Galaxy%20collisions%20

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Galaxy collisions galaxy formation

• Collisions of galaxies
• Formation of galaxies
• Dark Matter

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NGC4622
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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!

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M51
Credit Tony and Daphne Hallas
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The Antennae Galaxy
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Merger of two Spiral Galaxies
Chris Mihos Sean Maxwell
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Merger of a Spiral and an Elliptical Galaxy
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Merger of two Elliptical Galaxies
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When Spirals Collide
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The Antennae Galaxy
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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!

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Big Galaxies Tear up Small Ones
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Spiral Galaxy dining on a Dwarf Spheroidal (side
view)
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Spiral Galaxy dining on a Dwarf Spheroidal (top
view)
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The Cartwheel Galaxy
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Simulation of the Cartwheel Galaxy
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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

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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

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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.
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Las Campanas Redshift survey
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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

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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
• Msun2?1030 kg
• Lsun4?1026 W
• Msun/Lsun5000 kg/W
• From now on, we will use Msun/Lsun as a standard
  reference.

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Other stars

• Lets 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

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Measuring a Galaxys Mass

• Typically measure L1010 Lsun
• So, mass of stars is M1011 Msun
• But, theres another way to measure mass

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Keplers Third Law

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

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Velocity dependence on radius for a planet
orbiting a star
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Measuring a Galaxys Mass

• Apply same arguments to a galaxy

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Measuring a Galaxys 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)

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What do we see? Galactic Rotation Curves.
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Real measurements - Strange Rotation Curves
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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
  isnt 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.

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Called a dark matter halo
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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.

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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

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Supermassive Black Holes - Monsters in the Closet
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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

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• 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.

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• 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

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MCG-6-30-15
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• 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.

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III The Center of our Galaxy
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Theres 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

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• 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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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 galaxys bulge (but not the
  disk)

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• But, even better correlation with stellar
  velocity in bulge

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• Correlations crucially important!
• Argues for a connection between the formation of
  the galaxy and the supermassive black hole.
• Currently forefront of research