PHYS 1303, Stars & Galaxies, Chap. 25, Homework & Test Review, Prof. Kaim, DMC

temperature-energy relationship

- hotter objects emit more radiation than colder objects - temperature is directly proportional to the level of energy

Evolution and interactions among galaxies Most fully-formed galaxies reside in small groups and clusters, where they can interact with other galaxies. These interactions can have major effects on the characteristics of a galaxy. Use the labels to complete the descriptions of how interactions can cause galaxies to evolve over time. Match the words in the left column to the appropriate blanks in the sentences on the right. Make certain each sentence is complete before submitting your answer. a) a rapid and intense period of star formation b) a minor merger c) a tidal stream of stars d) a major merger e) galactic cannibalism

1. A starburst galaxy is characterized by (a) a rapid and intense period of star formation. 2. A minor galaxy merger in which a large galaxy consumes a smaller galaxy is called (e) galactic cannibalism. 3. As two galaxies interact and strip stars from each other, (c) a tidal stream of stars can form. 4. When two large galaxies collide, (d) a major merger occurs, turning spirals into ellipticals with no ongoing star formation. (Isolated galaxies will predictably evolve as the gas available for star formation is used up and as stars evolve into white dwarfs, neutron stars and black holes. However, the evolution of galaxies in groups and clusters is quite different because repeated interactions and mergers between galaxies in dense environments may alter their internal evolution. Simulations of major and minor mergers show that such interactions can cause bursts of new star formation, tidal streams, and galactic cannibalism. The idea that major and minor mergers affect galaxy evolution is supported by observations of physical consequences of merging events within galaxies. Observations have also shown that spiral galaxies were more common far in the past and in less crowded environments, whereas elliptical galaxies were more common in the recent past and in crowded environments.)

Part C - Evolution of Active and Normal Galaxies Astronomers today believe that normal bright galaxies may have evolved from active galaxies, which would explain the greater incidence of intense quasar activity during the quasar epoch 10 - 12 billion years ago. Using the figure below, complete each statement about the evolution of galaxies with the appropriate phrase. The figure shows a number of images connected by arrows from left to right. Leftmost are four small yellow light blurs. Each pair of them runs to the right into a yellow light circle, with black dot at its center. One of that circles is labeled as a. These two circles run into a bigger circle, labeled as b, with black dot at its center and cloudy tails around it. Two b circles run into a white, extremely bright circle, labeled as c, with two opposite smoke jets firing from it and a black dot. Finally, c stage run into white light circle, labeled as d, with black dot at its center. One arrow points from b stage circle somewhere beyond the figure. 1. At position A, small irregular galaxies merge to create ______________. 2. At position B, fueling of the supermassive black hole in a massive bulge produces ______________. 3. At position C, mergers of large galaxies produce ______________. 4. At position D, as nuclear fueling and activity winds down, _____________ is observed. a) a radio galaxy b) a normal elliptical galaxy c) intense quasar activity d) a large galaxy with a central black hole.

1. At position A, small irregular galaxies merge to create (d) a large galaxy with a central black hole. 2. At position B, fueling of the supermassive black hole in a massive bulge produces (c) intense quasar activity. 3. At position C, mergers of large galaxies produce (a) a radio galaxy. 4. At position D, as nuclear fueling and activity winds down, (b) a normal elliptical galaxy is observed. (Models for galaxy evolution propose that the universe began forming small irregular galaxies first. These small galaxies merged to form large galaxies with central black holes. Through successive merging of large galaxies into ever larger galaxies, massive bulges and supermassive black holes were built up, creating quasars, radio galaxies, and other active galaxies. Supermassive black holes sustained nuclear activity as they accreted infalling matter during the quasar epoch. As fuel for accretion ran out, the supermassive black holes within active galaxies became dormant, and the galaxies settled down into the normal elliptical and spiral galaxies.)

What is the threshold of fusion of hydrogen into heavier elements?

10 million Kelvins

How much does the visible matter of the Universe cover?

5%

Discuss some theories of how galaxies form and evolve. The formation and evolution of galaxies is not as well understood as the formation and evolution of stars. However, astronomers do have some general ideas about how galaxies form and evolve. Observational and theoretical evidence suggests that galaxies formed when smaller objects merged very early in the history of the universe in a process called hierarchical merging. The merging of larger galaxies in later epochs is thought to play a role in galaxy evolution. Part A - Mergers and their effects on galaxy properties Astronomers believe that early galaxies grew from the repeated mergers of smaller gas clouds. If this idea is true, then the properties of galaxies must have changed over time. Determine whether each property below increases or decreases over time, and then sort each property into the appropriate bin. a) number of galaxies b) disordered motions of stars within galaxies c) irregular structure of galaxies d) size of galaxies

Increase: d) size of galaxies b) disordered motions of stars within galaxies Decrease: c) irregular structure of galaxies a) number of galaxies (The theory of hierarchical merging predicts that galaxies in the early universe were significantly smaller, more irregular in shape, and more numerous. Small irregular galaxies merged into larger spiral and elliptical galaxies of more regular shape. Over time, the total number of galaxies decreased as a result of these mergers. This evolutionary progression is depicted in the following figure. The figure shows three stages of the universe. First is an array of small bluish objects of different shapes. Second is an array of middle-sized blue spiral objects and yellow blots. Third is an array of big light spirals and small yellow and blue irregular forms.)

Part A Rank the following items according to their size (diameter) from left to right, from largest to smallest. a) Jupiter b) the Sun c) the Local Group d) the local supercluster e) the universe f) our solar system g) Earth h) the Milky Way Galaxy

Largest e) the universe d) the local supercluster c) the Local Group h) the Milky Way Galaxy f) our solar system b) the Sun a) Jupiter g) Earth Smallest

How can observations of distant quasars be used to probe the space between them and Earth?

Light from quasars travel through the universe through a vast amount of space on the way to the Earth. The information gathered from the voyage the light experienced helps us to create a picture of what exists in our universe. For example, when we see quasars it may appear as though there are two next to each other - in reality, the dark matter in between the quasar and the next galaxy causes gravitational lensing (mirror imaging.

Part B Rank the following items that describe distances from longest distance (left) to shortest distance (right). (If two distances are equal, drag the second item on top of the first item.) a) the distance from the Sun to the center of the Milky Way Galaxy b) the distance across out solar system (to Neptune) c) one light-year d) one astronomical unit (AU) e) the average distance from Earth to the Sun f) the distance from Earth to Alpha Centauri g) the distance from the Milky Way Galaxy to the Andromeda Galaxy

Longest g) the distance from the Milky Way Galaxy to the Andromeda Galaxy a) the distance from the Sun to the center of the Milky Way Galaxy f) the distance from Earth to Alpha Centauri c) one light-year b) the distance across out solar system (to Neptune) d & e) the average distance from Earth to the Sun AND one astronomical unit (AU) Shortest (Be sure to remember that 1 AU is the average distance from Earth to the Sun of about 150 million kilometers, while 1 light-year is nearly 10 trillion kilometers. In other words, a light-year is a distance more than 60,000 times as large as an astronomical unit.)

Describe the distribution of galactic matter on very large (more than 100-Mpc) scales.

On a large scale matter in the universe is distributed unevenly or clumps. On very large scales - more than 300 Mpc - the distribution of galaxies seems to be roughly the same in all directions.

Because the original assumption about all matter interacting with electromagnetic radiation was proved incorrect for the Milky Way Galaxy, scientists next need to determine whether the assumption still holds true for other galaxies. To do this, scientists need a way to calculate the masses of individual galaxies and galaxy clusters, so they can compare those masses with the luminous matter we actually observe. Astronomers use rotation curves to measure the masses of individual spiral galaxies. The faster the stars orbit about the center of a galaxy, the more mass there is between them and the galactic center. Starting from the hypothesis that other galaxies are made up only of visible matter, astronomers can predict what the galaxy rotation curves would look like if the hypothesis were correct. Sort the galaxy rotation curves according to whether they would support or refute the hypothesis that galaxies are made up only of visible matter. a) Rotation Curve 4 b) Rotation Curve 3 c) Rotation Curve 1 d) Rotation Curve 2 e) Rotation Curve 6 f) Rotation Curve 5

Refutes hypothesis: b) Rotation Curve 3 c) Rotation Curve 1 d) Rotation Curve 2 Supports hypothesis: a) Rotation Curve 4 e) Rotation Curve 6 f) Rotation Curve 5 (If the hypothesis that galaxies are made only of visible matter is correct, then the galaxy rotation curves should show both an increase and a decline in rotational speed. There should be a peak in rotational speed near the region of concentrated starlight in the galaxy's center. There should then be a decline in rotational speed toward the galaxy's edge, where there is a smaller concentration of visible matter. The steady decline in rotational speed should occur because the stars are farther and farther from the galaxy's center. This distance acts to lessen the effect of gravity due to the central mass concentration. However, actual measurements of galaxy rotation curves show a flattening or steady increase in rotational speeds, even at large distances from the galaxy centers. This result implies that there must be matter within these galaxies that cannot be seen; in other words, dark matter must be present in other galaxies just as it is present in the Milky Way. Therefore, the data scientists collected refutes the hypothesis that galaxies are only composed of visible matter.)

Explain the role of active galaxies in current theories of galactic evolution. Astronomers have come to the remarkable conclusion that every bright galaxy - active or not - contains a central supermassive black hole. This unifying principle connects our theories of normal and active galaxies by indicating that every bright galaxy probably went through an active phase sometime in the past. In this tutorial, you will explore the evidence for these claims and learn more about the role of active galaxies in galaxy evolution. Part A - Evidence for Supermassive Black Holes In science, astronomers must analyze many observations and separate out those that provide evidence in support a specific theory. To reinforce the credibility of a theory, astronomers must look for multiple, independent pieces of evidence. Consider the following example: Astronomers believe that every bright galaxy harbors a supermassive black hole at its center. Sort the following observations according to whether they support or do not support this theory. a) Powerful jets of high-speed material b) Spiral arms in the outer disks of galaxies c) Fast motions of closely orbiting material in galactic nuclei d) Exceptionally bright galactic nuclei e) Stellar black holes in interacting binary systems f) Globular clusters in the outer galactic halos

Support: a) Powerful jets of high-speed material c) Fast motions of closely orbiting material in galactic nuclei d) Exceptionally bright galactic nuclei Do not support: b) Spiral arms in the outer disks of galaxies e) Stellar black holes in interacting binary systems f) Globular clusters in the outer galactic haloes (The rapid orbital motions of radiating material provide compelling evidence for supermassive black holes within the cores of both active and normal galaxies. The extreme luminosities and jets associated with active galactic nuclei further support the idea that supermassive black holes are present in these galaxies and are powering the activity there. Spiral galaxy NGC 4258 exhibits all of these traits: The visible-light image shows the bright nucleus, and the diagram shows the rapidly orbiting nuclear disk and outflowing jet inferred from observations at radio wavelengths.)

Plan B - Evidence for hierarchical merging Why do astronomers believe that galaxies were formed by mergers and acquisitions early in the history of the universe? This idea is supported by several pieces of theoretical and observational evidence. Sort each piece of evidence below according to whether it provides theoretical support, observational support, or no support for hierarchical merging. a) Computer simulations show merging taking place b) Red galaxies are found very distant from Earth c) Galaxies are located closer together in space d) Small irregular galaxies are found very distant from Earth e) Blue galaxies are found very distant from Earth f) Large, regular galaxies are found very distant from Earth

Theoretical Support: a) Computer simulations show merging taking place Observational Support: c) Galaxies are located closer together in space d) Small irregular galaxies are found very distant from Earth e) Blue galaxies are found very distant from Earth No Support: f) Large, regular galaxies are found very distant from Earth b) Red galaxies are found very distant from Earth (Astronomers study how the properties of galaxies, such as appearance, color, and size, change with distance in order to develop a theory of how galaxy evolution works. Observations and theoretical models support the idea that galaxies have evolved through repeated interactions and mergers over long periods of time. Galaxies at large distance are small, irregularly-shaped, blue galaxies that, through the process of hierarchical merging, become larger, redder, and more regular in shape over time.)

Part B What is the evidence that there must be vast amounts of dark matter? a) Both clusters of galaxies and galaxies themselves would fly apart if dark matter did not supply the extra gravity to keep them bound. b) Light from distant galaxies is blocked by the dark matter, so we can tell it's there. c) The dark matter is only dark in the visible part of the spectrum, but shines brightly in the radio.

a) Both clusters of galaxies and galaxies themselves would fly apart if dark matter did not supply the extra gravity to keep them bound.

Part B - Galactic Mergers Astronomers have found an important correlation between the masses of the central black holes and the properties of the galaxies in which they reside. The largest black holes tend to be found in the galaxies with the most massive bulges (see figure). The reason for this correlation is not fully understood, but most astronomers take it to mean that the evolution of normal and active galaxies must be very closely connected. By observing galaxies at different distances and corresponding look-back times, astronomers have concluded that supermassive black holes were created as galaxies merged during (or just before) the quasar epoch 10-12 billion years ago. How did galactic mergers contribute to the development of supermassive black holes? (Check all that apply.) a) Merging provided more mass to fuel the central black hole, leading to increased nuclear activity. b) Merging stabilized larger galaxies, preventing further nuclear activity. c) Merging lead to more massive bulges and hence to more massive central black holes. d) Merging decreased the number of small galaxies, thus suppressing nuclear activity.

a) Merging provided more mass to fuel the central black hole, leading to increased nuclear activity. c) Merging lead to more massive bulges and hence to more massive central black holes. (Small galaxies merged frequently in the early universe to create larger galaxies with larger bulges. The more massive bulges harbored proportionately more massive black holes. In turn, the more massive black holes hosted greater nuclear activity as quasars. Thus, the period of widespread galaxy merging was followed by the quasar epoch, in which newly-formed super-massive black holes produced high levels of nuclear activity.)

According to current theories of galactic evolution, quasars occur a) early in the evolutionary sequence b) near the Milky Way c) when elliptical galaxies merge d) late in the evolutionary sequence

a) early in the evolutionary sequence

Based on galactic rotation curves and motions in clusters of galaxies, dark matter a) makes up about 90% of the universe. b) is best detected by the largest optical telescopes. c) makes up about 10% of the matter in clusters of galaxies. d) exists but has no observable effects on galaxies. e) is the result of gas and dust.

a) makes up about 90% of the universe.

The rapid variation of brightness of quasars indicates a) the source of energy is very small. b) energy is coming from matter and antimatter. c) the energy source is rotating rapidly. d) a chain reaction of supernovae occurs. e) there are many separate sources of energy in the core.

a) the source of energy is very small. (The size of an object cannot be larger than the distance light can travel in the time it takes to change its brightness.)

The lensing of a distant quasar is produced in a foreground galaxy by its a) total mass of stars, gas, and dark matter. b) central supermassive black hole. c) globular clusters. d) magnetic fields. e) intergalactic gas.

a) total mass of stars, gas, and dark matter.

Many nearby galaxies a) were more active in the past b) have radio lobes c) will become black holes d) contain quasars

a) were more active in the past

What is an external galaxy?

any galaxy beyond our own

By measuring the rotation curves of galaxies beyond the Milky Way, astronomers have determined that dark matter exists in individual galaxies. Because dark matter is a part of galaxies, astronomers need to determine if dark matter also exists *between* galaxies. Considering a galaxy cluster as a gravitationally bound system, astronomers estimate the total mass of the cluster by measuring the movements of the individual galaxies relative to one another. The total mass of a cluster can then be compared to the estimated mass of the observable matter: individual galaxies and hot, intracluster gas. For example, by measuring the motions of galaxies in a particular cluster, the mass of the cluster is determined to be 800 MMW (where MMW is a unit of mass equal to the mass of the Milky Way Galaxy). When the mass of the visible matter is added up, the individual galaxies have a total mass of 40 MMW, and the hot intracluster gas between galaxies has a total mass of 80 MMW. What conclusions can be drawn about the nature of the galaxy cluster's mass? (Check all that apply.) a) Most of the cluster's mass is accounted for by the hot intracluster gas. b) Most of the cluster's mass cannot be seen as visible matter. c) Most of the cluster's mass is accounted for by the individual galaxies. d) The mass of the dark matter accounts for more of the gravitational force within the cluster than does the mass of the visible matter. e) The mass of the visible matter accounts for more of the gravitational force within the cluster than does the mass of the dark matter.

b) Most of the cluster's mass cannot be seen as visible matter. d) The mass of the dark matter accounts for more of the gravitational force within the cluster than does the mass of the visible matter. (In this example, the individual galaxies and the intracluster gas account for only 15 percent of the mass of the galaxy cluster, leaving the majority of the mass unaccounted for by visible matter. This example is typical of the calculations scientists have made when observing galaxy clusters. In general, the mass of galaxies and intracluster gas account for about 10-20 percent of the mass of galaxy clusters, which means something else must be present in the galaxy clusters other than the visible matter. Therefore, scientists think that dark matter exists between galaxies, and it accounts for 80-90 percent of a galaxy cluster's mass. This composite photograph illustrates the differences between visible galaxies, intracluster gas, and dark matter. The galaxies making up this pair of colliding galaxy clusters can be seen in true color. The pink-red color indicates intracluster gas, which emits X rays. The blue represents the inferred position of dark matter. We know the dark matter is there from its effect on the galaxies in the cluster and the light coming to us from behind the cluster, but astronomers cannot directly see it.)

Part A Why might you expect to find giant elliptical galaxies at the centers of old, dense clusters? a) Giant ellipticals were probably formed there intially and attracted all the other galaxies closer to it over time. b) There would be many interactions and collisions near the center, which would ultimately result in the formation of giant ellipticals. c) Most of the gas and dust would settle to the center of the cluster, and we know that giant ellipticals contain vast amounts of gas/dust butts.

b) There would be many interactions and collisions near the center, which would ultimately result in the formation of giant ellipticals. (When we look at the centers of rich galaxy clusters, we often see giant elliptical galaxies there.)

If light from a distant quasar did not pass through any intervening atomic hydrogen clouds, then the figure ("Absorption Line") would have to be redrawn to show The plot shows intensity of light as a function of wavelength. The wavelength is measured from about 610 to about 70 nanometers decreasing in the direction of the positive x-axis. The light intensity is measured from 0 to 100 percent on the y-axis. The intensity increases from 610 nanometers and 30 percent to about 560 nanometers and about 98 percent forming a convex curve. In this region it fluctuates slightly. Then, from 560 to 70 nanometers, it has very frequent and sharp dips and peaks. The envelope of intensity decreases from 560 nanometers and 98 percent to 470 nanometers and 20 percent forming a convex curve. Then it increases almost linearly to 160 nanometers and 42 percent having a peak of 52 percent at 170 nanometers. The wavelength of 122 nanometers is marked as the Lyman-Alpha line, and the region from this wavelength to higher values is marked as redshift. If light from a distant quasar did not pass through any intervening atomic hydrogen clouds, then the figure ("Absorption Line") would have to be redrawn to show a) more features at short wavelengths, but fewer at long wavelengths. b) few absorption features. c) more absorption features. d) single large absorption feature.

b) few absorption features.

When spiral galaxies collide, the greatest impact occurs on their a) globular cluster stars. b) giant molecular clouds. c) central bulge stars. d) open clusters. e) disk stars.

b) giant molecular clouds.

The large-scale distribution of galaxies in the universe reveals a) a smooth, continuous, and homogenous arrangement of clusters. b) large voids, with most of the galaxies lying in filaments and sheets. c) a large supercluster at the center of universe. d) a central void with walls of galaxies at the edge of the universe.

b) large voids, with most of the galaxies lying in filaments and sheets.

A galaxy containing substantial amounts of dark matter will a) appear darker b) spin faster c) have more tightly wound arms d) repel other galaxies

b) spin faster

The Hubble Deep Field (Figure 25.10 in the textbook) shows a patch of sky that has the same angular size as a) dime b) the thickness of a piece of string c) clenched fist d) basketball held at arm's length

b) the thickness of a piece of string

According to X-ray observations, the space between galaxies in a galactic cluster is a) completely devoid of matter b) very hot c) filled with faint stars d) very cold

b) very hot

What is the cause of the intense luminosity of active galaxies, such as quasars or Seyferts? a) The huge number of stars in these galaxies b) The interaction between hot gas clouds forming new stars c) Accretion of material by supermassive black holes in the galactic cores

c) Accretion of material by supermassive black holes in the galactic cores

A galaxy seen 1 billion light-years away means we see it a) as it was when the universe was 1 billion years old. b) as it will be 1 billion years from now. c) as it was 1 billion years ago. d) as it is today, but redshifted 10% of the speed of light. e) as it was just after the Big Bang.

c) as it was 1 billion years ago. (Looking farther away in space means looking back further in time, to when the object (and universe) was younger.)

Galaxies evolve by a) using up all their gas and eventually becoming ellipticals b) ejecting their gas and dust into intergalactic space c) merging to form larger galaxies d) fragmenting into smaller galaxies

c) merging to form larger galaxies

Due to the density and collisions among galaxies, ___________ are rare in the centers of clusters. a) giant ellipticals. b) irregulars. c) spirals. d) active galaxies. e) radio galaxies.

c) spirals. (The gas, dust, and disks of spiral galaxies are tidally disrupted, and even destroyed, in the centers of dense clusters, which are often dominated by giant elliptical galaxies.)

The more massive a galaxy is, a) the faster star formation in it occurs b) the larger the proportion of old stars it contains c) the faster it rotates d) the more distant it is

c) the faster it rotates

How did Hubble prove that the Andromeda nebula was in fact an external galaxy? a) He resolved it into stars. b) He measured Doppler shifts of some of its stars. c) He measured the trigonometric parallax of the nebula. d) He found Cepheids in the nebula, which allowed him to estimate the distance through the period-luminosity relation. e) He studied its globular clusters and compared them to those in the Milky Way.

d) He found Cepheids in the nebula, which allowed him to estimate the distance through the period-luminosity relation.

Collisions between galaxies a) are much rarer than collisions between stars. b) can transform elliptical galaxies into spirals. c) trigger Type II supernova explosions in the halo. d) cause gas and dust clouds to collide leading to rapid star formation.

d) cause gas and dust clouds to collide leading to rapid star formation. (Galaxies are relatively close compared with their size. In clusters of galaxies, collisions clearly occur.)

Relative to luminous stellar matter, the fraction of dark matter in clusters is a) unknown b) the same as the fraction in galaxies c) less than the fraction in galaxies d) greater than the fraction in galaxies

d) greater than the fraction in galaxies

Dark Matter

keeps the gravitational forces stable

Dark energy

pulls matter apart

What is lensing?

when light waves bend due to the gravitational pull of a dense object, such as a black hole