Active Galactic Nuclei

Astronomy 315Professor Lee

CarknerLecture 19

Strange Galaxies Some galaxies have a compact,

powerful source of energy at their core Among the most energetic objects in

the universe

Need multiwavelength observations to

understand them

M87

Jet From M87

Seyfert Galaxies Seyfert galaxies are spirals with

optically bright, concentrated nuclei

Can change in brightness very quickly

Small, but very bright

Other Seyfert Properties Some Seyferts have broad emission

lines

These same Seyferts also are bright in high energy X-ray and UV radiation

Called Type 1 Seyferts

Seyfert Clues

Are there any common properties shared by Seyfert galaxies?

Are Seyferts caused by gravitational interactions?

Radio Galaxies

Some galaxies are flanked on either side by a pair of radio lobes

Some central galaxies are also bright radio sources and some are not

Cygnus A Radio Galaxy

Mapping the Lobes Radio lobes often show hot spots of

enhanced emission

Lobes are material ejected from the star and impacting the intergalactic medium

Radio galaxies produce bipolar jets

(like young stars)

Source of Radio Waves

Radio emission is due to synchrotron radiation

Lobes must have magnetic field and galaxy must be ejecting electrons

Total energy stored in lobes is huge

Radio Galaxy Properties Central galaxy is often giant elliptical

and in a crowded cluster Often deformed

Jets sometimes are twisted

Some radio galaxies have broad and

narrow lines, some just narrow lines

BL Lac Objects BL Lac objects look like stars but show

rapid variations

They don’t show the broad or narrow lines we see in Seyfert galaxies

Don’t have radio lobes

Quasars Some sources of radio galaxies look like

stars

Quasars have very large red shifts and very large distances

Since quasars are billions of light years away, we are seeing what they looked like billions of years ago

Quasar Properties In most cases you can’t see the host galaxy

Core must be brighter than regular AGN

Quasars are younger than “normal” AGNs Type 1 quasars have broad emission lines,

Type 2 quasars do not

AGN Power Source

We have two questions about AGNs

Different types of AGNs are due to viewing matter falling into a black hole from different angles

Massive Black Holes AGN black holes are a million to a billion

times the mass of the Sun Why do we think they have black holes?

Computed densities indicate black hole

AGNs vary so rapidly that the emitting region must be very small (small+massive=BH)

Structure of the Core Black hole pulls matter into an accretion disk

Outer disk is thick can block view of center The moving material twists up the magnetic

field creating a magnetic flux tube that the jets follow out the poles

Unified Model

How does this model account for the basic properties of AGNs?

We thus see the jet and disk regions

in different ways, producing the observed type of AGN

Case 1 -- Face on

Can see the radio jets, but no lobes Can’t see broad or narrow lines

Type of AGN

Case 2 -- Inclined Can see radio emission and lobes

Can also see broad and narrow lines

Types of AGN:

Case 3 -- Edge on Can see radio emission and lobes

Only see narrow lines

Types of AGN

Unified Model for AGNs

Quasars and AGNs Quasars act very much like extra

powerful versions of radio galaxies or BL Lac objects

Quasars may be young active AGN,

low red shift active galaxies may be AGN that have been refueled

AGN and Non-AGN Why are some galaxies active and others

not? We think all galaxies have black holes

What makes a galaxy active is matter falling

into it

Collisions and tidal forces may disrupt the center of galaxies and move material into the black hole

Next Time

Read 24 .3, 25.1-25.4