Active Galaxies and Quasars
- The Quasar
- Hubble’s Law
- Active Galactic Nuclei & Seyfert Galaxies
- Radio Galaxies
The First Quasar
Each time we observe the sky in a new wavelength, we discover strange new things.
In early 60’s were the beginnings of radio astronomy. Hundreds of objects with no visible companions were found.
Including 3C 48 (Third Cambridge Catalog, object 48).
Looking deeper, found a faint blue "star."
Spectral lines looked unlike anything then known.
This came to be known as a quasi-star, or Quasar.
What were these strange Quasar lines?
Careful examination showed these lines to be familiar.
Hydrogen and Helium and other common spectral lines, but red-shifted by 37%
This kind of Doppler shift means this Quasar is moving away from us at almost 1/3 the speed of light. (100,000 km/s)
Much too fast to be something within our galaxy.
Quasi-Stellar Objects must be extra-galactic, or beyond our galaxy.
Hubble’s Law
Edwin Hubble measured velocities of galaxies in the 1920’s
Discovered Two Major Things
All galaxies, except a few local exceptions, are moving away from us.
The faster a galaxy is moving away from us, the FARTHER away that galaxy is.
Recessional Velocity = Ho x Distance
Ho @
70 km/s/Mpc
Major Implication: Universe is Expanding
(more on this next lecture)
Back to Our Quasar
If our Quasar is outside our galaxy, then it must obey the Hubble Law.
At 1/3 the speed of light, 3C 48 must be 1340 Mpc away, or roughly 5 billion light years away.
Remember though, this object was thought to be a star in our galaxy. If we put it at the predicted distance it must be putting out the equivalent of a 1000 Milky Way galaxies, or 20 trillion suns.
That is a typical Quasar. Some can be 100x brighter than that, making them the brightest known objects in the universe.
Variability and Size
Quasar light is highly variable, meaning it changes its brightness on the scale of days.
This makes Quasars inherently SMALL.
No signal can travel faster than speed of light.
So, if a Quasar wants to change brightness, signal making change must pass from one side to the other.
At most, Quasars are light days across, or solar system sized.
This means energy output of 100 galaxies coming out of region normally holding only one star.
Quasar Energy Source
So far, the main source of energy we have discussed has been Fusion.
Hydrogen into Helium loses 0.7% of its mass into energy.
This method, i.e. Stars, is no where near efficient enough to pump so much power out of so small an area.
Only matter falling into a black hole, yielding almost 10% of its mass into energy, is efficient enough.
Quasar Central Engine
In the center of the Quasar lies a supermassive black hole, possibly a billion solar masses in size, converting matter into energy as it is fed into it through a gigantic accretion disk.
This released energy powers the Quasar and drives JETS of material out its poles, sometimes stretching as much as a Mpc.
Active Galactic Nuclei
Quasars are the biggest and baddest of a class of galaxies known as Active Galactic Nuclei galaxies, or AGN.
In the local universe, these AGN fall into two main catagories:
Seyfert Galaxies
Radio Galaxies
All AGN are believed to be powered by a central supermassive black hole.
Seyferts
Superficially seem to resemble normal spiral galaxies.
However, most of the energy comes from its core, putting out 10x as much energy as the Milky Way.
Unlike normal galaxies, the majority of its radiation is coming out at long wavelegths:
Radio and infrared wavelengths
Spectral lines show extremely high energy lines, lines that even the brightest stars could not make.
Seyferts Explained
A Seyfert is essentially a low energy, nearby Quasar.
If you moved a Seyfert to Quasar distances, you wouldn’t be able to see the galaxy: Only the central nucleus.
Radio energy comes from synchrotron radiation.
Produced from high energy electrons whipping around in a magnetic field...the conditions you get in the central engine.
Only the intense energies of a supermassive black hole engine could make the high energy lines seen.
Radio Galaxies
AGN that emit most of their energy in the radio, even further out than the Seyfert.
Radio emission tends to be extremely extended: NOT confined to just the center.
Believed we are following the central JETS carrying energy vast distances from the galaxy’s central core.
Jets eventually crash into thin intergalactic medium, creating gigantic radio lobes, each carrying more energy than anything visible from central galaxy.
Fuel Consumption
or
Where Did All the Quasars Go?
The brightest quasars would need to eat about 1000 solar masses per year.
Over the age of the universe, 10 billion years, that would eat up ten galaxies worth of material.
Assuming the quasar only had one galaxy to work with, it can not last forever.
Starving Black Holes
So maybe Quasars didn’t go away...they just ran out of stuff to eat and became quiet.
In some galaxies their food supply is just 100x lower and now they are Seyferts.
In others, their food supply is so low they would look just like a "normal" galaxy.
In fact, maybe the majority of the "normal" galaxies we see are actually harboring a starving supermassive black hole in their cores...
Including our own galaxy.