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GalaxiesHubble deep field:Combination of 276 exposures taken over a 10 d i d f

More ellipticalsthan spirals, 25% are irregular.

Spiral Elliptical Lenticular Irregular

10-day period of a one arcminute area of sky in Ursa Major.

There are hundreds of billion of galaxiesin the observable

are irregular.

About 70% ofthose in catalogsAre spirals. Why?

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Irregulars

• Ellipticals: no spiral arms, no disk, old stars, little gas and dust.• S0: disk, little gas and dust• Spirals: spiral arms, disk, both young and old stars, lots of gas and dust.• Irregulars: irregular shapes, lots of gas and dust.

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28 Million light years from earth, 50 thousand light years diameter, 800 billion solar masses

M87 – a Giant Elliptical Galaxy

Long exposure, halo containing about 500 globular

Short exposure, jet in nuclear region.

globular clusters.

12M 3 10 M= ×

diameter 120 thousand light years≈ distance 60 million light years≈

M87 has a several billion solar mass black hole in its nucleus.

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The Large Magellanic Cloud

Copyright: AURA/NOAO/NSF

distance 180 thousand light years≈

NGC5886 – a type S0 (lenticular) galaxy

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Distance Measurements

• Trigonometric parallax several hundred parsecs)• Trigonometric parallax several hundred parsecs)• Spectroscopic parallax (~ 50 kpc)• Period – luminosity relations (50 Mpc)• Globular cluster sizes and luminosities.• Type Ia supernovae (~ 150 Mpc)• Galaxy luminosities (~ 500 Mpc)• v = Hd (approximation)

Hubble’s Law

Except for a few of the nearest galaxies, all galaxies in the universe are moving away from us. Edwin Hubble was the first astronomer to notice this. Using Einstein’s general theory of relativity, we understand that this phenomenon is actually due to the expansion of space itself. The following equation is known as Hubble’s Law.

Hdv Hdv = the radial speed of the galaxy, d = its distance from Earth, and H is the "Hubble constant".=

The currently accepted value of H is about 70 km/s/Mpc

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Measurement of Galaxy Masses• Rotation curve method

orbital speed at distance r gives mass inside radius r.

• Cluster methodradial speeds of many galaxies in a cluster and size of the cluster → How much mass is needed to prevent the galaxies with the highest speeds from l i th l t f l i th l tleaving the cluster → average mass of a galaxy in the cluster.

• Velocity dispersion methodMotion of matter in a galaxy → mass of the galaxy

The mass of stars, gas, and dust in a galaxy is only about 10% of its total mass. The

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Galaxy masses range from ~ 10-6 to ~ 50 times the mass of the Milky Way galaxy.

composition of the remaining 90% is unknown and is referred to as “dark matter”.

Interactions, Clustering, and Evolution

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Galaxy Interactions

Hubble Space Telescope Images

420 million light years away6000 galaxies in background

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300 million light years away

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Rich Galaxy Clusters• 1000 or more galaxies• ~ 3 Mpc diameter.• Galaxies are concentrated toward center• Galaxies are concentrated toward center• Many have one or more giant elliptical galaxies at the

center• X-ray observations show that many are filled with hot gas

(intracluster medium), partly due to supernovae explosions.

• Contain more elliptical and S0 than spiral galaxies -Contain more elliptical and S0 than spiral galaxies probably due to (a) “stripping” of gas and dust from moving spiral galaxies by “friction” with intracluster medium and (b) gravitational interactions between galaxies.

The Virgo Cluster2500 galaxies17 Mpc from Earth,with giant elliptical galaxy M87 atgalaxy M87 at center

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Poor Galaxy Clusters• Less than 1000 galaxies.• About the same size as rich clusters of galaxies.• No concentration toward cluster center.• Less dense intracluster medium - stripping has small

impact on spiral galaxies.• Collisions less common than in rich clusters.• Spiral galaxies are a larger fraction of the total number of

galaxies in a poor cluster than in a rich cluster.• Milky Way Galaxy is a member of a poor cluster called the

Local Group.

Hercules ClusterLess than 100 galaxiesAb 190About 190 Mpc from Earth.

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The Local GroupPicture and data fromhttp://www.anzwers.org/free/universe/localgr.html

3 large galaxies within 5 Mly36 dwarf galaxies within 5 Mly

At least 40 members

Superclusters• Clusters of clusters.• Typical dimensions are tens of Mpc• Typical dimensions are tens of Mpc.• Local Cluster belongs to the Local

Supercluster (diameter ~ 25 to 50 Mpc, disk-shaped, Virgo cluster near center).

• Superclusters are joined to form filaments, p j ,which surround vast empty spaces called voids. Sizes range from 75 to 150 Mpc.

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Filaments and VoidsNorth Slice

Great Wall

Southern Wall

South Slice

Southern Wall

Origin and Evolution of Galaxies• Elliptical galaxies contain almost no gas and dust and no new stars.• Spirals contain plenty of gas and dust as well as both old and new

stars.• Therefore elliptical galaxies can’t evolve to become spiral galaxies;• Therefore, elliptical galaxies can t evolve to become spiral galaxies;

the absence of gas and dust won’t permit it.• Spiral and irregular galaxies can’t evolve to become elliptical galaxies;

the presence of both old and new stars in them shows that they are not young.

• Spiral galaxies retain lots of gas and dust; these would have been expelled and used up in star formation if they had experienced any major collisions.Tid l f i j lli i ld h h hi di k f• Tidal forces in major collisions would have torn apart the thin disks of spiral galaxies.

• We conclude that spiral galaxies haven’t experienced any major collisions.

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Starburst Galaxies

M82Effect of Collisions and Mergers:

ejection of gas and dust in the collision

Deep space imaging with modern telescopes reveals that billions of

processthe consumption of gas and dust in rapidbursts of star formationthe ejection of gas and dust as a result ofsupernovae

Deep space imaging with modern telescopes reveals that billions of years ago

there were more spirals than ellipticals.galaxies were more compact and more irregular.galaxies were closer together; ~30% were in pairs, compared to ~7% now.This suggests that the present galaxies probably formed by merging.