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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 l.jpg
Galaxy collisions & galaxy formation

  • Collisions of galaxies

  • Formation of galaxies

  • Dark Matter



Collisions of galaxies l.jpg
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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Merger of two Spiral Galaxies

Chris Mihos & Sean Maxwell






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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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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

    • 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.


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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


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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…


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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.



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Measuring a Galaxy’s Mass star…

  • Apply same arguments to a galaxy…


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Measuring a Galaxy’s Mass star…

  • 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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How Can this Be? star…

  • 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.



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What is Dark Matter? star…

  • 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? star…

  • 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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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… studies

  • 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 studies

    • 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 studies


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  • Case III : MCG-6-30-15 studies

    • “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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There’s something strange at the center of our galaxy… studies

  • 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 studies

    • 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? studies

  • 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)…



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  • Correlations crucially important! bulge…

    • Argues for a connection between the formation of the galaxy and the supermassive black hole.

    • Currently forefront of research…


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