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

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)

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

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

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.

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

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

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