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Chapter 25: Galaxy Clusters and the Structure of the Universe

• Distribution of galaxies

• Evolution of galaxies

• Study of distant galaxies – Distance derived from redshift– Hubble’s constant – age of the Universe: 14 billion years – Example: galaxy at distance 6 billion LY – seeing it as it was

when the Universe was 8 billion years old

• Exploring the Universe almost back to the time it began

A Million Galaxies

The known part of the Universe - to a distance of ~ 4000 Mpc

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Clusters of galaxies• Galaxy - huge group of stars, dust, gas, and other celestial bodies

bound together by gravitational forces.

• Single galaxies (field galaxies): less than 8 % of total number of galaxies

• Groups of galaxies - the smallest aggregates of galaxies (N=10 to 100 members in a diameter of 2 Mpc)

• Cluster of galaxies - contain up to one thousand galaxies (N=102 to 103)

• Superclusters of galaxies - contain up to thousands of galaxies (N=103 to 104)

• Enormous clouds of extremely hot intergalactic gas and Dark matter

Clusters of galaxies• Classification of clusters

– Shape: regular or irregular– Richness: based on number of members – Elliptical dwarfs most numerous – observational selection affects the classification

• at distances > 30-50 Mpc the elliptical dwarfs are not visible• statistical approaches to count them

• Average distance between clusters – r ~(10 x Cluster’s Diameter ) -- much larger spatial concentration in comparison to

the concentration of stars in a given galaxy ( r between stars = 105 to 106 x Dstar )

• Contemporary theory– No principal difference between groups and clusters – Hierarchical structure of the Universe – the field galaxies form groups and the

groups form clusters

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Clusters typically have the following properties:

• Content– 10 to 1000 galaxies– hot X-ray emitting gas– large amounts of dark matter

• The distribution of these three components is approximately the same from one cluster to another

• Total mass: 1014 to 1015 solar masses

• Diameter: 3 to 8 Mpc

• The spread of velocities for the individual galaxies is about 800-1000 km/s.

• The average distance between clusters is approximately 30-40 Mpc

Poor Clusters & Groups

N~100 visible objects

NGC 2300

MKW 4

AWM 7

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Rich clusters

N~1000 - 10000 visible objectsD~ 3-4 MpcTotal known number ~ 4000

Cluster dominant galaxies

The brightest members – 1 or 2 giant E or S0:

L~ 3x1011 L Sun

Mv = -24 magmass ~ 1013 mass Sun

Galaxies of type cD (cluster dominant)

Standard candles – distance indicator – all tests of the expansion of the Universe are done using cD galaxies

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Elliptical Galaxies• Red color• Smooth Profile• High Surface Brightness• Egg shaped• Many globular clusters• Little or no dust line• Absorption lines only• No rotation• Found in clusters• -22 < M < -18

Cluster dominant galaxies• multinuclear structure – formed by capturing other galaxies or by

repeatedly merging with other cluster members

• These mergers occur at the centre of the cluster • dynamical friction makes galaxies lose kinetic energy:

– The motion of a galaxy creates an enhanced "wake" of galaxies behind it

– The excess gravitational pull of this wake slows the motion of the galaxy

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Dynamical friction

• The net effect of this force is to make a galaxy slowly spiral in toward the centre of the cluster

• Once there, it will merge with all the galaxies that have proceeded it.

• Evidence for this scenario comes from the large number of "multiple nuclei" seen in cDgalaxies:

• These secondary condensations of light are thought to be the partially digested remains of merging latecomers.

The Local Group

• Members and Size

• The Canis Major galaxy

• Dominant galaxies (M31, MW, M33)

• Dwarf elliptical galaxies

• 50 similar groups in a sphere with a radius 20 Mpc

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The Local Group

The Local Group

NGC 6822

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Clusters of galaxies

The closest clusters – Vigro(20 Mpc) and Coma (90 Mpc)

Virgo

Coma

The Virgo Cluster

Moving away from the Local group at 1000 km/s (should be 1300 km/s)The Local group belongs to the Virgo Supercluster.

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Fig.25.12

M 87

The Coma Cluster

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Clusters of galaxies

The Perseus Cluster of Galaxies

The cluster CL 0939+4713

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The formation and evolution of galaxies:galaxies in clusters vs. galaxies between clusters

How do clusters of galaxies form?

The pancake model: very large clumps of gas collected to form protoclusters

Fragmentation to form individual galaxies

Pancakelike shape

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How do clusters of galaxies form?

The hierarchical model: small clouds of gas formed clusters of stars, which later formed individual galaxies and clusters of galaxies

How do clusters of galaxies form?

The explosion model: gigantic explosions in the early Universe swept up the surrounding matter into dense shells. Individual galaxies formed in the shells;clusters of galaxies formed at the intersections of the expanding shells.

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“Pre-galactic Blobs”

Our picture of galaxy evolution

Observations + Computer simulations

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First 0-0.5 billion years:

very smooth distribution of matter directly after the Big Bang gravity of the more massive clumps of stars starts to attract

more matter

Our picture of galaxy evolution

0.5 - 1 billion years larger :

clumps grow from merging of smaller once

Our picture of galaxy evolution

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After 1-2 billion years:

after growing to a fraction of the size of our own galaxy, the clumps are large enough for the Hubble Space Telescope to see them

Our picture of galaxy evolution

After 2-4 billion years:larger irregular looking objects form through collisions

and mergers between these sub-galactic sized clumps

Our picture of galaxy evolution

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After 4-13 billion years:

Galaxies as we see them today form and take their final shape. The elliptical and the spiral galaxies with old red stellar populations in

their centers form firstThe spiral galaxies disks form later from infalling surrounding gas.

Our picture of galaxy evolution

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Evolution of Galaxies

1. All galaxies have formed at the same time; they are equally old ~ 14 -15 x 109 yrs

2. Different galactic types are at different evolutionary stages• Elliptical Galaxies formed remarkably early in the universe – all stars born

long ago during giant single star-forming process, exhausting all interstellar star-forming material

• Spiral Galaxies took much longer to form – ongoing star-forming processes

3. Reason for this difference – different physical parameters • Compactness • Rotation• Environment• Hidden matter