Galaxies And the Foundation of Modern Cosmology - PowerPoint PPT Presentation

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Galaxies And the Foundation of Modern Cosmology

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Title: Galaxies And the Foundation of Modern Cosmology


1
Galaxies And the Foundation of Modern Cosmology
2
What are the three major types of galaxies?
3
Hubble Ultra Deep Field
4
Hubble Ultra Deep Field
5
Hubble Ultra Deep Field
Spiral Galaxy
6
Hubble Ultra Deep Field
Spiral Galaxy
7
Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
8
Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Spiral Galaxy
9
Hubble Ultra Deep Field
Elliptical Galaxy
Elliptical Galaxy
Irregular Galaxies
Spiral Galaxy
10
halo
disk
bulge
Spiral Galaxy
11
Disk Component stars of all ages, many gas clouds
Type Sa Galaxy
Spheroidal Component bulge halo, old
stars, few gas clouds
12
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

13
Sb Galaxies
  • Moderate nuclear bulge
  • Intermediate spiral pattern
  • Some evidence for massive young stars, HII
    regions, star formation

14
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
15
Sc Galaxies
(Some classify Messier as as Type Sd)
  • Small to nearly non-existent nuclear bulge
  • Open spiral pattern
  • Active star-formation

16
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
17
Barred Spiral Galaxy
Has a bar of stars across the bulge
18
Barred Spiral Types
SBa SBb
SBc
19
S0 Lenticular Galaxy
Has a disk like a spiral galaxy but very little
dust or gas (intermediate between spiral and
elliptical)
20
S0 Edge-on
Note the clear presence of a disk, but absence of
dust band in this S0 galaxy NGC 3115
21
Elliptical Galaxy
All spheroidal (bulge) component, no disk
22
Elliptical Galaxy All spheroidal component,
virtually no disk component
Red-yellow color indicates older star population
23
Irregular Galaxies
Irregular I Galaxy
Blue-white color indicates ongoing star formation
24
Irr II Galaxy - Messier 82
25
Hubbles Galaxy Classes
Spheroid Dominates
Disk Dominates
26
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27
How are galaxies grouped together?
28
Spiral galaxies are often found in groups of
galaxies (up to a few dozen galaxies)
29
Our Galaxy Andromeda belong to a small Local
Group
of about 20 or so galaxies
30
Elliptical galaxies are much more common in huge
clusters of galaxies (hundreds to thousands of
galaxies)
31
How do we observe the life histories of galaxies?
32
Deep observations show us very distant galaxies
as they were much earlier in time (Old light
from young galaxies)
33
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34
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35
Denser regions contracted, forming protogalactic
clouds H and He gases in these clouds formed
the first stars
36
Supernova explosions from first stars kept much
of the gas from forming stars Leftover gas
settled into spinning disk Conservation of
angular momentum
37
Why do galaxies differ?
M87
NGC 4414
But why do some galaxies end up looking so
different?
38
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39
Why dont all galaxies have similar disks?
40
Nature Conditions in Protogalactic Cloud?
  • Spin Initial angular momentum of protogalactic
    cloud could determine size of resulting disk

41
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

42
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

43
We must also consider the effects of collisions
44
Collisions were much more likely early in time,
because galaxies were closer together
45
Many of the galaxies we see at great distances
(and early times) indeed look violently disturbed
46
The collisions we observe nearby trigger bursts
of star formation
47
Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an
elliptical
48
Modeling such collisions on a computer shows that
two spiral galaxies can merge to make an
elliptical
49
Shells of stars observed around some elliptical
galaxies are probably the remains of past
collisions
50
Collisions may explain why elliptical galaxies
tend to be found where galaxies are closer
together
51
Giant elliptical galaxies at the centers of
clusters seem to have consumed a number of
smaller galaxies
52
What is the evidence for dark matter in galaxies?
53
  • We measure the mass of the solar system using the
    orbits of planets
  • Orb. Period
  • Avg. Distance
  • Or for circles
  • Orb. Velocity
  • Orbital Radius

54
Rotation curve A plot of orbital velocity versus
orbital radius Solar systems rotation curve
declines because Sun has almost all the mass
55
Rotation curve of merry-go-round rises with radius
56
Rotation curve of Milky Way stays flat with
distance Mass must be more spread out than in
solar system
57
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!
58
Mass within Suns orbit 1.0 x 1011 MSun
Total mass 1012 MSun
59
The visible portion of a galaxy lies deep in the
heart of a large halo of dark matter
60
We can measure rotation curves of other spiral
galaxies using the Doppler shift of the 21-cm
line of atomic H
61
Spiral galaxies all tend to have flat rotation
curves indicating large amounts of dark matter
62
Broadening of spectral lines in elliptical
galaxies tells us how fast the stars are
orbiting These galaxies also have dark matter
63
Clusters of Galaxies
We can measure the velocities of galaxies in a
cluster from their Doppler shifts
64
The mass we find from galaxy motions in a cluster
is about 50 times larger than the mass in stars!
65
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
66
Gravitational lensing, the bending of light rays
by gravity, can also tell us a clusters mass
67
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68

All three methods of measuring cluster mass
indicate similar amounts of dark matter
69
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

70
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

71
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

72
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

73
MACHOs occasionally make other stars appear
brighter through lensing
74
MACHOs occasionally make other stars appear
brighter through lensing but not enough
lensing events to explain all the dark matter
75
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
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