Astronomy 100


Lectures Table of Contents Astro 100

Quasars and Black Holes


  1. Quasars
  2. Black Holes That Shine -- Gravity Power

Terms to Know

active galaxy
active galactic nucleus
quasar = QSO
host galaxy
accretion disk

1. Quasars

In the early 1960's, some strange objects were discovered that looked like stars in optical images, but had spectra very different from any star's spectrum; they were dubbed "Quasi-Stellar Objects," or QSOs. Some of the objects also appeared on radio maps; these were called "quasars." (Now we use the terms more or less interchangeably; we specify "radio-loud" or "radio-quiet" when necessary.) It was finally recognized that the spectra showed common emission lines of hydrogen, carbon, magnesium, etc., but redshifted by many thousands of km/s, implying tremendous distances (from the Hubble Law). Modern-day extragalactic astronomy was born!

The great distances to quasars was only half the wonder: the distance implies phenomenal luminosities. Most quasars are brighter than the entire Milky Way Galaxy! And yet they are very, very small. How small? Since some quasars are seen to vary in intensity over a few days' time, we can safely conclude that the quasars themselves must be less than a few light-days across. This is because if the quasars were larger, they wouldn't be able to coordinate their rapid fluctuations in such a short time (since no coordinating message can travel faster than the speed of light). Compare this to the Milky Way Galaxy, which is as bright as a QSO but is 75,000 light-years across -- a trillion times larger than a QSO!

We now know that quasars are a common form of "active galactic nucleus" (other types are radio galaxies and Seyfert galaxies). Very sensitive, long time-exposures with large telescopes actually show the "host" galaxies that surround many QSOs; they range from spirals like the Milky Way to giant ellipticals to huge, strange-looking irregulars. Galaxies that host an active nucleus are called "active galaxies."

As we use large telescopes to look farther and farther away -- and farther back in time -- we see more and more quasars. If we look at a typical box of space about 10 billion light years away -- when the Universe was only 1/4 as old as it is now -- we see about 1000 times more quasars than we do in a similar box today! Quasars were very common in the early Universe, but are very rare today.

Quasars are important for several reasons:

  1. They are among the brightest objects known in the Universe, and therefore are visible to the greatest possible distances.
  2. They appear to live in galaxies, so they can help us understand galaxy formation and evolution.
  3. They are so far away, and there are so many faint, distant galaxies between our Milky Way Galaxy and distant QSOs, that the QSOs often shine through the other galaxies, like a flashlight shining through a series of clouds. The gas in those galaxies often leaves absorption lines in the QSOs' spectra -- so we can study the galaxies without ever seeing them directly!
  4. Finally, QSOs appear to be powered by a strange and potent source of energy. What could it be?

2. Black Holes That Shine -- Gravity Power

The only process we know of that can release a whole galaxy's worth of radiation from an area only a few times larger than the solar system is closely related to X-ray binary stars: a cosmic burst of energy from material that is about to be swallowed up by a black hole due to its intense gravitational field.

The standard picture of a QSO, based on their small sizes, high luminosities, spectra, and locations at the heart of galaxies, consists of a massive black hole -- up to a billion MSun -- surrounded by a swirling accretion disk of gas that is being devoured by the black hole. The gas whips around faster and faster as it nears the black hole, like water going down a drain, and the faster it goes the hotter it gets, until -- just before it disappears from our view -- it is hot enough to emit X-rays or even gamma rays.

Additional evidence for the black hole accretion disk model of QSOs comes from the "jets" that are sometimes seen associated with active galaxies: like fire hoses shooting hot gas far out from the nucleus, jets are probably formed along the axis of rotation of the central accretion disk, since this is the most likely escape route for material that is manages to avoid getting swallowed by the black hole. How the QSO or other active galaxy appears depends on whether we are looking straight down the jet (a very bright, intense view) or looking at the jet sideways (a less intense view).

Where did the black holes come from? And which came first -- galaxies or quasars? We're not sure! QSOs appear to be closely linked to galaxy formation, so they may be the first part of a typical galaxy to form. Our own Milky Way Galaxy may have a black hole in its center -- a dead quasar! If there is such a massive black hole at the galactic center, evidently it ran out of fuel long ago and no longer is surrounded by a glowing, swirling accretion disk.

Lectures Table of Contents Astro 100

Houjun Mo Astronomy 100