1. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong

14. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
How is the observed large-scale structure formed?
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?

15. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
How is the observed large-scale structure formed?
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?

16. Distribution of Nearby Galaxies
 Mollweide projection of our Galaxy and the sky distribution of nearby galaxies.

17. Mollweide Projection of the Earth
 Projection of a sphere onto a 2-dimensional plane so as to accurate preserve
proportions in area.

18. Our Galaxy
 Our Galaxy, also known as the Milky Way, is the bright band along the equator.

19. Our Galaxy
 Our Galaxy, also known as the Milky Way, is the bright band along the equator.

20. Our Galaxy
 Our Galaxy, also known as the Milky Way, is the bright band along the equator.

21. Our Galaxy
 We live on a planet orbiting a star at the outskirts of a spiral galaxy.

22. Our Galaxy
 We live on a planet orbiting a star at the outskirts of a spiral galaxy.

23. Our Galaxy
 Our Galaxy looks like a milky band of light across the sky.

24. Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

25. Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

26. Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

27. Large Magellanic
Cloud
Small Magellanic Cloud
Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

28. Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

29. Large Magellanic Cloud
Small Magellanic Cloud
Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

30. Large Magellanic Cloud
Small Magellanic Cloud Supernova 1987A
Neighboring Galaxies
 The nearest galaxies are the Large and Small Magellanic Clouds.

31. Neighboring Galaxies
 The next nearest galaxy is M31, the Andromeda galaxy. (250 m away)

32. Neighboring Galaxies
 The next nearest galaxy is M31, the Andromeda galaxy.
M31 (Andromeda galaxy)

33. Local Group
 Our galaxy is part of a group of over 50 galaxies.

34. More Distant Galaxies, Groups, and Clusters
 At distances up to 140 million light years, we see … (5 km away; Wan Chai)

35. < 140 million light years
 At distances up to 140 million light years, we see …
More Distant Galaxies, Groups, and Clusters

36. < 140 million light years
 Clusters comprising hundreds to thousands of galaxies such as the Virgo cluster.
More Distant Galaxies, Groups, and Clusters

37. Virgo Cluster
 Clusters comprising hundreds to thousands of galaxies such as the Virgo cluster.
More Distant Galaxies, Groups, and Clusters

38. < 140 million light years
 Clusters comprising hundreds to thousands of galaxies such as the Fornax cluster.
More Distant Galaxies, Groups, and Clusters

39. Fornax Cluster
 Clusters comprising hundreds to thousands of galaxies such as the Fornax cluster.
More Distant Galaxies, Groups, and Clusters

40. < 140 million light years
 Galaxies (individual, in groups and clusters) arranged in long filaments.
More Distant Galaxies, Groups, and Clusters

41. < 140 million light years
 Galaxies (individual, in groups and clusters) arranged in long filaments
(superclusters).
More Distant Galaxies, Groups, and Clusters

42.  At distances from 140-280 million light years, we see … (5-10 km away; Wan
Chai and Fortress Hill)
More Distant Galaxies, Groups, and Clusters

43. < 280 million light years
 More clusters like the Centaurus cluster.
More Distant Galaxies, Groups, and Clusters

44. < 280 million light years
Centaurus Cluster
 More clusters like the Centaurus cluster.
More Distant Galaxies, Groups, and Clusters

45. < 280 million light years
 More superclusters.
More Distant Galaxies, Groups, and Clusters

46. < 280 million light years
 The Perseus-Pisces supercluster spans the constellation Perseus to Pisces.
More Distant Galaxies, Groups, and Clusters

47. < 280 million light years
Perseus
 The Perseus-Pisces supercluster spans the constellation Perseus to Pisces.
More Distant Galaxies, Groups, and Clusters

48. < 280 million light years
Perseus Cluster
 The Perseus-Pisces supercluster is anchored by the Perseus cluster to the east.
More Distant Galaxies, Groups, and Clusters

49. < 280 million light years
 More galaxies (individual, in groups and clusters) arranged in long filaments.
More Distant Galaxies, Groups, and Clusters

50.  At distances from 280-420 million light years, we see … (10-15 km away;
Fortress Hill and Shau Kei Wan)
More Distant Galaxies, Groups, and Clusters

51. < 280 million light years< 420 million light years
 More clusters, such as the Coma cluster.
More Distant Galaxies, Groups, and Clusters

52. < 280 million light years
Coma Cluster
 More clusters, such as the Coma cluster.
More Distant Galaxies, Groups, and Clusters

53. < 280 million light years< 420 million light years
 Galaxies (individual, in groups and clusters) arranged along “cell walls.”
More Distant Galaxies, Groups, and Clusters

54.  At distances from 420-560 million light years, we see … (15-20 km away; Tai
Wan and Fo Tan)
More Distant Galaxies, Groups, and Clusters

55. < 560 million light years
 More superclusters, and galaxies (individual, in groups and clusters) arranged
along “cell walls”.
More Distant Galaxies, Groups, and Clusters

56.  At distances from 560-700 million light years, we see … (20-25 km away; near
Lok Ma Chau)
More Distant Galaxies, Groups, and Clusters

57. < 700 million light years
 More superclusters, and galaxies (individual, in groups and clusters) arranged
along “cell walls”.
More Distant Galaxies, Groups, and Clusters

58. More Distant Galaxies, Groups, and Clusters
 At distances from 700-840 million light years, we see … (25-30 km away;
Shenzhen)

59. < 560 million light years< 700 million light years< 840 million light years
 More superclusters, and galaxies (individual, in groups and clusters) arranged
along “cell walls”.
More Distant Galaxies, Groups, and Clusters

60. More Distant Galaxies, Groups, and Clusters
 At distances from 840-980 million light years, we see … (30-35 km away; beyond
Shenzhen)

61. < 980 million light years
 More superclusters, and galaxies (individual, in groups and clusters) arranged
along “cell walls”.
More Distant Galaxies, Groups, and Clusters

62. More Distant Galaxies, Groups, and Clusters
 At distances from 980-1220 million light years, we see … (35-44 km away;
Pinghu residential district)

63. < 1220 million light years
 More superclusters, and galaxies (individual, in groups and clusters) arranged
along “cell walls”.
More Distant Galaxies, Groups, and Clusters

76.  Closeup of a … beads (galaxies) along a string (filament)?
The Cosmic Web

77.  Closeup of a … beads (galaxies) along strings (filaments)?
The Cosmic Web
< 280 million light years

78.  Closeup of a … beads (galaxies) along connected strings (cell walls)?
The Cosmic Web

79.  You were looking at closeups of a spider’s web.
The Cosmic Web

80. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
In connected filaments called the Cosmic Web.
How is the observed large-scale structure formed?
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?

81. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
In connected filaments called the Cosmic Web.
How is the observed large-scale structure formed?
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?

82. What is the Universe made of?
 The Universe is made of
- 4.6% atoms (ordinary matter), such as people, Earth, Sun, stars, …
- 23% Dark matter, which cannot be seen and only interact with ordinary matter
through their mutual gravity
- 72% Dark Energy, which repulses ordinary and Dark matter
 Dark matter therefore dominates gravity in the Universe.

83. How did the Universe begin?
 The Universe expanded from a dense and hot state in a “Big Bang.”
Approximately 10−37 seconds into the expansion, the Universe went through a
period of exponential expansion known as inflation.

84. State of the Universe after Inflation
 The Universe was almost perfectly smooth following Inflation. If draped over the
Earth’s surface, the height from the deepest to tallest points would only be 850 m.

85. State of the Universe after Inflation
 The Universe was almost perfectly smooth following Inflation. If draped over the
Earth’s surface, the height from the deepest to tallest points would only be 850 m.

86. The Cosmic Microwave Background
 The CMB comprises radiation from the epoch when the Universe first became
transparent, ~380,000 years after the Big Bang. The CMB is smooth to about 1
part in about 100,000, but is not perfectly smooth.
 Regions of higher-than-average density have higher-than-average gravity,
attracting both Dark and ordinary matter to form galaxies distributed in a Cosmic
Web.
Dunkley+09

87. Gravity on a Uniform Distribution of Matter
 If particles are uniformly distributed in space, there would be not be a net
gravitational force between particles. Particles would not clump together.

88. Gravity on a Non-Uniform Distribution of Matter
 If particles are not uniformly distributed in space, there exists a net gravitational
force between particles. Particles would start to clump together.

89. Gravity on a Non-Uniform Distribution of Matter
 Particles that clump together exert a stronger gravitational force on neighboring
particles, attracting ever more particles towards the clump.

90. Modeling the Formation of Large-Scale Structure
 The Millennium Simulation Project, an N-body simulation comprising over 109
particles each with a mass of ~8.6 108/h M and having a spatial resolution of
5/h kpc.
z Time since
Big Bang
20 0.18 Gyr
15 0.27 Gyr
10 0.48 Gyr
8 0.65 Gyr
6 0.95 Gyr
5 1.20 Gyr
4 1.57 Gyr
3 2.19 Gyr
2 3.34 Gyr
1 5.94 Gyr
0.5 8.65 Gyr
0.2 11.26 Gyr
0.1 12.38 Gyr

92. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
In connected filaments called the Cosmic Web.
How is the observed large-scale structure formed?
Through gravitational collapse of initial density inhomogeneities.
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?

93. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
In connected filaments called the Cosmic Web.
How is the observed large-scale structure formed?
Through gravitational collapse of initial density inhomogeneities.
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?

94. Galaxy Formation
 The excellent overall agreement between observations and theoretical models of
large-scale structure indicates that we
understand in a global sense how
galaxies form.
Springel+06

95. Galaxy Formation
 The excellent overall agreement between observations and theoretical models of
large-scale structure indicates that we
understand in a global sense how
galaxies form.
Springel+06
 But, theoretical models simply
represent galaxies as a
collection of gravitating point
masses. Real galaxies contain
stars and gas (along with dark
matter), where gas turn into
stars and stars expel gas that
turn into stars …
 One of the frontiers of
astrophysics: formation and
evolution of galaxies.
Andromeda galaxy

96. Formation of Large-Scale Structure and Galaxies
Jeremy Lim, Associate Professor, University of Hong Kong
How are galaxies distributed in space (large-scale structure)?
In connected filaments called the Cosmic Web.
How is the observed large-scale structure formed?
Through gravitational collapse of initial density inhomogeneities.
Does our ability to reproduce (in our computers) the observed
large-scale structure mean we understand galaxy formation?
No. We understand in a global sense how galaxies come into
existence, but how galaxies actually form and evolve to assume
their present-day properties is one of the most important frontiers in
modern-day astrophysics.

98. < 140 million light years
 At distances up to 140 million light years, we see …
More Distant Galaxies, Groups, and Clusters