1. The Active Galactic Nuclei Zoo

Radio Galaxies:

• have some kind of strong radio emission:
  large radio-loud 'lobes', jets, or compact cores
• sometimes optical jets
• appear to be only in elliptical galaxies

One notable sub-type: BL Lac's

These objects have 'featureless' spectra; i.e. continuum only, no emission lines or absorption lines. Most likely a projection effect.

Seyferts:

• don't have as much radio emission
• don't have radio lobes
• don't have optical jets
• in spiral galaxies

Quasi-stellar objects:

• Extremely rare and luminous
• look like stars in images taken from ground-based telescopes
• spectra are not those of stars -- non-thermal radiation.
• radio loud: Quasars
• radio quiet: QSOs

A connection? i.e. are QSOs related to active galaxies?

radio galaxies / ellipticals Quasars

spiral galaxies / seyferts QSOs

Are ``quasi-stellar'' objects

• active galactic nuclei that out-shine the rest of the ('host') galaxy.

or . . .

• active galactic nuclei that form before the the rest of the galaxy.

The one thing all 'animals' in the zoo have in common is that their central engines are all presumably massive black holes.

2. Luminosities, Masses and Lifetimes

(i) Luminosities

10³⁸ - 10⁴² Watts

10⁴⁰ corresponds to 20 trillion suns

(a lot of light bulbs!)

(ii) Masses

Black hole masses:

10⁸ - 10⁹ M for luminous quasars

(10⁶ - 10⁷ M for low-luminosity active galaxies)

Amount of fuel needed to produce luminosities:

- 0.1 - 1000 M per year!

depending on luminosity and black hole mass

(iii) Lifetimes

Universe is roughly 10 Gyr (10¹⁰ years)

black hole mass grows to over 10¹³ M!

This is more massive than most galaxies.

A problem?

3. Evolution

Some QSOs are found locally.

More are found at larger distances.

In general, QSOs are Extremely rare and luminous

Roughly 1 quasar (or AGN) for every 1000 'normal' galaxies nearby.

The most luminous quasars are up to 1000 times brighter than the brightest 'normal' galaxy!

Observational claim:

'Long ago and far away there were many luminous quasars, but today (nearby), there are very few and they are less luminous'

True? Many possibile explanations. Here are 2:

1. quasars are long-lived (billions of years), rare events.

- consumed all their fuel

- now lie dormant in 0.1% of normal galaxies as super-massive, central black holes

2. quasars are short-lived (a billion years) common events

- consumed all their fuel

- now lie dormant in almost all normal galaxies as moderately massive central black holes

How can we figure out which one?

Observations:

Look down the throats of nearby galaxies with the Hubble Space Telescope and see what lies within.

(Measure mass of nucleus from Doppler widths of spectral lines)

4. Revelation

Do quasars really evolve? Almost certainly, but . . .

What do we actually observe?

Recall:

We observe 'apparent-brightness-limited' samples, not volume-limited samples.

The farther away you look, the more volume you sample -- an ever widening cone.

In a given volume, however . . .

- bright objects are intrinsically rare

- faint objects are intrinsically common

Distant objects observed are over-luminous

Nearby objects observed are under-luminous

Are we comparing ``apples to apples?''

Only if we can push to very faint apparent brightness can we see under-luminous objects at great distances, and make a fair comparison.

To do this we need big telescopes.

Summary

(a) Observe a quasar for a long time.

(b) Observe quasars at different times.

(c) Measure the masses of central black holes in nearby galaxies.

(d) Measure the age of a black hole.

(e) Measure the total mass of nearby galaxies.

(a) at large distances where there is more volume.

(b) at short distances.

(c) equally at all distances.

(d) luminous quasars are only found nearby.

(e) luminous quasars would not be found in an apparent-brightness-limited sample.