Question 1

How does the unified model describe an active galactic nucleus?

Accepted Answer

The unified model describes an active galactic nucleus as a supermassive black hole surrounded by an accretion disk and a dense disk of gas. This model explains the various observed properties of AGNs, such as their emission lines and variability, as being due to the orientation of the black hole's accretion disk with respect to our line of sight.

Question 2

According to the unified model of active galactic nuclei, what is observed if our line of sight is parallel to the plane of the dense disk of an active galactic nucleus?

Accepted Answer

If our line of sight is parallel to the plane of the dense disk of an active galactic nucleus, we would observe narrow spectral lines due to the Doppler shift of the gas in the disk. This is because the gas is moving in circular orbits around the central supermassive black hole, and the Doppler shift causes the spectral lines to appear narrower than they actually are. The broad spectral lines are observed when our line of sight is inclined to the disk, allowing us to see the gas moving in other directions as well. The thin accretion disk is a component of the unified model of active galactic nuclei, but it is not directly related to the orientation of our line of sight.

Question 3

Which of the following emits large amounts of energy, but photographically appears to be a single point of light, much like a star?

Accepted Answer

Quasars emit large amounts of energy and appear as single points of light, similar to stars, due to their immense distance from Earth, despite being extremely luminous and powerful sources often associated with supermassive black holes at the centers of distant galaxies.

Question 4

What of the following could be referred to as "galactic cannibalism" ?

Accepted Answer

Galactic cannibalism refers to the process where a larger galaxy, through gravitational forces, merges with and absorbs a smaller galaxy. This phenomenon is a common occurrence in the evolution of galaxies in the universe.

Question 5

In the unified model of active galactic nuclei, what is the broad line region in an active galactic nucleus?

Accepted Answer

The broad line region is composed of clouds of gas and stars moving at very high orbital velocities near a supermassive black hole. These clouds emit broad emission lines in the spectrum of an active galactic nucleus.

Question 6

What inflates the lobes of radio emission in a radio galaxy?

Accepted Answer

Jets of excited gas flowing from the central galaxy are responsible for inflating the lobes of radio emission in a radio galaxy. These jets, emanating from the vicinity of a supermassive black hole at the galaxy's center, push out and interact with the surrounding intergalactic medium, creating the extended radio lobes observed.

Question 7

Why were quasars so mysterious when first discovered?

Accepted Answer

Quasars emitted radio energy like radio galaxies, but looked like stars in visible light, which was very mysterious and confusing when they were first discovered.

Question 8

If the redshifts of quasars arise from the expansion of the universe, yet they have brighter magnitudes than galaxies with the same redshifts, what must be generally true of quasars?

Accepted Answer

The fact that quasars have brighter magnitudes than galaxies with the same redshifts and are still visible at extreme distances suggests that they must emit a very large amount of energy per second, making them incredibly luminous. This emission could come from the accretion disk surrounding the supermassive black hole at the center of the quasar.

Question 9

The hydrogen Balmer line Hβ has a wavelength of 486.1 nanometres in the laboratory. The same line is observed in a quasar at 2430.5 nm. What is the quasar's redshift?

Accepted Answer

The redshift (z) is calculated using the formula z = (observed wavelength - rest wavelength) / rest wavelength. Here, z = (2430.5 nm - 486.1 nm) / 486.1 nm = 4.

Question 10

Mathematical models indicate that one type of galaxy is produced by high-speed collisions in which a smaller galaxy passes through a larger galaxy almost perpendicular to the disk of the larger galaxy. What is this type of galaxy called?

Accepted Answer

Ring galaxies are formed as a result of high-speed collisions where a smaller galaxy passes through a larger one, creating a ring-like structure due to the gravitational effects of the collision.

Question 11

Why are supermassive black holes believed to be located at the centre of many galaxies?

Accepted Answer

The orbital motion of material near the center of galaxies is observed to be very fast, indicating the presence of a very massive object, such as a supermassive black hole, influencing the motion through its gravitational pull.

Question 12

Which of the following is a feature of Seyfert galaxies?

Accepted Answer

Seyfert galaxies are characterized by their active nuclei, which emit strong, high-energy radiation. This radiation comes from gas that is very hot and moving at high velocities in the galaxy's core, which is observed through their spectra. This is a defining feature of Seyfert galaxies, distinguishing them from other types of galaxies.

Question 13

Which of the following indicates that quasars must be small?

Accepted Answer

If quasars fluctuate rapidly on time scales as short as a few hours, then they must be small because otherwise, it would take longer for any changes to be visible.

Question 14

What would happen to the Sun if the supermassive black hole in the centre of our galaxy could be instantly removed?

Accepted Answer

The supermassive black hole at the center of our galaxy has a significant gravitational influence, but its sudden removal would not drastically alter the Sun's orbit. The Sun's orbit around the galaxy is primarily determined by the overall mass distribution of the galaxy, not just the mass of the central black hole. Therefore, if the black hole were instantly removed, the Sun would continue on its current orbit around the galaxy's center.

Question 15

What kind of radiation is produced by the radio lobes that are on each side of some radio galaxies, and also by the jets from them?

Accepted Answer

The radio lobes and jets from radio galaxies produce mainly synchrotron radiation, which is produced by electrons that are spiraling around magnetic field lines at nearly the speed of light.

Question 16

Which type of active galaxy can be seen at the greatest distance?

Accepted Answer

Quasars are the most luminous objects in the universe and emit high-energy radiation, making them visible at the greatest distances.

Question 17

What is the major observational difference between a Seyfert galaxy and a normal spiral galaxy?

Accepted Answer

The central region of a Seyfert galaxy is typically much brighter than the central region of a normal spiral galaxy. This is due to the presence of an active galactic nucleus (AGN) powered by a supermassive black hole at the center of the Seyfert galaxy, which produces intense radiation and outflows. Normal spiral galaxies may host a supermassive black hole at their center, but they are not typically active and do not produce the same level of radiation as Seyfert galaxies. The length of spiral arms and the presence of star formation are not distinguishing characteristics between Seyfert and normal spiral galaxies.

Question 18

Which of the following describes the way that ring galaxies are believed to form?

Accepted Answer

Ring galaxies are believed to form when a smaller galaxy collides with a larger galaxy directly through the center of the larger galaxy, along a direction perpendicular to the larger galaxy's disk. This collision causes the stars and gas in the larger galaxy to be displaced, creating a ring-like structure.

Question 19

What produces the energy from an active galactic nucleus?

Accepted Answer

The energy from an active galactic nucleus is produced by matter flowing into a supermassive black hole at the center of the galaxy, which heats up and emits powerful radiation and jets of high-energy particles.

Question 20

Which of the following are spiral galaxies that have small luminous nuclei?

Accepted Answer

Spiral galaxies with small luminous nuclei are commonly known as Seyfert galaxies.

Quiz 10: Galaxies