Ch 28 Evolution and Distribution of Galaxies.pdf - Chapter...

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Chapter Outline28.1Observations of Distant Galaxies28.2Galaxy Mergers and Active Galactic Nuclei28.3The Distribution of Galaxies in Space28.4The Challenge of Dark Matter28.5The Formation and Evolution of Galaxies and Structure in the UniverseThinking AheadHow and when did galaxies like our Milky Way form? Which formed first: stars or galaxies? Can we see directevidence of the changes galaxies undergo over their lifetimes? If so, what determines whether a galaxy will“grow up” to be spiral or elliptical? And what is the role of “nature versus nurture”? That is to say, how muchof a galaxy’s development is determined by what it looks like when it is born and how much is influenced by itsenvironment?Astronomers today have the tools needed to explore the universe almost back to the time it began. The hugenew telescopes and sensitive detectors built in the last decades make it possible to obtain both images andspectra of galaxies so distant that their light has traveled to reach us for more than 13 billion years—more than90% of the way back to the Big Bang: we can use the finite speed of light and the vast size of the universe as acosmic time machine to peer back and observe how galaxies formed and evolved over time. Studying galaxiesso far away in any detail is always a major challenge, largely because their distance makes them appear veryfaint. However, today’s large telescopes on the ground and in space are finally making such a task possible.Figure 28.1 Colliding Galaxies.Collisions and mergers of galaxies strongly influence their evolution. On the left is a ground-based image oftwo colliding galaxies (NCG 4038 and 4039), sometimes nicknamed the Antennae galaxies. The long, luminous tails are material torn out of thegalaxies by tidal forces during the collision. The right image shows the inner regions of these two galaxies, as taken by the Hubble SpaceTelescope. The cores of the twin galaxies are the orange blobs to the lower left and upper right of the center of the image. Note the dark lanesof dust crossing in front of the bright regions. The bright pink and blue star clusters are the result of a burst of star formation stimulated by thecollision. (credit left: modification of work by Bob and Bill Twardy/Adam Block/NOAO/AURA/NSF; credit right: modification of work by NASA,ESA, and the Hubble Heritage Team (STScI/AURA)-ESA/Hubble Collaboration)28THE EVOLUTION AND DISTRIBUTION OF GALAXIESChapter 28 The Evolution and Distribution of Galaxies995
28.1OBSERVATIONS OF DISTANT GALAXIESLearning ObjectivesBy the end of this section, you will be able to:Explain how astronomers use light to learn about distant galaxies long agoDiscuss the evidence showing that the first stars formed when the universe was less than 10% of itscurrent ageDescribe the major differences observed between galaxies seen in the distant, early universe and galaxiesseen in the nearby universe todayLet’s begin by exploring some techniques astronomers use to study how galaxies are born and change overcosmic time. Suppose you wanted to understand how adult humans got to be the way they are. If you werevery dedicated and patient, you could actually observe a sample of babies from birth, following them throughchildhood, adolescence, and into adulthood, and making basic measurements such as their heights, weights,and the proportional sizes of different parts of their bodies to understand how they change over time.

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