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Quasars
Chapter 17
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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
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Furthest known quasar (April, 2000) -- red shift 5.8
Age ~ less than 1 billion years after the universe began
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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
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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
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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.
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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.
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Was this quasar ejected from the galaxy? No, it probablywas not ejected but just happens to appear along the same line of sight.
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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.
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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.
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Einstein cross