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Chapter 21: Problem 26

Choose the best answer to each of the following. Explain your reasoning with one or more complete sentences. A collision and merger of two large elliptical galaxies will eventually produce (a) a large elliptical galaxy. (b) a large spiral galaxy. (c) a large irregular galaxy.

Short Answer

Expert verified
The result is (a) a large elliptical galaxy.

Step by step solution

01

Understand the Nature of Galaxies

Galaxies come in various shapes, primarily elliptical, spiral, and irregular. Elliptical galaxies have a smooth, homogeneous appearance without defined structures like spirals. They often result from collisions because the random motions of stars eliminate any previous orderly rotation.
02

Analyze the Outcomes of Galaxy Collisions

When two galaxies collide, their stars pass by each other due to the vast distances between them, but their gravitational pulls affect each other significantly. Gas within the galaxies can collide, ignite star formation, and potentially form different shapes.
03

Consider Collisions Between Elliptical Galaxies

Elliptical galaxies mainly consist of older stars and have little gas and dust compared to spiral galaxies. Therefore, when two large elliptical galaxies collide and merge, there isn't typically enough gas to form new stars or structures like a spiral galaxy.
04

Determine the Likely Result of the Collision

Given that two elliptical galaxies merge and there is minimal gas to form extensive new structures or star formation, the resulting galaxy is likely to remain elliptical. The stars will redistribute to form a larger elliptical galaxy without forming the distinct arms of a spiral galaxy.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Elliptical Galaxies
Elliptical galaxies are a distinct type of galaxy characterized by their smooth, featureless appearance. Unlike their spiral siblings, elliptical galaxies lack the grand spirals or well-defined structures. They are often more spherical or elongated in shape. Here are some key aspects of elliptical galaxies to better understand their nature and behavior:

  • Composition: They predominantly contain older, red stars. These stars have low metal content, signifying that not much new star formation is occurring within their bounds.

  • Stellar Population: The stars within elliptical galaxies move in random orbits, compared to the organized disk of a spiral galaxy. This lack of order is primarily due to past mergers and collisions.

  • Gas and Dust: Elliptical galaxies have little gas or dust, essential components for creating new stars. Consequently, they seldom engage in new star formation activities.

Understanding these characteristics helps explain why the outcome of their collision tends to result in a larger elliptical galaxy. Since these galaxies are largely devoid of gas and dust, there鈥檚 less potential for substantial new star formation or the creation of spiral arms indicative of spiral galaxies.
Stellar Dynamics
Stellar dynamics play a pivotal role in the life and behavior of galaxies, including elliptical ones. In essence, it deals with how stars interact with each other on a gravitational level. Here鈥檚 a deeper dive into stellar dynamics as it pertains to galaxy evolutions and mergers:

  • Random Orbits: In elliptical galaxies, the stars follow random paths rather than orderly orbits. These paths are the result of gravitational interactions during galaxy formation and subsequent mergers.

  • Gravitational Influence: During galaxy collisions, although individual stars rarely collide due to the vast distances between them, the overall gravitational interactions are significant. These interactions dictate how galaxies will merge and what shape they might take afterward.

  • Energy and Motion: Merging processes conserve energy and momentum. The stars are redistributed rather than disappeared, thus as elliptical galaxies merge, they form an even larger elliptical galaxy due to the energy scales involved.

Stellar dynamics influence the final shape of merged galaxies. When elliptical galaxies, with their randomized stellar motion, merge, the resulting merger generally maintains this characteristic due to the conservation of energy and momentum.
Galactic Evolution
Galactic evolution is the process by which galaxies form and change over time. Mergers, like those of elliptical galaxies, are significant events that drive this evolution. Here's how they contribute to galactic evolution:

  • Merger Process: Galaxy mergers, a critical phase of galactic evolution, involve the integration of two or more galaxies. These events can initiate different evolutionary paths depending on the components of the merging galaxies.

  • Transformation and Growth: Although new stars form predominantly from gas-rich spiral mergers, elliptical galaxy mergers lead to size growth rather than new star creation. Over time, large ellipticals may form via multiple mergers.

  • Long-term Stability: Post-merger, the dynamics settle into a stable elliptical form. With little gas present for triggering star formation, the galaxy's development focuses on existing stellar populations.

Understanding galactic evolution through the lens of mergers provides insight into why elliptical galaxies typically remain elliptical after a merger. These events highlight the importance of initial galactic composition and dynamics in determining the resultant galaxy structure.

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Most popular questions from this chapter

Be sure to show all calculations clearly and state your final answers in complete sentences. Distances Between Galaxies. If you were to divide the present-day universe into cubes whose sides are 10 million light-years long, each cube would contain, on average, about one galaxy similar in size to the Milky Way. Now suppose you travel back in time, to an era when the average distance between galaxies is one-quarter of its current value, corresponding to a cosmological redshift of \(z=\) 3\. How many galaxies similar in size to the Milky Way would you expect to find, on average, in cubes of that same size? In order to simplify the problem, assume that the total number of galaxies of each type has not changed between then and now. Based on your answer, would you expect collisions to be much more frequent at that time or only moderately more frequent?

Be sure to show all calculations clearly and state your final answers in complete sentences. Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all these have definitive answers, so your explanation is more important than your chosen answer. Galaxies that are more than 10 billion years old are too far away for us to see even with our most powerful telescopes.

Orbits Around Supermassive Black Holes. The data in Figure 21.22 show the Doppler shifts of emission lines from gas at a distance of 60 light-years from the center of the galaxy M87. Suppose you observed emission lines from gas 30 light-years from the center. How would you expect the Doppler shifts of those lines to be different, assuming that the gas really is orbiting a supermassive black hole? What about gas at 120 light-years from the center?

Future Observatories. Galaxy evolution is a very active area of research. Look for information on future observatories that will investigate galaxy evolution (such as the James Webb Space Telescope). How big are the planned telescopes? At what wavelengths will they look? When will they be built? Write a short summary of one or two proposed missions.

Briefly describe the discovery of quasars. What evidence convinced astronomers that the large redshifts of quasars really do imply great distances? Why can we learn more about quasars by studying nearby active galactic nuclei and radio galaxies?
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