Scholars Online Astronomy - Chapter 24: Quasars and Active Galaxies

Homework

• Reading Preparation
  • Study Guide/Reading Notes
• Key Formulae to Know
• WebLecture
• Study Activity
• Chat Preparation Activities
• Chapter Quiz
• Lab Work

Reading Preparation

Reading: Astronomy, Chapter 24: Quasars and Active Galaxies

Study Guide

• Section 1: Quasi-stellar radio objects (quasars) are galaxies that appear as star-like points of light because they are so distant that only the light from the nucleus reaches us. Unlike strs or even nearby galaxies, which have absorption lines due to materials that intercept light emitted from the stars, quasars have strong hydrogen emission lines red-shifted by 15% of the speed of light or more. Using the Hubble Law to determine distance, quasars are at least 2 billion light years distance from Earth, and thousands of times more luminous than the entire Milky Way. Quasars represent objects that were common in the galaxy 10-13 billion years ago, but which no longer exist.
• Section 2: All active galactic nuclei (including quasars) must have very small core regions since they cannot vary in brightness faster than light can travel across the region: a quasar that fluctuates from peak brightness to dimness in one day canot be more than one light-day across. Given the calculated masses of quasar objects, the matter-to-volume ratio is so great that the AGN must be a black hole. The Eddington limit quantifies the luminosity maximum for material falling into a black hole of a given radius; if the luminosity of the black hole is greater than this limit, the heat and energy represented by the luminosity would expel material away from the black hole. A large enough mass need not be superdense to form a supermassive black hole.
• Section 3: Supermassive black holes would have enough gravity to attract accretion disks that accelerate accreting matter to very high velocities and condense them very quickly, causing high temperatures emitting radiation in the UV range. Because the disk is spread out over a large area of space, pressures on the accreting material differ, causing material near the center to be expelled back into space as jets of charged particles with relativistic speeds, confined by magnetic fields. The apparent superluminal motion of some quasar jets can be explained by looking at the angle of motion of the jets relative to the line of sight from earth.
• Section 4: Observed AGNs can be grouped into six major types, listed below. Those with strong hydrogen emission lines are marked type 1, and those with weak hydrogen emission lines are type 2.
  • Blazars 1: Quasars with jets pointing directly at Earth
  • Radio-loud quasars 1: Quasars with jets and lobes
  • Radio-loud galaxies 2: Radio galaxies without visible quasars
  • Radio-quiet quasars 1: Quasars without jets or lobes
  • Seyfert Galaxies 1: Quasars in visible galaxies (broad hydrogen)
  • Seyfert Galaxies 2: Quasars in visible galaxies (narro hydrogen)
  All AGN observations can be explained using a unified model of a galaxy surrounded by a dense gas and dust torus in the accretion disk plane. Differences in spectra depend on the angle of observation, determined by whether the observation is dominated by the jet, the accretion disk, or the torus.
• Section 5: Since AGN are thought to be the result of collisions between galaxies, we can explain their current scarcity as the result of time, during which most of the nearby gas and dust has been absorbed by the black hole, so that little remains to fall on and power the high luminosity of the object. Observed quasars are very far away, meaning their light left the object billions of years ago. Nearby galaxies and our own Milky Way have supermassive black holes at their nucleus, indicating they were once quasars, but are no longer active.

Key Formulae to Know

Web Lecture

Read the following weblecture before chat: Quasars ad Active Galaxies

Study Activity

Chapter Quiz

• Required: Complete the Mastery exercise with a passing score of 85% or better.
• Go to the Moodle and take the quiz for this chat session to see how much you already know about astronomy!

Lab Work

Read through the lab for this week; bring questions to chat on any aspect of the lab, whether you intend not perform it or not. If you decide to perform the lab, be sure to submit your report by the posted due date.

• Lab instructions: Active Galaxy Classification

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