Chapter 16

Quasars and Active Galaxies

What do you think?

• What do quasars look like?

• What powers a quasar?

Galaxy Cygnus A

This galaxy has a red shift corresponding to 6% the speed of light or 750 million light-years away

Quasars look like stars but have huge redshifts

• object with a spectrum much like a dim star

• highly red shifted

• enormous recessional velocity

• huge distance (ala Hubble’s Law)

• must be enormously bright to be visible at such a great distance

• Quasi-stellar object

• QSO or Quasar

This object that looks like a star must be

enormously luminous - its redshift indicates it is 4 billion light years

away!!

3C 273’s spectral lines are greatly redshifted

This change implies a distance of 2 billion light years

A quasar emits a huge amount of energy from a small volume

Such rapid changes in brightness can only result from changes small objects

Active Galaxies bridge the energy gap between ordinary

galaxies and quasars• peculiar galaxies (pec)

– appear to be blowing themselves apart

Active Galaxies bridge the energy gap between ordinary

galaxies and quasars• peculiar galaxies (pec)

– appear to be blowing themselves apart

• Seyfert galaxies– luminous, star-like nuclei with strong emission

lines

Active Galaxies bridge the energy gap between ordinary

galaxies and quasars• peculiar galaxies (pec)

– appear to be blowing themselves apart

• Seyfert galaxies– luminous, star-like nuclei with strong emission

lines

• BL Lacertae objects (BL Lacs)– featureless spectrum with a brightness that can

vary by a factor of 15 times in a few months.

Radio image of Cygnus A showing a small but very bright radio galaxy in the middle of the 320,000 ly wide lobes

Active galaxies lie at the center of double radio sources

Supermassive black holes lurk at the centers of some galaxies

• High resolution spectroscopy allows astronomers to peak at the motion of gas near centers of galaxies

• Some galaxies exhibit high-velocity jets of material leaving the center

• Observations suggest that the centers of some galaxies are incredibly massive

• All of this suggests the existence of supermassive black holes

Galaxy (which is actually

quite large)

Intergalactic gas jet

Giant Gas Clouds

(surrounding the galaxy)

Jets of matter ejected from around a black hole

may explain quasars and active galaxies

Jets of matter ejected from around a black hole may explain quasars and active galaxies

From where you observe it might make all the difference ...

What did you think?

• What do quasars look like?They look like stars but spew out immense

energy

• What powers a quasar?A quasar is a galaxy powered by a supermassive

black hole at its center.

Self-Check

1: Describe the observable properties of quasars and indicate how they are identified.

2: Explain why intrinsic properties of quasars are so difficult to explain.

3: List the various types of active galaxies and discuss how each might be related to quasars.

4: Discuss the observations that seem to be consistent with the presence of supermassive black holes in most galaxies.