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Lectures
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Astro 100 |
Quasars and Black Holes
Outline
- Quasars
- 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:
- They are among the brightest objects known in the Universe,
and therefore are visible to the greatest possible distances.
- They appear to live in galaxies, so they can
help us understand galaxy formation and evolution.
- 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!
- 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
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Table of Contents
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Astro 100 |
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