Active Galaxies and Quasars

Fig. 25.15

3C 175, radio jets and central quasar

1500 Mpc away

Radio lobes span a million light years!

Normal Galaxies

Milky Way has a luminosity of 10³⁷ W

Energy emission we measure comes from the accumulated light from a billion stars

Spectrum peaks in the visible

Seyfert Galaxies

Seyfert galaxies look like a normal spiral galaxy in visible light

However the nucleus is 10,000 times brighter than a normal nucleus; nucleus is 10 times brighter than a normal spiral

Most of the emission from the nucleus is in radio and IR bands

Rapid variability of nucleus

M87- Jets in Nuclues

Giant Elliptical galaxy

Fig. 25-9: optical and radio jet

Centaurus A- Radio Galaxy

Giant Elliptical galaxy

Fig. 25-4: optical and radio images, not to same scale

Centaurus A- Radio Galaxy

Fig. 25-5: Giant elliptical galaxy recently collided with dusty spiral

Radio lobes are 10 times size of Milky Way, same size as Local Group, no optical emission from lobes

Head -Tail Radio Galaxy

Fig. 25-7

Radio image on left

Optical image on right, with radio contours superposed

Why are jets bent?

AGNs and Radio Galaxies

Fig. 25.16

Fig. 25-6

Radio Galaxies

Fig. 25.8: Core- Halo radio galaxy

Perspective

Fig. 25.10, what you see depends on your viewing angle

Superluminal Motion

Fig. MP25-1

Motion appears faster than the speed of light

This is due to the geometry and near c velocity of the jet

Quasar

Quasar is the fast way of saying QSO (quasi-stellar object)

Spectrum of quasar is not star-like

Objects appear star-like (point-like) with no extended emission until recent high-resolution images

Fig. 25-11

Central Engine

The spectrum shows the same line measured at two different places in the nucleus of M87

What does the spectrum tells us about matter in the nucleus?

Fig. 25-18

Properties of the Central Engine

High luminosity (much greater than luminosity of Milky Way 10³⁷ W)

The energy emission is mostly nonstellar

Energy output can be highly variable- implies that the emitting region is less than a parsec across!

Often have jets or other signs of explosive activity

Optical spectra may show broad emission lines characteristic of rapid internal motion

Lifetime of Quasar

1 star per year can feed a 10³⁸ W (10⁸ -10⁹ solar mass) black hole

10 stars per year can feed 10⁴⁰ W black hole, 100 times brighter

Lifetime of universe ~10 billion years

Need 10¹³ stars to feed a quasar over that time

We don't see galaxies this massive

We don't see any 10¹³ solar mass black holes

Quasars are short-lived phenomena

Nonthermal Radiation

Nonthermal radiation (synchrotron) is caused by accelerating electrons that spiral around magnetic field lines emitting photons

Thermal radiation (blackbody) is caused by thermal (kinetic) energy of atoms jostling each other

Fig 25-20

Gravitational Lensing

Lensing yields mass and idea of distribution of mass

Fig. 25-222 and Fig 25-23

Quasars and Look Back Time

Quasar hosts: Mostly irregular in shape, Fig. 25-27

Redshift gives recessional velocity, velocity gives distance (Hubble's Law), light travels at finite speed- distance equals light years.

Light years can be thought of as the years it takes light to travel the distance= look back time

Galaxy Formation

Fig. 25-28

Summary

Difference between active and normal galaxies

Properties of Seyferts and radio galaxies

Properties of quasars and the radiation emitted

Properties of central Engine

Use of Quasars as probes of distant Universe

Describe how AGNs and quasars fit into galactic evolution