Lecture 27: Quasars and Active Galaxies (AGN's)

Normal Galaxies Versus Active Galaxies

Properties of "Normal" Galaxies

  • Most of the galaxies which we have looked at so far in this course are normal galaxies.
  • All of the galaxies in the Local Group (including the Milky Way and Andromeda) are normal galaxies.
  • Normal galaxies have total luminosities up to about 1011LSun.
  • The spectrum of a galaxy is mainly the sum of the spectra of all the stars (absorption spectrum) as well as the dust and Hydrogen gas.
  • The luminosity of a normal galaxy does not change much over short periods of time.

Properties of Active Galaxies

  • Active Galaxies are typically more luminous than normal galaxies. Typical luminosities range from 1011LSun to 1015LSun.
  • The spectrum of an active galaxy includes features which are usually not seen in the spectrum of stars including: Hydrogen emission lines; radio synchrotron emission; x-rays
  • The active galaxies often have a nucleus which is much brighter than the nucleus of a normal galaxy.
  • The active galaxies often have gigantic jets pointing out of the galaxy.
  • The luminosity of an active galaxy can change by a factor of 2 over a short period of time (such as a few days).
  • Some different types of active galaxies are: Radio Galaxies, Seyfert Galaxies, BL Lac Objects (also known as Blazars), and Quasars.
Figure 27-14

Rapid Variability

  • This gamma-ray picture of the active galaxy PKS0528 shows large changes over only 3 months of observations.

Emission Lines

  • Instead of seeing absorption lines (as in a regular galaxy) we see emission lines in the spectrum of the quasar 3C 273.
  • In addition, the Balmer series of Hydrogen lines have all been redshifted.
  • Hubble's law would imply that this quasar is very far away, so it must be very luminous since we can see it.

Radio Galaxies

  • All galaxies emit radio waves. For normal galaxies, radio emission corresponds to a small fraction of the total energy emitted by the galaxy.
  • For Radio Galaxies, the energy emitted at radio wavelengths is 0.1 to 10 times the energy emitted at visible wavelengths.
  • Most radio galaxies are either elliptical or giant elliptical galaxies.
  • The radio emission comes from a tiny bright source in the nucleus of the galaxy as well as large jets which are often larger than the galaxy.
  • The radio emission comes from the synchrotron mechanism where rapidly moving electrons spiral around magnetic field lines.

Centaurus A

  • The galaxy Centaurus A at a distance of about 3 million pc is the closest active galaxy to us.
  • This galaxy is an elliptical galaxy with a dusty disk.
  • These images are taken at visible wavelengths.
  • This image shows a close-up of the nucleus showing the star-formation occuring inside the dusty nucleus.

Radio Emission from Centaurus A

  • This image shows the combined light from visible, infrared and radio wavelengths.
  • The infrared light is coloured red, and shows that the centre of the elliptical galaxy appears very similar to a small barred spiral galaxy with a radius of about 3 kpc!
  • The contour lines show where radio waves are emitted.
  • The radio emission originates from giant jets which point in directions perpendicular to the dusty disk.
  • The radio emission extends out much further than the visible part of the elliptical galaxy.
  • The length of the radio jets are about 10 kpc.

X-ray Emission from Centaurus A

  • blue = x-ray emission (Chandra space X-ray Observatory)
  • yellowish = submm (APEX, Chile)
  • This X-ray image of Centaurus A shows the bright nucleus and a jet extending outwards.
  • The X-ray emission is in the same location as the emission from the radio jets.

Seyfert Galaxies

  • Seyfert Galaxies are spiral galaxies with an unusually bright nucleus.
  • The nucleus is much bluer than a normal nucleus.
  • Emission lines are present, showing that very hot ionized gas is present.
  • The gas in the central region orbits the centre very rapidly.
  • The luminosity of the nucleus changes rapidly over the course of days and weeks.
  • A Seyfert Galaxy is an example of a galaxy with an active nucleus.

Some Examples of Seyfert Galaxies (Visible Light)

NGC 7742 Circinus Galaxy

Blazars (BL Lac Objects)

  • The first Blazar identified was first thought to be a variable star and given the name BL Lacertae.
  • All objects similar to this first blazar are called either BL Lac objects or Blazars.
  • BL Lac objects appear to be star-like point sources in early photos.
  • Their spectra show no emission lines, only continous synchrotron emission.
  • Longer exposure photos show that the Blazars are the very bright nuclei of faint elliptical galaxies.
Figure 27-15


  • Quasar is short for Quasi-Stellar Object also abbreviated as QSO.
  • These objects were first discovered in the 1960's and looked like stars (point sources) on photographic plates.
  • These objects emitted radio waves and had strong emission lines, which is not a normal spectrum for a star.
  • In 1963 Martin Schmidt identified the emission lines of Hydrogen in the QSO named 3C 273 and showed that they were redshifted.
  • The redshift for 3C 273 is z = 0.158 which corresponds to motion of the galaxy at a speed (with respect to us) of 16% the speed of light.
  • At the time, this was the largest redshift ever recorded.
  • Using Hubble's redshift law, we can find the distance to 3C 273.
  • d = v/H0 = cz/H0
  • c/H0 = 3 x 105 km/s / 70 km/s/Mpc = 4.3 x 109 pc
  • The distance to an object with redshift z is then
  • d = z x 4.3 x 109 pc
  • This means that the distance to the Quasar 3C 273 is 680 Mpc.
  • With this large distance, the luminosity of the quasar must be very large.
Figure 27-2

Quasars are Active Galactic Nuclei (AGN)

  • Later more powerful telescopes have shown that the QSOs have faint galaxies surrounding them, suggesting that the Quasars are the very bright nuclei of active galaxies.
  • The brighter quasars have a luminosity of 1015 LSun ~ 100,000 x more luminous than the Milky Way.
Figure 27-8

A Gravitationally Lensed Quasar

  • Since quasars are far away from us, they can appear to be gravitationally lensed by a nearby galaxy.
  • This picture is called the "Einstein Cross".
  • The central bright spot in the centre is a "nearby" galaxy with redshift z=0.0394
  • The four spots around the centre are four images of a background quasar with z=1.695
  • The spectra of the four images are the same, showing that this is a gravitational lens.
  • When the spectrum of the quasar changes, there is a time delay between when the changes occur due to the different distances that light has to travel for the different images.

Models of Active Galactic Nuclei

  • All of the Active Galaxies (Seyferts, Radio Galaxies, Blazars, Quasars) have properties in common: bright nuclei and rapid variability.
  • What can cause the nucleus of an active galaxy to be so powerful?

Time Variability

  • The short time scales over which the nucleus changes its luminosity gives us information about its size.
  • If the nucleus is a sphere with radius R, it takes a period of time
    t = R/c
    for light to cross the radius of the sphere.
  • By measuring the period of time over which the nucleus changes its luminosity, we can find an upper limit to the size of the nucleus.
  • For instance, if the nucleus changes its luminosity over the course of one day, the bright region must be smaller than one light-day in radius.
Figure 27-19
  • The shortest variability timescales seen for active galaxies are on the order of hours, suggesting that the nuclei of these galaxies are smaller than a few light-hours.
  • One light-hour corresponds to 7.2 AU.
  • This means that the source of energy in the nucleus must be very small.

Orbital Speeds Near the Nucleus

  • Gas moving in circular orbits near the nuclei of active galaxies has been observed.
  • An example is the giant elliptical galaxy M87, which is also a radio galaxy with a jet.
  • In the Hubble Space telescope of the central region, a disk and a jet are seen.
  • A series of Radio images of the centre of the galaxy show that the bright energetic region is smaller than a light-year in diameter.

Evidence for a High-Mass Object in the Nucleus

  • Spectra of gas clouds at a distance of
    20 pc = 6.2 x 1014 km = 4,100,000 AU
    from the centre were measured.
  • The Doppler shifts showed that the gas orbits the centre at a speed of 550 km/s.
  • Kepler's law gives us the mass of the central region.
  • Orbital Period = P = 2 pi a/v
    = 2 pi x 6.2 x 1014 km/ 550 km/s = 7 x 1012 s
    = 220,000 years.
  • Mass = a3/P2 = (4,100,000)3/(220,000)2
    = 1.4 x 109 MSun
  • A gigantic mass is found inside of a small region.
Figure 19.9

Black Holes as Engines for Active Galaxies

  • a gigantic black hole (or Supermassive black hole) with a mass in the range of one million to one billion solar masses can also have an accretion disk which emits radiation.
  • The basic model for all active galaxies is that a rotating supermassive black hole lies at the centre of the galaxy.
  • The black hole has an accretion disk which emits radiation.
  • The rapid rotation can also funnel material into jets which point along the axis of rotation, similar to what is seen in the formation of stars.

Black Hole Model

  • In more detailed black hole models, the types of phenomena are a result of the viewing angle.
  • Blazars correspond galaxies with jets pointing right at us.
  • Radio galaxies and Seyferts correspond to views of active galaxies which are inclined.
  • The luminosity of the black hole's accretion disk is proportional to the rate that mass falls onto the disk.
  • The black holes which are surrounded by the most material should be brightest.
Figure 27-25

Evolution of Galaxies

  • When we look at galaxies, we find that the closest galaxies are more likely to be normal galaxies.
  • Active galaxies are more likely to be found further away from us.
  • The quasars have large redshifts and are the objects furthest from us.
  • If an object is a distance "d" from us, it takes the light an amount of time t = d/c to reach us.
  • When we see an object we see it as it looked when the light was emitted, not as it is today.
  • For example, when we view a galaxy which is one billion light-years away, the light was emitted one billion years ago.
  • When we look at galaxies which are far away, we are seeing what galaxies looked like when they were much younger.
  • The fact that galaxy properties seem to change when we look out to large distances suggests that there was an early epoch of galaxy formation about 10 billion years ago.
  • The most distant objects (quasars) correspond then to very young galaxies.
Figure 27-5

Aging Quasars

  • As the quasars evolve in time, they become less luminous and appear as either Seyfert galaxies or Radio Galaxies.
  • Over time, these active galaxies become less bright and appear as regular galaxies.
  • In this evolutionary picture, the young galaxies (or quasars) begin with large amounts of gas in their nuclei which spirals onto the black hole's accretion disk, causing it to be very luminous.
  • Over time, the black hole consumes gas in the inner regions and less gas is available to accrete onto the disk causing it to be less luminous.
  • Observations of nearby galactic centres (such as the Milky Way and Andromeda) have shown evidence that most galaxies have a supermassive black hole at their centre.
  • The Milky Way's central black hole is only one million times the mass of the Sun, so it may not have been as bright as a quasar when it was young.

The Centre of the Milky Way

The innermost 1,500 light years of the Galactic Centre (at radio wavelengths)

  • The bright radio source called SGR A (Sagitarius A) is the centre of the galaxy.
  • The bright region of SGR A seen in this image is about 100 light-years across.

The Central light-year of the Milky Way in Infrared

  • Time-lapse photography over 8 years shows motion of stars.
  • There are about 100 stars in a region about one light-year cubed in volume.
  • A typical velocity of a star 0.3 pc from the the centre is 260 km/s (in a circle around the centre).
  • The central region of the Milky Way is smaller than 20 AU in radius. (Too small to be seen in this image.)
  • On Assignment 8, we looked at the motion of one of these stars.
  • We showed that the motion of these stars implies that a mass of 2 million solar masses lies at the galactic centre.
  • We see hundreds of stars in this small region, not millions.
  • It is difficult to understand how such a large mass can be held into a stable configuration of stars or a gas cloud.
  • The most likely object is a black hole.

How large is a black hole with such a large mass?
  • On Assignment #7, you showed that the event horizon of a black hole has a radius of
    REH = 3 km M/MSun
  • If the mass of the black hole is 2 x 106 MSun, the event horizon radius is
    REH = 3 km x 2 x 106 = 6 million km = 0.04 AU.
  • Since the central source is smaller than 20 AU, and the black hole is smaller than this size, the black hole hypothesis is consistent with the data.

Next lecture: Gamma-Ray Bursts