1. Active Galaxies

Define:

Galaxies with nuclei (centers) that are overly luminous, and have emission indicative of a non-thermal source.

non-thermal: not a black-body

(so, not stars, or not just stars)

What, then, causes this luminosity?

More definitions:

Quasars - quasi-stellar radio sources

QSOs - quasi-stellar objects

I will refer to ``quasars'' and ``QSOs'' interchangeably, except when noted.

Are QSOs related to active galaxies?

Almost certainly

QSOs are likely to be 'naked' active galaxies, where the nucleus out-shines the rest of the ('host') galaxy.

I will use AGN to mean all of above, except when noted.

Where do we find quasars and AGN?

Some are found locally.

More are found at larger distances.

QSOs are Extremely rare and luminous

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

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

2. The Central Engine

(i) The facts:

AGN have high luminosities, typically larger than 'normal' galaxies (like the Milky Way).

- most of this emission is at infrared and radio wavelengths

energy emission is non-stellar (non-thermal)

- spectral continuum is NOT shaped like a black-body - continuum radio emission is largely due to electrons spinning in magnetic fields

``synchotron'' radiation

energy emission is variable on time-scales of one year or less.

- objects rapidly get brighter and fainter

small sizes

(of order a light-year in size)

AGN often are associated with jets of material extending over large distances (up to Mpc)

- a powerful engine!

spectra show broad emission-lines that indicate rapid internal motions in the energy-producing region

- a massive engine!

(ii) The speculations:

Could the central engine consist of . . .

(a) . . . an incredibly massive star?

Possibly, but it would burn up quickly and lead to a massive black hole.

(b) . . . many very massive stars?

Possibly, but they would burn up quickly and lead to many black holes that would eventually collapse into a single massive black hole.

Also, neither of the above cases naturally produce the right time variability, jets, and large non-thermal emission.

(c) . . . a massive black hole surrounded by a hot, glowing accretion disk fed by in-falling material

fits the bill!

- small size

rapid variability

- hot accretion disk, with magnetic fields and lots of free electrons

non-thermal emission

jets

- massive = powerful

high luminosity (a little fuel goes a long way)

3. The Zoo

Radio Galaxies:

- have some kind of strong radio emission:

large radio 'lobes', or

radio-loud jets, or

compact radio cores

- sometimes have 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.

Seyferts

- don't have as much radio emission

- don't have radio lobes

- don't have optical jets

- in spiral galaxies

Quasi-stellar objects

radio loud: quasars

radio quiet: QSOs

A connection?

radio galaxies quasars

spiral galaxies QSOs

. . . perhaps. But we don't know for sure.

(a) radiation is non-thermal.

(b) central luminosities vary on rapid time-scales.

(c) some AGN have jets.

(d) some AGN have strong radio emission.

(e) some AGN are in spiral galaxies.

(a) Only active galaxies have radio emission.

(b) Only QSOs have radio emission.

(c) One can only see the host galaxy in QSOs.

(d) One can see the host galaxy in active galaxies.

(e) The emission is thermal in QSOs.