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Normal and Active Galaxies. Three different types of Galaxies. Spirals. Hubble devised a classification scheme to describe different shapes. Spirals typically have a center of older stars, and spiral arms which are sites of star formation. . Ellipticals.

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Presentation Transcript
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Three different types of Galaxies

Spirals

Hubble devised a classification scheme to describe different shapes. Spirals typically have a center of older stars, and spiral arms which are sites of star formation.

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Ellipticals

Ellipticals have no spiral arms, no flattened disk, very little dust or star formation in progress.

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Irregulars

Irregular galaxies don’t fit into nice categories. They tend to be rich in dust, gas, and active star formation regions. Believed to be the result of “collisions” of normal spiral galaxies.

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We have two irregular galaxies in orbit around the Milky Way called the Magellanic Clouds which were named after the famous explorer.

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Since more massive galaxies are likely to be brighter, and because more massive galaxies are likely to rotate faster. The Tully-Fisher relation allows us to estimate the luminosity of the galaxy by its rotation speed.

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Distribution of Galaxies in Space

Distribution is not random. Galaxies are concentrated in some regions which are beside regions which are empty or void.

Local Group of about 20 galaxies.

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When Hubble began to take spectra of the galaxies, he eventually noticed that the spectral lines where extremely red shifted. Using, primarily Cepheid variables to determine their distance, he noticed a tight correlation between velocity and distance

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The spectrum from normal galaxies is dominated by black body radiation from stars.

The spectrum from active galaxies have an additional component due to synchrotron radiation.

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Galaxy Interactions

When galaxies “collide” they pass through each other without stars actually colliding. We think this is how irregular galaxies are made.

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Galaxy Interactions

When galaxies “collide” they pass through each other without stars actually colliding. We think this is how irregular galaxies are made.

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Synchrotron radiation is caused by high speed electrons spiraling around strong magnetic field lines. The radiation is stronger in the radio part of the spectrum and these are called radio galaxies

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Radio loud galaxies were found in radio surveys but their visible image looked faint and star like and were called Quasi Stellar Radio Sources or Quasars for short.

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Because active galaxies vary in brightness over a period of months, they must be at most a few light months in size.

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We can measure the Doppler velocities and combine this with distances to use Kepler’s laws to determine the mass of the “engine”. Answer (3 billion suns).

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The source of the radio emission is called Synchrotron radiation which is produced by accelerating electrons. This radiation has a fundamentally different spectrum than black body radiation.