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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 billions of galaxiesin the observable

are irregular.

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Irregulars

• Ellipticals: no spiral arms, no disk, old stars, little gas and dust.• S0: disk, no spiral arms, 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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Distance Measurements

• 1 Radar (solar system distances)1. Radar (solar system distances).• 2. Trigonometric parallax (hundreds of light years).• 3. Main sequence fitting(thousands of light years).• 4. Cepheid period – luminosity relations (millions of light

years).• 6. White dwarf supernovae (hundreds of millions of light

years).• 7. Galaxy luminosities (~ 500 Mpc).• 8. Hubble’s Law( billions of light years).

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 publish this. According to Einstein’s general theory of relativity, we understand that this phenomenon is actually due to the expansion of space itself.

Hdr G

r G

v 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 22 km/s/Mly

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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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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

i i f h2v rMG

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”.

Causes of the Variety of Galaxy Types

• High density regions→ rapid collapse→ rapid star• High density regions→ rapid collapse→ rapid star formation→ depletion of gas needed for star formation→ elliptical galaxy.

• High angular momentum regions→ slower collapse, formation of a disk→ spiral galaxy.

• Galaxy interactions→ distortion of galaxy shapes• Galaxy interactions→ distortion of galaxy shapes and rapid star formation→ irregular galaxies and starburst galaxies→ elliptical galaxies.

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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.

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

Galaxies with Active Nuclei

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Seyfert Galaxies• Broad emission lines of highly ionized atoms.• Velocities of matter near center ~ 10,000 km/s ⇒ supermassive black holes.• 2% of all galaxies are Seyferts.• Seyfert 1: very luminous at X and UV, broad emission lines with sharp,

narrow cores.narrow cores.• Seyfert 2: No strong X-ray emission, width of lines greater than normal

galaxies but smaller than Seyfert 1.• Rapid fluctation of core brightness ⇒ nucleus smaller than a few light

minutes.• Brightest have luminosities about

100 times that of the Milky Way Galaxy.• Activity due to interactions with other

l igalaxies.

Radio Galaxies

• Double lobed radio sources with a galaxy between the two lobes.• Deformed or interacting with other galaxies.• Complex shapes of some radio lobes related to motions of galaxies.

D t tt fl i i t ti di k d bl k h l• Due to matter flowing into an accretion disk around a black hole.• Emit synchrotron radiation, indicating the presence of relativistic charged

particles moving in a magnetic field.• “Hot” spots at outer extremes of the lobes.

Cygnus A

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Centaurus A

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Supermassive Black Holes

• M87: Accretion disk + jet along rotation axis.60 ly from center gas orbits at 750 km/s ⇒ 2.4 billion solar mass black hole.mass black hole.

• NGC7052: 300 million solar mass black hole.• Milky Way galaxy: 2.6 million solar mass black hole.• Most galaxies observed so far contain black holes with

mass equal to about 0.05 times the mass of their nuclear bulges.

h f ll i h i di k d• When matter falls into the accretion disk around a supermassive black hole, about 50% of the mass is converted into energy. The rest is assimilated by the black hole.

Unified Model

• Supermassive black hole.• Hot, thick inner disk.• Outer disk, cooler, thick, dusty, and doughnut shaped.• Inclination of accretion disk ≠ inclination of the galaxy.• Accretion disk seen edge-on – Seyfert 2.• Accretion disk seen tipped at a small angle – Seyfert 1.• Line of sight into the central cavity – Blazar (BL Lac object) – 10,000 times

more luminous than the Milky Way Galaxy.

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Quasars (QSO’s)

• Initially observed as star-like points of light.• Very large redshifts (as large as 6+): Hubble lawVery large redshifts (as large as 6+): Hubble law ⇒ very distant.

• First one discovered: 3C48 (redshift = 0.37).• L = 10 – 1000 times the L of a large galaxy.• Rapid fluctuations ⇒ small objects.• Quasar “fuzz” with spectrum like that of a normal

galaxy (stellar absorption lines) suggests thatquasars are distant galaxies.

• About 100,000 have been found.