Galaxy collisions & galaxy
formation
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Collisions of galaxies
Formation of galaxies
Dark Matter
NGC4622
Collisions of galaxies
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Galaxy collisions are comparatively
common (and spectacular!)
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Major collision
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collision of 2 big galaxies
Quite rare
Minor collision
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Collision of a large galaxy with a small
“dwarf” galaxy
Very common!
M51
Credit : Tony and Daphne Hallas
The Antennae Galaxy
Merger of two Spiral Galaxies
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Chris Mihos & Sean Maxwell
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
Collisions of galaxies
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Galaxy collisions are comparatively
common (and spectacular!)
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Major collision
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collision of 2 big galaxies
Quite rare
Minor collision
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Collision of a large galaxy with a small
“dwarf” galaxy
Very common!
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
Simulation of the Cartwheel Galaxy
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Internal evolution
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Galaxy collision can drive “internal evolution” of
galaxies…
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Rapid star formation
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Galactic collisions makes gas clouds collapse and turn into
stars
Makes galaxy look blue (since there can be many young,
hot stars)
Quasar activity
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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
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How did galaxies form?
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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)
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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?
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“Bottom-up” formation scenario…
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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
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First think about
stars…
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we want mass, but
see light
Construct the “massto-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
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Let’s use star-light to weigh a whole galaxy…
have to average M/L over all stars.
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Different types of stars have different massto-light ratios
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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
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M/L  10 Msun/Lsun
Measuring a Galaxy’s Mass
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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
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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.
Velocity dependence on radius for
a planet orbiting a star…
Measuring a Galaxy’s Mass
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Apply same arguments to a galaxy…
Measuring a Galaxy’s Mass
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Consider a star in the
galaxy at distance D
from center at speed V
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Then, mass of the
galaxy within distance
D, Msun(inside D)
What do we see? Galactic
Rotation Curves.
Real measurements Strange “Rotation” Curves
How Can this Be?
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Orbital velocity of stars/gas stays flat as
far out as we can track it
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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.
Called a dark matter “halo”
What is Dark Matter?
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Is most dark matter normal Dust/Gas? What
about Black Holes, Neutron Stars, Planets?
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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?
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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 Matter?
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WIMPs (Weakly Interacting Massive Particles)?
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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
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Various possibilities suggested by Particle Physics
Theory…
 Super-symmetric particles
 Gauge bosons
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Many experiments currently on-going
Supermassive Black Holes Monsters in the Closet
II : Evidence for supermassive
black holes – three case studies
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Case I : M87
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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
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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
MCG-6-30-15
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Case III : MCG-6-30-15
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“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.
III : The Center of our Galaxy
There’s something strange at
the center of our galaxy…
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Modern large
telescopes can track
individual stars at
Galactic Center
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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
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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?
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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…
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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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But, even better correlation with stellar velocity in
bulge…
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Correlations crucially important!
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Argues for a connection between the
formation of the galaxy and the
supermassive black hole.
Currently forefront of research…