Download QUASARS and ACTIVE GALAXIES

QUASARS and ACTIVE
GALAXIES
- a Detective Story
Twinkle, twinkle, quasi-star,
Biggest puzzle from afar.
How unlike the other ones,
Brighter than a trillion Suns.
Twinkle, twinkle, quasi-star,
How I wonder what you are!
- George Gamow
Remember How Redshifts Arise
Light can be red-shifted (shifted to longer
wavelengths) in three ways.
1.
By the motion of objects through space.
Example: stars near us. (This can yield
blue or red shifts, since stars can be
moving towards or away from us.)
Typically Small Velocities and Shifts
Second
2.
Gravitational redshift.
This occurs when light ‘climbs away’ from a
very dense body and loses energy.
Not Generally
Observationally Important
It’s a very small effect
for ordinary stars, even
dense white dwarfs.
It’s much bigger for
neutron stars, but they
are too faint to observe.
It’s infinite for black
holes – which means
that no light reaches us!
Third
3.
The Expansion of the Universe
We can see very large redshifts, depending
on the distance of the object. It is being
carried by the expansion of space itself,
carrying the galaxies; it is not really a
‘velocity’ of an object moving through space.
…as found
by Hubble
Notice:
There are No Big Blueshifts!!
We detect no big blueshifts – only small
ones, like Andromeda approaching us at
300 km/sec. This is only 1/1000 of c.
(That could change in the very remote future – umpteen
trillion years from now! -- if the universal expansion
were to come to a halt and turn into a contraction. Then
we would start to see systematic blueshifts.)
One Logical Conclusion
If we find an object that has a very large
red-shift, we conclude that it is being
carried along by the expansion of the
universe -- and must be at an immense
distance.
At such large distances, only extremely
luminous objects (as bright as whole
galaxies) can be seen.
Some History
In the 1960s, radio
telescopes (then
new!) found many
hitherto unknown
sources in the sky.
What were they?
One Such Source: 3C273
The 273rd radio
source in the
Cambridge
Catalogue.
In visible light, it
looks like a
star – except for
that wisp of light!
What is It?
Take a Spectrum
(Note: this is a negative image)
What we see:
unknown lines, in emission (like a neon lamp)
What Elements Produce These Lines?
They Don’t Correspond to Any That We Know!
There were lots of imaginative suggestions,
like
pure uranium stars, or
some bizarre leftover from a strange
supernova
Meet Maarten Schmidt
Consider the Spectrum of Hydrogen
These are Emission Lines of Hydrogen
- but at huge redshifts!
So What?
Well, to have these large redshifts, the sources
must be very far away.
(The current record is a redshift of ~8!)
They must be fantastically bright objects to
show up so well! They certainly aren’t stars,
despite their appearance.
What to Call Them?
Quasi-stellar Radio sources = QUASARs
However, we later discovered many more that are
seen in visible light but NOT at radio
wavelengths. So the name was modified to
Quasi-stellar Objects = QSOs
[many astronomers just call them all ‘quasars’]
A Reminder
[objects with large redshifts are very far away!]
What’s The Problem?
To show up so conspicuously, despite being
so very far away, these objects must be
thousands of times as as bright as a whole
galaxy!
What can produce that kind of energy?
There’s a Further Problem:
Old photographs show that these objects
vary in brightness quite quickly
Quasar Variability
Since they vary dramatically on short
timescales (like one year), the luminous
region must be rather small (a light-year
in size, or less -- much smaller than a
galaxy!)
It seemed impossible to generate enough
power from so small a volume.
How Can We Explain This?
The ‘power’ problems go away if the quasars are nearby –
they don’t need to be particularly bright. But if they are
nearby, why do they have big redshifts?
Some imaginative new ideas were forthcoming:


Maybe quasars are small lumps shot out of the centre of
the Milky Way at very high speed (to explain the redshift).
But if other galaxies do the same, you’d expect some
quasars to be shot ‘towards us’, yet we see no big
blueshifts in any spectra. Is our galaxy unique? – that’s
unpalatable.
Maybe there’s ‘new physics’ of some unknown sort,
overthrowing standard theories of physics and gravitation!
Problem Solved
We now understand the power source: rapidly rotating
supermassive black holes (SMBHs) (very efficient energy
producers) in the central cores of galaxies
More recently, we have been able to show that some
quasars are indeed embedded in faint fuzzy patches (the
host galaxies); and there is considerable direct evidence
of the ubiquity of SMBHs in big galaxies in general
By the way, the SMBH in the Milky Way is not active in this
way, but may have been (mildly) so at one time
In Retrospect…
We over-reacted to the unusual quasars in
the 1960s because nothing like them had
ever been seen.
There are in fact related objects (AGN:
‘active galactic nuclei’) that span the gap
between ordinary galaxies and the
quasars
Radio Galaxies
Some strong
radio sources are
associated with
bright galaxies.
Double-Lobed Radio Galaxies
- two regions of strong emission
Some of These Sources are Huge!
[Our galaxy is sketched to scale at the lower left.]
What Produces the Long Jets?
Material (principally fast-moving electrons) is
‘squirted out’ along the spin axis of a rapidly
rotating supermassive black hole in the core of a
galaxy.
Stability is provided by the rapid spin, thanks to
the conservation of angular momentum. The jet
maintains its orientation, producing the very
long feature – up to millions of light years!
The
Central
Engine
An accretion disk is formed by gas and stars that are
spiralling into the black hole. Not all the material is
swallowed up, partly because it is guided ‘up and out’ by
strong magnetic fields. As a result, material is ejected at
‘relativistic’ velocities (near ‘c’) along the rotation axis.