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Galaxies

Collection of stars, gas and dust bound together by their common gravitational pull.

Galaxies range from 10,000 to 200,000 light-years in size.

21920’sClassification based on appearance

MW

Handy Way to “Weigh” Spirals

MInside Orbit depends on V2 x D

Type: Spiral Elliptical

Stars Pop I & II Pop II only

ISM Gas/Dust Almost none

Rotation Spinning Disk None

Mass 109-1012 105-1013

Diameter 15,000-150,000 lyr

3,000 -600,000 lyr

Luminosity 108-1011 106 - 1011

DISTANCE !

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1920’s: Hubble begins measuring distances to galaxies (mostly using brightest star method)

1912: Vesto Slipher begins taking spectra of galaxies• “Star-like” (absorption lines)• Very faint - 20/40 hour exposures!• 20 years = 40 galaxies• Most are redshifted (all but closest)

Largest redshift = 1,800 km/s

Edwin Hubble

1925 - Trying to get distance to “Great Spiral Nebula” in Andromeda

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Doppler Effect• Christian Johann Doppler (1803-1853)

• Information about the motion of the object

• Example with sound– Rest frequency– Source approaching the observer: waves bunched up ahead of it –

frequency increases– Source receding from the observer: waves are stretched out –

frequency decreases

lightofspeedwavelengthreal

wavelengthrealwavelengthshiftedobjectofspeed ×−

=

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Fig.06.06

Similar Effect for Light Blue light – small wavelengthRed light – large wavelength

Redshift

lightofspeedwavelengthreal

wavelengthrealwavelengthshiftedobjectofspeed ×−

=

wavelengthrealwavelengthrealwavelengthshiftedredshift −

=

0

−=

λλλ 0z

zcv =Late 1920’s:

Hubble plots his distances versus Slipher’s velocities

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1929 1931

Distance and Velocity (away) are related!v = H x D

Distances to Far Away Galaxies

v = H x D

Measure v from spectrum (redshift), Solve for D:

D = v/H

Example: v = 40,000 km/s and H = 70 km/s/Mpc

Universal recessional motion

• Redshift in almost all galactic spectra• Hubble flow

– Nearby galaxies – Distant galaxies

• Hubble law– Redshift as a distance indicator

• Hubble constant– Current value– Meaning– Significance

The Expansion of the Universe

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Evolution of Galaxies – Current Theory

1. Groups, Clusters, Superclusters of galaxies

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

3. Different galactic types are at different evolutionary stages• Elliptical Galaxies – all stars born long ago during giant single star-forming

process, exhausting all interstellar star-forming material • Spiral Galaxies – ongoing star-forming processes

4. Reason for this difference – different physical parameters of different galactic types

• Compactness • Rotation• Environment• Hidden matter 18

First galaxies - 13.6 billion years ago

0.5 - 1 billion years later larger clumps grow from merging of smaller once

Our picture of galaxy evolution

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1 billion years later - 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

2-4 billion years later – larger irregular looking objects form through collisionsand mergers between these sub-galactic sized clumps

“Pre-galactic Blobs”

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Our picture of galaxy evolution

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Evolution of GalaxiesDifferent for different types

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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

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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

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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

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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

• multinuclear structure – formed by capturing other galaxies or by repeatedly merging with other cluster member

The Local Group

• Members and Size

•Dominant galaxies (M31, MW, M33)

• Dwarf elliptical galaxies

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

•Virgo Cluster at 20 Mpc: moving away from the Local group at 1000 km/s (should be moving at 1300 km/s)

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The Local Group Clusters of galaxies

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

Virgo

Coma

The Coma Cluster

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Obtaining the mass of clusters of galaxies

• First method– Orbits of galaxies in a

cluster are randomly oriented

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

– Average orbital speed– Mass contributing to

gravity of cluster

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Obtaining the mass of clusters of galaxies

• Second method– Count the galaxies – Multiply by the average mass of one galaxy– Add the mass of gas– Cluster’s visible and X-ray-emitting mass

Gravitational vs. Visible Mass

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Gravitational vs. Visible Mass

Fritz Zwicky(1933)

The Coma Cluster

Gravitational / Visible Mass = 10/1

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HST Image of a gravitational lens

Obtaining the mass of clusters of galaxies

Fig. 25.23

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Fig. 25.26

Other SuperclustersSuperclustersVirgoComaCentaurus…

Large Sheets of GalaxiesGreat WallPerseus-Pisces Great Wall…

Voids

“Great Wall”Large single structure Dimensions are about 600x250x30 million light yearsGiant quilt of galaxies across the sky

Fig. 25.27

Mapping the UniverseA slice of the Universe

Redshift surveys are ways of mapping the distribution of galaxies around us

Redshift - radial velocity –distance

Distance + coordinates = location within Universe

Spherical coordinate system centered on the Milky Way

Location of galaxies in slices of the Universe centered on the Earth

43Each point represents a galaxy in the northern celestial hemisphere that is brighter than an apparent blue magnitude of 15.5

Red color: v < 3000 km/s (Virgo Cluster: the core of the Local Supercluster) Blue color: 3000 < v < 6000 km/s (Pisces-Perseus Supercluster)Magenta: 6000 < v < 9000 km/s Cyan: 9000 < v < 12000 km/s Hercules SuperclusterGreen: 12000 < v km/s

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Why are there voids and sheets?• Large scale structure of the Universe: stars - galaxies - clusters and superclusters,

separated by voids

• Prior to 1989 it was commonly assumed that the superclusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction

• In 1989, "Great Wall", a sheet of galaxies more than 500 million light years long and 200 million wide, but only 15 million light years thick

• Distribution of visible matter, Actual distribution of matter

• Are the voids empty?

• Sheets form along and follow web-like strings of dark matter

• It is this dark matter which dictates the structure of the Universe on the grandest of scales

• Dark matter gravitationally attracts normal matter and it is this normal matter that we see forming thin long walls of Super-Galactic clusters.

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The Great Attractor

• The galaxies pictured here are part of a cluster of galaxies called ACO 3627 (or the Norma cluster) near the center of the Great Attractor