Chapter 25 Galaxies and Dark Matter · 2009. 4. 17. · 25.5 The Universe on Large Scales The Sloan Digital Sky Survey 25.1 Dark Matter in the Universe 25.2 Galaxy Collisions 25.3

Chapter 25Galaxies and Dark Matter

25.5 The Universe on Large Scales

The Sloan Digital Sky Survey

25.1 Dark Matter in the Universe

25.2 Galaxy Collisions

25.3 Galaxy Formation and Evolution

XX25.4 Black Holes in Galaxies

Units of Chapter 25

Galaxy clusters join inlarger groupings, calledsuperclusters. This is a 3-D map of the LocalSupercluster, of whichour Local Group is apart. It contains tens ofthousands of galaxies.


This slice of a larger galactic survey shows that, onthe scale of 100–200 Mpc, there is structure in theUniverse—walls and voids.


This survey, extendingout even farther, showsstructure on the scale of100–200 Mpc, but nosign of structure on alarger scale than that.

The decreasing density ofgalaxies at the farthestdistance is due to thedifficulty of observingthem.


Quasars are all very distant, and the light comingto us from them has probably gone through manyinteresting regions. We can learn about theintervening space by careful study of quasarspectra.


This “Lyman-alpha forest” is the result of quasarlight passing through hundreds of gas clouds, eachwith a different redshift, on its way to us:


This appeared at first to be a double quasar, but oncloser inspection the two quasars turned out to be notjust similar, but identical—down to their

luminosityvariations. This isnot two quasarsat all—it is twoimages of thesame quasar:


This could happen via gravitational lensing. Fromthis we can learn about the quasar itself, as there isusually a time difference between the two paths.We can also learn about the lensing galaxy byanalyzing the bending of the light.


Here, the intervening galaxy has made four images ofthe distant quasar:


These are two spectacular images of gravitational lensing:On the left is distant galaxies being imaged by a wholecluster. On the right is a cluster with images of what isprobably a single galaxy.


On the left is a visible image of a cluster of galaxies:on the right, to the same scale, is the dark matterdistribution inferred from galaxy motion:



This composite image shows two clusters ofgalaxies colliding, with the galaxies in white andthe intracluster gas in red:

Discovery 25-1:The Sloan Digital Sky Survey

The Sloan Digital Sky Survey is being done by adedicated telescope situated in New Mexico. Itspurpose is to measure hundreds of millions ofcelestial objects, with five intensity pointsspanning the visible and near-infraredwavelengths. Approximately one million of thesealso have their redshifts measured, makingpossible very detailed redshift maps.

• Galaxy masses can be determined by rotationcurves and galaxy clusters.

• All measures show that a large amount of darkmatter must exist.

• Large galaxies probably formed from the merger ofsmaller ones.

• Collisions are also important.

• Merger of spiral galaxies probably results in anelliptical galaxy.

Summary of Chapter 25

• Quasars, active galaxies, and normal galaxies mayrepresent an evolutionary sequence.

• Galaxy clusters are gravitationally bound intosuperclusters.

• The Universe has structure up to 100–200 Mpc;beyond that, there is no sign of it.

• Quasars can be used as probes of interveningspace, especially if there is galactic lensing.

Summary of Chapter 25 (cont.)

Other galaxies haverotation curves similar toours, allowingmeasurement of theirmass:


Another way to measurethe average mass ofgalaxies in a cluster is tocalculate how much massis required to keep thecluster gravitationallybound.


Galaxy mass measurements show that galaxies needbetween 3 and 10 times more mass than can beobserved to explain their rotation curves.

The discrepancy is even larger in galaxy clusters, whichneed 10 to 100 times more mass. The total needed ismore than the sum of the dark matter associated witheach galaxy.


This image may show a galaxy interacting with anunseen neighbor—a “dark galaxy”:


There is evidence forintracluster superhotgas (about 10 millionK) throughout clusters,that is densest in thecenter:


This head–tail radio galaxy’s lobes are beingswept back, probably because of collisions withintracluster gas:


It is believed this gas is primordial—dating fromthe very early days of the Universe.

There is not nearly enough of it to be the neededdark matter in galaxy clusters.


The separation between galaxies is usually not largecompared to the size of the galaxies themselves, andgalactic collisions are frequent.

The “cartwheel” galaxyon the left appears tobe the result of a head-on collision withanother galaxy,perhaps one of thoseon the right.


This galaxy collision has led to bursts of star formationin both galaxies; ultimately they will probably merge:


The Antennae galaxies collided fairly recently,sparking stellar formation. The plot on the right isthe result of a computer simulation of this kind ofcollision:


Galaxies are believed to have formed from mergersof smaller galaxies and star clusters. Image (c)shows large star clusters found some 5000 Mpcaway. They may be precursors to a galaxy.


This Hubble Deep Field view showssome extremely distant galaxies. Themost distant appear irregular,supporting the theory of galaxyformation by merger.


These starburst galaxiesshow evidence ofmassive, recent activity.Image (a) is clearlygalaxies in collision.Image (b) appears tohave a supermassiveblack hole at its center.Image (c) is also theresult of a collision.


Here, many small galaxies are in the process ofmerging into one larger one:



The Milky Way galaxy also contains stars in its halothat appear to have been the result of the captureof smaller galaxies:

This simulation shows how interaction with asmaller galaxy could turn a larger one into a spiral:



Mergers of two spiralgalaxies probablyresult in an ellipticalgalaxy; the merger ofa spiral galaxy and adwarf galaxy probablyresults in a largerspiral galaxy.

This galaxy is viewed inthe radio spectrum,mostly from 21-cmradiation. Doppler shiftsof emissions from thecore show enormousspeeds very close to amassive object—a blackhole.

25.4 Black Holes in Galaxies


The mass of the central black hole is wellcorrelated with the mass of the galactic bulge,for those galaxies where both have beenmeasured.

These visible and X-rayimages show twosupermassive black holesorbiting each other at adistance of about 1 kpc.

They are expected tomerge in about 400million years.


The quasars we see are very distant, meaning theyexisted a long time ago. Therefore, they mayrepresent an early stage in galaxy development.

The quasars in thisimage are shownwith their hostgalaxies; manyappear to beinvolved incollisions.


The end of the quasar epoch seems to have beenabout 10 billion years ago; all the quasars we haveseen are older than that.

The black holes powering the quasars do not goaway; it is believed that many, if not most,galaxies have a supermassive black hole at theircenters.


This figure shows how galaxies may have evolved, fromearly irregulars through active galaxies, to the normalellipticals and spirals we see today.


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Chapter 25 Galaxies and Dark Matter · 2009. 4. 17. · 25.5 The Universe on Large Scales The Sloan Digital Sky Survey 25.1 Dark Matter in the Universe 25.2 Galaxy Collisions 25.3