Quasars

Quasars

Bucknell University

Astronomy 102: Stars, Galaxies and the Universe

Professor Ran Sivron

To the unaided eye quasars appear almost like stars. Quasars do differ from stars in that they are located in the farthest reaches of the universe and very important in understanding the realms of the universe. Quasars are very strong sources of radio sources even though they are faint in visible light.The word quasar originated from QSR, “quasi-stellar radio source”.Another important characteristic of quasars is that they are traveling away from the earth at very high speeds.

The History of Quasars

Quasars were discovered thanks to a combination of radio and optical astronomy.Single-dash radio telescopes were sufficient in giving accurate positions of objects in the sky so interferometers were adapted to be used in these observations.In Australia a large telescope was used to observe the phases of the moon and while this occurred a bright radio source, 3C 273, was observed.This radio source was monitored for several months and it was derived that it was not star-like in appearance, and a luminous jet appeared to be connected to the point nucleus.With these results the radio source was labeled quasi stellar.

In 1963 quasars were truly defined and understood.Maarten Schimdt photographed 3C 273 and realized that it had the same patterns as lines of hydrogen under normal terrestrial conditions.He then made a very important discovery as he pondered whether he was just observing a hydrogen spectrum that had its wavelength changed by the Doppler effect.This idea meant that 3C 273 was moving away from the earth at 16 percent of the speed of light.This number was concluded from the evidence that proved each wavelength would have to shifted 16 percent to the red to account for the spectrum.Schimdt’s colleague, Jesse Greenstein, also recognized this in the spectrum of 3C 48, with a 37 percent shift.

Absorption lines were also discovered in quasars, in addition to the emission lines that were already known about.Absorption lines differ among each quasar; some also may have differing redshifts and emission lines.The absorption lines are formed in clouds of gas of differing velocities surrounding the quasars, and others are formed as light travels from the quasars to the earth.It has been derived that the absorption lines of ionized carbon, silicon, magnesium and other heavy metals in quasars might have formed in the halos of unseen galaxies.

Quasars and Redshifts

Red shift is said to occur by relative motion, and this is an idea that is accepted by numerous astronomers. For velocities that are small in comparison to the velocity of light, the shift in spectrum is this: The change in rest wavelength divided by the wavelength is equal to v/c. (v = velocity and c = the speed of light). The Doppler effect causes the redshifts of quasars, and if Hubble’s law is applied it can be learned that the quasars with the largest redshifts are the farthest objects in the universe. Although, if quasars didn’t satisfy Hubble’s law then whenever an object was observed, the distance derived wouldn’t be accurate. Overall, this means that the quasar’s redshifts and distances are proportional with the same Hubble constant that we find for other galaxies. If quasars are accepted as being the farthest objects away in the universe then they will help us understand the earliest phases of the universe.

Quasars - Implied Distance

Consider 3C 48. (Click here) Z = 0.37 so v = 0.37c. Using Hubble's expansion law we get an implied distance of d = v/H =

(0.37)(300 000 km/s)/(75 km/s/Mpc) = 1470 Mpc = 4.8 billion light years

Quasars - Implied Brightness

3C 48 demonstrates that it is a great energy source just by the fact that it can be seen from such a great distance.The distance modulus formula shows that:

M = m + 5 - 5log(d)

= 16.6 + 5 -5 log (2000 Mpc) = -24.3This is about 30,000 times as luminous as a normal galaxy.

Active Galactic Nuclei

This is a picture of optical and radio images of the active galaxy NGC 4261. These are the radio jets, which occur at the speed of light.Radio galaxies, quasars, and blazars are AGN with strong jets, which can travel outward into large regions of space.

Two New Features About Quasars

1.Their spectra show different Z values for different elements and systematic differences between Z-emission and Z-absorption. The absorption line Z values are usually smaller.

2.The light out-put of a quasar can vary considerable over periods as short as days. This is really significant since it sets an upper limit on how big the quasar can be.

Quasars are also often found in clusters of galaxies and this has helped promote the suggestion that quasars are infant galaxies. There is observational evidence showing that quasars are found in the core regions of galaxies. Thus, the quasars that we see are the intense core regions of young, active galaxies. This makes sense on several accounts:

1.The look-back time places quasars at an earlier epoch during which galactic formation would be more intense

2. Another class of galaxies - the Seyfert galaxies bear many resemblances to quasars - perhaps quasars evolve into Seyfert galaxies. There are many unanswered questions about quasars but astronomers are confident that at least some of the details are correct.

The Energy of Quasars

Through optic variability it can be derived that quasars are very compact, and aren’t much larger than our galaxy.One favored model that supports this the super massive black hole scenario. A very massive black hole is surrounded by an accretion disk of swirling matter. As the matter dribbles onto the BH intense x-ray radiation is released which is absorbed and re-emitted by the accretion disk. Also, intense particle beams could be produced and propelled far out into space. This occurrence explains quasars, Seyfert galaxies, cosmic jets, and radio galaxies.

Quasars and Galaxy Cores

Questions have arisen concerning whether quasars are really the bright nuclei of galaxies.This would prove the fact that we can see both the nuclei and the spiral arms of nearby Seyferts; only the nuclei would be visible if such galaxies were far away.Seyfert galaxies have bright nuclei and also have broad emission lines from various stages of ionization in their spectra, signs that hot gas is present.These results show that quasars are somehow related to the cores of galaxies.This linkage makes quasars seem more normal, and at the same time galaxies appear to be more complex than scientists thought.An example of a link between quasars and galaxies is the galaxy Cygnus A.This is the second brightest radio source in the sky and its spectrum reveals the typical emission lines of a quasar.These emission lines are very broad due to the gas in the nucleus that is swirling at high speeds.

The Interaction of Quasars and Galaxies

There is evidence that quasars and galaxies interact with each other gravitationally—this would explain why some quasars that are close could be seen.The occurrence of Seyfert galaxies, with their bright nuclei, is more common in galaxy pairs than in individual galaxies.Quasars that have the largest redshifts tend to be the farthest away, and are using up their original storage of fuel.Closer quasars may be experiencing a second life as they acquire energy from other neighboring galaxies.At least 30% of quasars with redshifts up to .6 are interacting with neighboring galaxies.This evidence portrays the idea that Seyfert galaxies may have smaller black holes or have less fuel to consume than quasars.With all of this information it can be said that starburst galaxies, Seyfert galaxies, and quasars all may be a part of an evolutionary sequence.Also, radio galaxies may simply be older quasars that never received enough fuel.

The A Real Life Merger of a Quasar and a Galaxy

This picture was taken by the NASA Hubble Space telescope and pictures the merger of a quasar and a companion galaxy. This occurrence is very interesting and goes against many scientists’ theories.The bright object in the center of the picture is the quasar—it is most likely 7 billion light years away. The two smaller objects to the left of the quasar are parts of a bright galaxy that has been affected by gravitation between the quasar and the companion galaxy.

Double Quasars

The discovery of double quasars began when the Palomar Sky Survey charts revealed a close pair of 17^th-magnitude objects at a radio source position in 1979.These objects were then found to be quasars with identical spectras and redshifts.These results seemed improbable so it was suggested that the objects were actually just one quasar.This occurrence was explained by describing how a massive object between the quasar and the earth was acting as a gravitational lens, and the radiation from the quasar bended it one way on one side and the opposite way on the other.This lens-like object was said to be a giant elliptical galaxy with a redshift about ¼ of that of a quasar.Radio maps of this have been made with interferometers--the two objects that were found directly coincide with the two sharpest radio peaks.Also, if the brightness of this mass was observed the Hubble constant could also be measured.Since the discovery of this double quasar, more than a dozen other double quasars have been discovered.

Other Links:

http://heasarc.gsfc.nasa.gov/docs/objects/objects.html

http://www.kingsu.ab.ca/~brian/astro/chp14.htm

http://www.encyclopedia.com/articles/10687.html

http://www.eg.bucknell.edu/physics/astronomy/as102-spr00/