1

Which of the following is least easily explainable as a result of interaction between galaxies? a) Some galaxies have long "tails" of stars. b) Rich, regular clusters are dominated by central giant ellipticals. c) Both spiral and elliptical galaxies are seen at very high redshift. d) Some galaxies seems to be undergoing bursts of star formation.

Rich and Poor Galaxy Clusters• Rich clusters:

– Contain hundreds tothousands of member galaxies

– Are roughly spherical, withthe largest galaxies near thecenter

– Contain mostly elliptical andtype S0 galaxies

– Lots of hot gas and dust• Poor clusters

– Contain only tens of galaxies– Have a ragged, irregular

appearance– More spiral and irregular

galaxies

2

Superclusters• Clusters of clusters are called

superclusters– Contain a few to many dozen

clusters of galaxies– Can be Mpc across!– The Local Group is part of the

Local Supercluster, shown at left.• The Local Supercluster is heading

toward a region of space knownas the Great Attractor, where thereare a large number of massivesuperclusters

• There may be super-superclusters!

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The most likely reason that clusters of galaxies havemore elliptical than spiral galaxies is that in the highdensity cluster environment

a) spirals merge to form ellipticals.b) intracluster gas strips galaxies of the gas needed for

star formation.c) near-misses between galaxies makes them rounder.d) galaxies are older and their brighter disk stars have

burned out.

Large Scale Structure in the Universe

• Using modern technology,astronomers have mapped thelocation of galaxies andclusters of galaxies in threedimensions

• Redshift is used to determinedistance to these galaxies

• Galaxies tend to form longchains or shells in space,surrounded by voidscontaining small or dimgalaxies

• This is as far as we can see!

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Seyfert Galaxies

• Seyfert galaxies arespiral galaxies withextremely luminouscentral bulges

• Light output of thebulge is equal to thelight output of thewhole Milky Way!

• Radiation from Seyfertgalaxies fluctuatesrapidly in intensity

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Radio Galaxies

• Radio Galaxies emitlarge amounts ofenergy in the radio partof the spectrum

• Energy is generated intwo regions– Galactic nucleus– Radio lobes on either

side of the galaxy• Energy generated by

energetic electrons– Synchrotron radiation– Electrons are part of the

gas shooting out of thecore in narrow jets

Quasars

• Quasars are small, extremelyluminous, extremely distantgalactic nuclei– Bright radio sources– Name comes from Quasi-Stellar

Radio Source, as they appeared tobe stars!

– Can have clouds of gas near them,or jets racing from their cores

– Spectra are heavily redshifted,meaning they are very far away

– Energy output is equivalent to onesupernova going off every hour!

• The HST was able to image aquasar, showing it to be the activecore of a distant galaxy

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Energy Source for ActiveGalactic Nuclei

• Active galactic nuclei emit atremendous amount ofradiation over a broad rangeof wavelengths

• A black hole can be bothvery small, and have anaccretion disk that can emitenough radiation

• Likely that at the centers ofthese galactic nuclei, thereare supermassive black holes

• Intense magnetic fields inthe accretion disk pumpsuperheated gas out into jetsthat leave the nucleus

• There are still manyquestions to be answered…

Figure 77.08b