cluster (Virgo consortium) This movie zooms in on one patch of a larger simulation where we know that a galaxy cluster is about to form.

  20. Las Campanas Redshift survey

  21. How do Galaxies Form? • “Bottom-up” formation scenario… • All driven by gravitational collapse • Some small things form first • Collisions/mergers cause bigger things to grow… • Dwarf galaxies  galaxies  galaxy clusters  superclusters and so on. • “Bottom-up” formation scenario…

  22. III : The mass of galaxies and the need for dark matter • First think about stars… • we want mass, but see light • Construct the “mass-to-light” ratio • Msun=21030 kg • Lsun=41026 W • Msun/Lsun=5000 kg/W • From now on, we will use Msun/Lsun as a standard reference.

  23. Other stars • Let’s use star-light to weigh a whole galaxy… have to average M/L over all stars. • Different types of stars have different mass-to-light ratios • Massive stars have small M/L. • Low-mass stars have large M/L. • Neutron stars and black hole hardly shine at all (very high M/L) • Averaging stars near to the Sun, get • M/L  10 Msun/Lsun

  24. Measuring a Galaxy’s Mass • Typically measure L=1010 Lsun • So, mass of stars is M=1011 Msun • But, there’s another way to measure mass…

  25. Kepler’s Third Law • Use same laws of motion as for planets going around a star… • Remember Kepler’s Third Law for Planets. • We can use this as an approximate formula for a star’s motion around the Galactic Center.

  26. Velocity dependence on radius for a planet orbiting a star…

  27. Measuring a Galaxy’s Mass • Apply same arguments to a galaxy…

  28. Measuring a Galaxy’s Mass • Consider a star in the galaxy at distance D from center at speed V • Then, mass of the galaxy within distance D, Msun(inside D)

  29. What do we see? Galactic Rotation Curves.

  30. Real measurements - Strange “Rotation” Curves

  31. How Can this Be? • Orbital velocity of stars/gas stays flat as far out as we can track it • Means that enclosed mass increases linearly with distance… even beyond point where starlight stops • So, in these outer regions of galaxies, the mass isn’t luminous… • This is DARK MATTER. • All galaxies seem to be embedded in giant dark matter balls (called halos) • At least 10 time more dark matter than visible stuff.

  32. Called a dark matter “halo”

  33. What is Dark Matter? • Is most dark matter normal Dust/Gas? What about Black Holes, Neutron Stars, Planets? • No!! No enough of this stuff! Solid arguments from cosmology limit the amount of “normal” matter to less than that needed for dark matter halos. • So, this is something new… non-baryonic matter. (matter not based on protons and neutrons). • 80-90% of matter in universe is non-baryonic dark matter!! • Neutrinos? • They are part of the “standard model” of particle physics… they have been detected and studied. • No… each neutrino has very small mass, and there are not enough of them to explain dark matter.

  34. What is Dark Matter? • WIMPs (Weakly Interacting Massive Particles)? • Generic name for any particle that has a lot of mass, but interacts weakly with normal matter • Must be massive, to give required mass • Must be weakly interacting, in order to have avoided detection • Various possibilities suggested by Particle Physics Theory… • Super-symmetric particles • Gauge bosons • Many experiments currently on-going

  35. Supermassive Black Holes - Monsters in the Closet

  36. II : Evidence for supermassive black holes – three case studies • Case I : M87 • Large elliptical galaxy • Black Hole suspected due to presence of prominent jet • Target of early study by Hubble Space Telescope

  37. HST found… • Rotating gas disk at galactic center • Measured rotation implied a central object of 3 billion solar masses! • Mass cannot be due to normal stars at center… not enough light is seen. • Good evidence for 3 billion solar mass black hole.

  38. Case II : M106 • Contains central gas disk • Disk produces naturally occurring MASER emission • Radio telescopes can measure position & velocity of MASERs to great accuracy. • Velocity changes with radius precisely as expected if all mass is concentrated at center! • 30 million solar mass black hole

  39. MCG-6-30-15

  40. Case III : MCG-6-30-15 • “Active galactic nucleus” • Bright X-ray source • Find signature of a gas disk in X-ray spectrum • This disk is orbiting something at 30% speed of light! • Also see strong “gravitational redshifts” • Strong evidence for a very massive black hole in this object.

  41. III : The Center of our Galaxy

  42. There’s something strange at the center of our galaxy… • Modern large telescopes can track individual stars at Galactic Center • Need infra-red (to penetrate dust?) • Need very good resolution. • We have been observing for past 10 years…

  43. The central object is • Very dark • Very massive (3 million solar masses) • Must be very compact (Star S2 gets within 125 AU of the center) • Currently the best case for any supermassive black hole

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