11. AGNs and galaxy evolution

Question 1

Describe how quasars were discovered

Answer 1

• The first quasars (3C 48 and 3C 273) were discovered in the late 1950s
• Discovered as radio sources in all-sky radio surveys
• They were first noted as radio sources with no corresponding visible object
• Using small telescopes and the Lovell Telescope as an interferometer, they were shown to have a very small angular size.

Question 2

Outline the main features of quasars

Answer 2

• Variable light
• Large ultraviolet flux of radiation
• Broad emission lines in the spectra with absorption lines in some cases
• Large redshift

Question 3

Describe why we believe super massive black holes are the power source behind quasars

Answer 3

Both active galactic nuclei and quasars derive their energy from material falling toward, and forming a hot accretion disk around, a massive black hole. This model can account for the large amount of energy emitted and for the fact that the energy is produced in a relatively small volume of space

Question 4

Outline some evidence that quasars are associated with Active galaxies

Answer 4

• Observations with the Hubble Space Telescope show that quasars lie at the centres of galaxies and that both spirals and ellipticals can harbour quasars.
• The redshifts of the underlying galaxies match the redshifts of the quasars embedded in their centres, thereby proving that quasars obey the Hubble law and are at the great distances implied by their redshifts.

Question 5

Describe an Active Galactic Nucleus

Answer 5

An active galactic nucleus (AGN) is a compact region at the centre of a galaxy that has a much-higher-than-normal luminosity over at least some portion of the electromagnetic spectrum with characteristics indicating that the luminosity is not produced by stars.