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:

n  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.

n  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!

Remember that some quasars have small jets.

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.