1. ACTIVE GALACTIC NUCLEI
Pinpoint nuclei emitting intense light
can be 100 x entire galaxy!
Incredibly compact: < size of Solar System
“Nonstellar” (“nonthermal”) spectrum of radiation
Spectral lines smeared out in wavelength
interpret by Doppler shift atoms move at ~10% c
randomly moving gas clouds, not thermal motion
High-speed jets of gas
close to speed of light

2. ULTRAVIOLET SPECTRUM of a SEYFERT GALAXY
Hydrogen
Carbon
Oxygen
NGC 4151:
Digital Sky Survey
UV spectrum of the Seyfert galaxy NGC 4151
observed with Hopkins UV Telescope (Kriss et al. 1992)

3. NGC 1068: An Obscured Seyfert
This bright cloud has a direct view of the intense nucleus
Radiation and (possibly) a wind escapes through a cone-shaped funnel
Nucleus is hidden in here

4. Schematic of an AGN
C.M. Urry and P. Padovani

5. MAX. SIZE FROM VARIABILITY
The “light travel time” argument:
Suppose source doubles (or halves) brightness in 1 day
If source is bigger than 1 light-day then no signal can coordinate the variation
Therefore source must be be smaller than 1 light-day
1 light-day
A catch: Random (uncoordinated) fluctuations may make source
appear to vary more quickly than light travel time - but only rarely.

6. THE REAL CATCH: RELATIVISTIC MOTION
Suppose source emits flashes 1 day apart, while moving toward you at
Flash 1
0.2 lt-day
Flash 1
Flash 2
1 lt-day
Flash 2
0.8c
Flashes emitted 1 day
apart, received
0.2 days apart.
0.8 lt-day
Flash 1
Day 0
Day 1

7. MIN. MASS FROM ENERGY OUTPUT
We see lots of active galaxies
Their lives can’t be too short (> few million yr)
Luminosity X Lifetime = Total energy output
Total energy output < Total mass
“Central Engine” > few million solar masses

8. PUTTING IT TOGETHER Variability small size Energy output large mass
COMBINATION OF SMALL SIZE AND
LARGE MASS BLACK HOLE