Quasars

Chapter 17

Topics

• Quasars– characteristics– what are they?– what is their energy source?– where are they?– how old are they?– interactions of quasars and their light with other

objects

Furthest known quasar (April, 2000) -- red shift 5.8

Age ~ less than 1 billion years after the universe began

Quasar Characteristics• Starlike appearance• High redshift! (Hmm...)• Luminosities up to1000 times

normal galaxies• In visible light, they are brighter

than brightest giant elliptical galaxies

• Brighter than all other types of galaxies in gamma rays, X-rays, and infrared radiation

• First discovered because of high radio emission; “radio quiet” quasars actually outnumber the “radio loud” quasars

• Many exhibit emissions that vary in periods of days or weeks (Hmm...

• Spectrum shows emission lines of elements such as carbon, magnesium, oxygen, helium, and hydrogen

• Most also possess narrow absorption lines, usually with redshifts less than or equal to those of emission lines (Hmm...)

• Discovered in 1960s

What are they?

• Supermassive blackholes at the centers of faint galaxies, perhaps 10-100 billion solar masses

• Why do we think this?– quasars are similar to nearby active galactic nuclei

– a “fuzz” can be seen around nearby quasars

– nothing else can produce such a high luminosity in such a relatively small volume

– x-ray and radio jets are produced; this is consistent with a spinning, massive black hole

What is their energy source?

• Black holes do not emit light.

• A supermassive black hole accretes matter in a disk only a few light days wide.

• As the charged particles are accelerated due to the gravitational and magnetic fields of the black hole, they give off high energy radiation.

Where are they?

• High red shifts imply that they are very far away.• Most are 1 billion light years or more from Earth.• Some are more than 10 billion light years from

Earth.• Since none are close by, we believe that quasars

are a product of the “early” universe and that most have since died out.

• But if they are a result of supermassive black holes gobbling up stars and gas within its event horizon, why can’t a quasar form today? I don’t know.

Was this quasar ejected from the galaxy? No, it probablywas not ejected but just happens to appear along the same line of sight.

Gravitational lensing

• Sometimes a quasar is behind a galaxy or cluster of galaxies.

• The nearer object acts as a gravitational lens, bending the light of the quasar and forming multiple images of the quasar.

• This was further evidence that Einstein’s General Relativity was correct.

• Quasar is definitely more distant than the galaxy; thus, its redshift is cosmological.

• The galaxy’s gas creates the absorption spectrum; explains why the absorption spectrum has different redshift than the quasar’s emission spectrum.

Time delays of the light forming the four images produced by the gravitation of the red elliptical galaxy can be used to calculate the Hubble constant.

Einstein cross