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


Credit : Tony and Daphne Hallas

merger of two spiral galaxies
Merger of two Spiral Galaxies

Chris Mihos & Sean Maxwell

collisions of galaxies13
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!
internal evolution
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
iii galaxy formation
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
zoom in on a forming galaxy cluster virgo consortium
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.


Las Campanas

Redshift survey

how do galaxies form
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…
iii the mass of galaxies and the need for dark matter
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.
other stars
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
measuring a galaxy s mass
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…
kepler s third law
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.
measuring a galaxy s mass31
Measuring a Galaxy’s Mass
  • Apply same arguments to a galaxy…
measuring a galaxy s mass32
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)
how can this be
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.
what is dark matter
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.
what is dark matter38
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
ii evidence for supermassive black holes three case studies
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
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.
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
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.
there s something strange at the center of our galaxy
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…
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
iv a supermassive black hole in every galaxy
IV : A Supermassive Black Hole in Every Galaxy?
  • Black holes exist in centers of some galaxies…
  • But how widespread are they?
  • Does every galaxy have a supermassive central black hole?
  • Several teams set out to answer that question…
    • Use best resources (HST, large telescopes on ground etc.) to gather lots of data on many nearby galaxies.
    • Systematic search for black holes
    • They found them, and discovered interesting patterns…
    • Correlation between size of black hole and the brightness of the galaxy’s bulge (but not the disk)…
Correlations crucially important!
    • Argues for a connection between the formation of the galaxy and the supermassive black hole.
    • Currently forefront of research…