Galaxies And the Foundation of Modern Cosmology

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Galaxies And the Foundation of Modern Cosmology
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What are the three major types of galaxies?
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Hubble Ultra Deep Field
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Hubble Ultra Deep Field
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Hubble Ultra Deep Field
Spiral Galaxy
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Hubble Ultra Deep Field
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
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Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy
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halo
disk
bulge
Spiral Galaxy
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Disk Component stars of all ages, many gas clouds
Type Sa Galaxy
Spheroidal Component bulge halo, old
stars, few gas clouds
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Sa Galaxies

• Sa Galaxies
• Dominant nuclear bulge
• Tightly wound spiral pattern
• Few (but some) newly formed stars, HII regions
  or other evidence of active star formation

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Sb Galaxies

• Moderate nuclear bulge
• Intermediate spiral pattern
• Some evidence for massive young stars, HII
  regions, star formation

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Type Sc Galaxy
Blue-white color indicates ongoing star formation
Disk Component stars of all ages, many gas
clouds Spheroidal Component bulge halo, old
stars, few gas clouds
Red-yellow color indicates older star population
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Sc Galaxies
(Some classify Messier as as Type Sd)

• Small to nearly non-existent nuclear bulge
• Open spiral pattern
• Active star-formation

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Disk Component stars of all ages, many gas clouds
Blue-white color indicates ongoing star formation
Spheroidal Component bulge halo, old
stars, few gas clouds
Red-yellow color indicates older star population
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Barred Spiral Galaxy
Has a bar of stars across the bulge
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Barred Spiral Types
SBa SBb
SBc
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S0 Lenticular Galaxy
Has a disk like a spiral galaxy but very little
dust or gas (intermediate between spiral and
elliptical)
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S0 Edge-on
Note the clear presence of a disk, but absence of
dust band in this S0 galaxy NGC 3115
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Elliptical Galaxy
All spheroidal (bulge) component, no disk
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Elliptical Galaxy All spheroidal component,
virtually no disk component
Red-yellow color indicates older star population
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Irregular Galaxies
Irregular I Galaxy
Blue-white color indicates ongoing star formation
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Irr II Galaxy - Messier 82
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Hubbles Galaxy Classes
Spheroid Dominates
Disk Dominates
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How are galaxies grouped together?
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Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
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Our Galaxy Andromeda belong to a small Local
Group
of about 20 or so galaxies
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Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
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How do we observe the life histories of galaxies?
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Deep observations show us very distant galaxies
as they were much earlier in time (Old light
from young galaxies)
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Denser regions contracted, forming protogalactic
clouds H and He gases in these clouds formed
the first stars
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Supernova explosions from first stars kept much
of the gas from forming stars Leftover gas
settled into spinning disk Conservation of
angular momentum
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Why do galaxies differ?
M87
NGC 4414
But why do some galaxies end up looking so
different?
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Why dont all galaxies have similar disks?
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Nature Conditions in Protogalactic Cloud?

• Spin Initial angular momentum of protogalactic
  cloud could determine size of resulting disk

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Conditions in Protogalactic Cloud?

• Density Elliptical galaxies could come from
  dense protogalactic clouds that were able to cool
  and form stars before gas settled into a disk

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Distant Red Ellipticals

• Observations of some distant red elliptical
  galaxies support the idea that most of their
  stars formed very early in the history of the
  universe

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We must also consider the effects of collisions
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Collisions were much more likely early in time,
because galaxies were closer together
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Many of the galaxies we see at great distances
(and early times) indeed look violently disturbed
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The collisions we observe nearby trigger bursts
of star formation
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Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an
elliptical
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Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an
elliptical
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Shells of stars observed around some elliptical
galaxies are probably the remains of past
collisions
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Collisions may explain why elliptical galaxies
tend to be found where galaxies are closer
together
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Giant elliptical galaxies at the centers of
clusters seem to have consumed a number of
smaller galaxies
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What is the evidence for dark matter in galaxies?
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• We measure the mass of the solar system using the
  orbits of planets
• Orb. Period
• Avg. Distance
• Or for circles
• Orb. Velocity
• Orbital Radius

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Rotation curve A plot of orbital velocity versus
orbital radius Solar systems rotation curve
declines because Sun has almost all the mass
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Rotation curve of merry-go-round rises with radius
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Rotation curve of Milky Way stays flat with
distance Mass must be more spread out than in
solar system
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Mass in Milky Way is spread out over a larger
region than the stars Most of the Milky Ways
mass seems to be dark matter!
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Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
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The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
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We can measure rotation curves of other spiral
galaxies using the Doppler shift of the 21-cm
line of atomic H
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Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
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Broadening of spectral lines in elliptical
galaxies tells us how fast the stars are
orbiting These galaxies also have dark matter
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Clusters of Galaxies
We can measure the velocities of galaxies in a
cluster from their Doppler shifts
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The mass we find from galaxy motions in a cluster
is about 50 times larger than the mass in stars!
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Clusters contain large amounts of X-ray emitting
hot gas Temperature of hot gas (particle
motions) tells us cluster mass 85 dark
matter 13 hot gas 2 stars
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Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
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All three methods of measuring cluster mass
indicate similar amounts of dark matter
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Does dark matter really exist?
Either

• Dark matter really exists, and we are observing
  the effects of its gravitational attraction
• Something is wrong with our understanding of
  gravity, causing us to mistakenly infer the
  existence of dark matter

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Bottom Line

• What is the evidence for dark matter in galaxies?
• Rotation curves of galaxies are flat, indicating
  that most of their matter lies outside their
  visible regions
• What is the evidence for dark matter in clusters
  of galaxies?
• Masses measured from galaxy motions, temperature
  of hot gas, and gravitational lensing all
  indicate that the vast majority of matter in
  clusters is dark

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Our Options

• Dark matter really exists, and we are observing
  the effects of its gravitational attraction
• Something is wrong with our understanding of
  gravity, causing us to mistakenly infer the
  existence of dark matter
• Because gravity is so well tested, most
  astronomers prefer option 1

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Two Basic Options

• Baryonic (Ordinary) Dark Matter (MACHOS)
• Massive Compact Halo Objects
• dead or failed stars in halos of galaxies
• Extraordinary Dark Matter (WIMPS)
• Weakly Interacting Massive Particles
• mysterious neutrino-like particles

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MACHOs occasionally make other stars appear
brighter through lensing
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MACHOs occasionally make other stars appear
brighter through lensing but not enough
lensing events to explain all the dark matter
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Why Believe in WIMPs?

• Theres not enough ordinary matter
• WIMPs could be left over from Big Bang
• Models involving WIMPs explain how galaxy
  formation works