quasars – unsolved mysteries?

March 7, 2006 Astronomy 2010 1

Quasars – Unsolved mysteries?

• 2 basic things used for studying far away galaxies – the data encoded in the light received; – our creative minds to interpret what is seen using the

laws of physics. • Some blue star-like objects appeared to violate those

rules.


Discovery of Quasars

• Stars do not produce much energy in the radio band.• When strong radio emission coming from some blue

stars was spotted in 1960, astronomers were puzzled.

• They quickly took spectra of the stars in the visible (optical) band to find out the conditions in these strange objects.


Quasars – Unsolved mysteries?

• Line patterns did not match any of the patterns seen in 1000’s of stellar spectra gathered over a 100 years.

• Spectra of the blue radio sources did not have absorption lines, but broad emission lines!

• What a mystery!


Quasars

• Maarten Schmidt solved the mystery in 1963.• Constructed an energy level diagram from the pattern of the

emission lines. • As a test he compared the spectrum of 3C 273 with the

spectrum of hydrogen. • He was shocked because the pattern was the same but

greatly red-shifted! • 3C 273 is moving at a speed of 47,400 kilometers/second

(almost 16% the speed of light!).


Quasars

• The Hubble Law says that this blue radio object is far outside the Galaxy.

• Other radio “stars” were also at great distances from us.

• Called quasi-stellar radio sources or quasars.• Later, other blue star-like objects at large red-shifts

were discovered to have no radio emission, but they are also called quasars.


Strangeness of Quasars

• What is strange about the quasars is not their great distance, but, rather, their incredible luminosities.

• They are 100 to 1000 times more luminous than ordinary galaxies.

• Yet, all of this energy is being produced in a small volume of space.


Strangeness of Quasars…

• Their luminosity varies on time scales of a few months to as short as a few days.

• The quasars that vary their light output over a few months are about the size of our solar system.

• This is 10,000 times smaller than a typical galaxy!


Star/Galaxy Spectra

• The shape of the continuous part of a quasar spectrum is also quite unusual.

• Stars are luminous primarily in the visible (optical) band of the electromagnetic spectrum.

– Hot stars also emit a significant fraction of their light in the ultraviolet band – Cool stars emit a significant fraction of their light in the infrared band.

• Star spectra, and the spectrum of a normal galaxy, are thermal, they rise to a peak at a wavelength determined by the temperature and drops off at shorter or longer wavelengths.


Quasar Spectra

• Quasars have a non-thermal spectrum– luminous in the X-ray, ultraviolet,

visible, infrared, and radio bands.

• Have same power at all of the wavelengths down to the microwave wavelengths (shortwave radio wavelengths).

• Spectrum looks like the synchrotron radiation from charged particles spiraling around magnetic field lines at nearly the speed of light


Quasar Mystery

• Quasars are found in clusters of galaxies. • Galaxies are much fainter than the quasars

– Only the largest telescopes can gather enough light to create a spectrum for those far away galaxies.

– Their spectra also have the same large redshift of the quasars in the cluster.

• Some quasars are close enough to us that some fuzz is seen around them. – Color of the “fuzz” is like that of normal galaxies. – Spectra show that the light from the “fuzz” is from stars.


Top left: core of normal spiral, Bottom left: core of normal elliptical, Top center: spiral galaxy hit face-on to make a quasar+starburst galaxy,Bottom center: quasar merging with a bright galaxy and maybe another one,Top right: tail of dust and gas show that the host galaxy collided with another one Bottom right: merging galaxies create a quasar in their combined nucleus.


Quasars

• Quasars are the exceptionally bright nuclei of galaxies!


Active Galaxies: a Clue to Quasars

• Not all active galaxies blaze with the strength of a quasar.

• They do exhibit a non-thermal spectrum that has no peak and does not depend on the temperature.

• And the energy is generated in their nucleus.• Active galaxies are less energetic cousins of the quasars. – Luminosity between that of typical galaxies and the powerful quasars.

– Whatever is going on in quasars, is going on in active galaxies to a lesser extent.


Seyfert Galaxy

• Type of active galaxy named after Carl Seyfert – first to discover their peculiar spectra.

• Spiral galaxy with compact, very bright nucleus– produces a non-thermal continuous spectrum with broad

(fat) emission lines on top.• Some emission lines produced by multiply ionized

atoms.


Seyfert Galaxy

• Such highly ionized atoms are found only in regions of intense energy.

• Many Seyfert nuclei are in disks with distorted spiral arms and a companion galaxy nearby that is probably gravitationally interacting with the galaxy.

• The energy of Seyfert galaxy nuclei fluctuates quickly like the quasar fluctuations, so the energy generator must be quite small.


Seyfert Galaxy (2)

• Broad emission lines are produced by gas clouds moving at about 10,000 km/s.

• Doppler shifts of the gas moving around the core widens the emission lines.


Radio Galaxy

• Yet another type of active galaxy…• Emit huge amounts of radio energy.• Radio emission from the core AND very large

regions on either side of the optical part of the galaxy called “radio lobes”.

• Radio lobes can extend for millions of LY from the center of the galaxy.


Radio Galaxy (2)

• Radio emission from normal galaxies is thousands to millions of times less intense and is from the gas between the stars.

• Most radio galaxies are elliptical galaxies.• Spectrum of the radio emission has the same non-thermal

(synchrotron) shape as the quasars and Seyferts. • Radio lobes produced from electrons shot out from the

nucleus in narrow beams called jets.• When the electrons in the beam hit the gas surrounding the

galaxy, the beam spreads out to form the lobes.


Examples of Radio Galaxies

quasars can have huge radio lobes also.


Power Source for Active Galaxies and Quasars

• Problem: – how does nature produce objects that are luminous over a large range of wavelengths and generate the energy in a very small volume?

– Number of stars needed to produce the tremendous luminosity could not be packed into the small region and neither would they produce the peculiar non-thermal radiation.


Production Scenario - 1

• Production of radiation by hot gas in an accretion disk around a black hole.

• Black hole must be super massive. • The intense radiation from the disk would drive the gas outward if the black hole did not have enough gravity to keep the gas falling onto the black hole.


Production Scenario - 2

• In order to keep the gas spiraling in and heating up, the mass of the black hole must be hundreds of millions to several billion solar masses.

• The accretion disk is a few trillion kilometers across (a few light months) but most of the intense radiation is produced within a couple of hundred billion kilometers from the black hole.


Massive Black Holes•HST has imaged the nuclei of several active galaxies.

The black event horizon of the super-massive black hole too small to be resolved from our distance.

Surrounding the core of the radio galaxy NGC 4261 is a ring of dust and gas about 400 light years in diameter and the jets emerge perpendicular to the plane of the dust/gas ring.


• Doppler shifts of the disk material close to the center show that the gas is moving at speeds of 100 km/s.

• Blueshifted/Redshifted lines produced from opposite parts of the disk – clear proof of rotation.

• Base on the observed speeds and distance the gas is from the center, the central object must have a mass of 2.5 billion solar masses.

• Only a black hole could be this massive+compact. • The jet coming from the nucleus (visible in the

wider-field view at right) is also seen to be perpendicular to the plane of the disk.

Core of active galaxy M87 has a disk of hot gas moving very quickly around the center.


Where Quasars are…

• Found at great distances from us– no nearby quasars.

• We see them as they were billions of years ago. • Number of quasars increases at greater distances.

– they were more common long ago. • Number of quasars peaked at a time when the universe was

about 20% of its current age. • Back then the galaxies were closer together and collisions

were more common than today.


Where Quasars are… (2)

• Also, the galaxies had more gas that had not been incorporated into stars yet.

• The number of quasars was hundreds of times greater then than now.

• At very great distances the number of quasars drops off.

• The light from the most distant quasars are from a time in the universe before most of the galaxies had formed, so fewer quasars could be created.


Dead Black Holes

• Predicts there should be many dead quasars lurking at the cores of “old” galaxies.

• Astronomers beginning to find the inactive super massive black holes in some galaxies.

• In most galaxies the central BH would have been smaller than the billions of solar mass black holes for quasars. – This is why the less energetic active galaxies are more common than quasars.

• Our galaxy harbors a super massive black hole in its core that has a mass of “only” 2.5 million solar masses.

• Astronomers are studying the cores of other normal galaxies to see if there are any signs of super massive black holes that are now “dead”.


Whirlpool Galaxy

• X marks the spot in the core of the Whirlpool Galaxy!

• Darkest bar may be the dust ring seen edge-on.

• The jet seen in wider fields of view is perpendicular to the darkest dust ring.

• The lighter bar may be another disk seen obliquely.

• A million solar mass black hole is thought to lurk at the center.


Some closing comments…

• An important implication of the fact that there were more quasars billions of years ago than now, is that the universe changes over time.

• The conditions long ago were more conducive to quasar activity than they are today.

• Also, the sharp drop in the quasar number for the earliest times is evidence for a beginning to the universe.

quasars – unsolved mysteries?

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