What are Quasars?

Wednesday 20 September 2017 - 00:08:44 Posted by  Bobby

What are Quasars

  Quasars are active galaxies with highly luminous nuclei. Their phenomenal luminosity somehow originates in the immediate vicinity of the supermassive black holes that they harbor. They stand out as the beacons of the cosmos.

Circinus Galaxy

  This Hubble Space Telescope image shows the black hole core of the Circinus galaxy resembling a swirling witches cauldron of glowing vapors.
This galaxy is designated a type 2 Seyfert, a class of mostly spiral galaxies that have compact centers and are believed to contain massive black holes.
The Galaxy lies 13 million light-years away, in the southern constellation Circinus.
  The energy output is not only visible light; it probably originated as high energy X-rays, and cascades down the electromagnetic spectrum, making the nuclei apparent in X-ray, ultraviolet, visible, infrared, and occasionally radio wavelengths. While they often exhibit jets, like radio galaxies, clearly most of the energy released has been diverted into luminosity.


  The radiation output often shows variations on time scales as short as a week. To show such coherent variation, the region responsible cannot be larger than light-weeks in size, further indication that something as compact as a black hole must be involved. In physical size, the active nucleus must be tiny compared to the galaxy that hosts it.
  Yet, astonishingly, the light from the nucleus may out shine all the stars in the galaxy many times over, so much so that the host galaxy is often lost in its glare. Suffice to say that when quasars were first discovered in the 1960s, they were first mistaken for foreground stars within our own Galaxy.
  The word quasar is an acronym for "quasi-stellar radio source" named because the first quasars were identified by their radio emission. Nowadays, we know that the majority, ironically, are radio quiet.

NGC 4258

  Combination of ground-based and X-ray images of NGC 4258 (M106). Two prominent arms emanate from the bright Seyfert 2 nucleus and spiral outward. These arms are dominated by young bright stars, which light up the gas within the arms.

Quasar Types

  Not all quasars are "super luminous". Seyfert galaxies - first identified by Carl Seyfert in 1943 - are recognizable as normal galaxies, predominantly spirals, but possessing highly luminous nuclei. The so-called Seyfert 1 nuclei are identical in all respects to quasars, save for their more modest luminosity.
  Today we recognize that the same phenomenon extends down to low luminosity - very mild quasar nuclei are sometimes barely detectable in nearby spiral galaxies. There are also related galaxies with low-luminosity nuclei known as Seyfert 2 galaxies, and low-excitation liners.
  Our best insight into the physical conditions in the immediate vicinity of the nucleus is by spectroscopy, displaying the amount of emitted light against wavelength - effectively color. Active nuclei stand out because of their conspicuous emission lines of hydrogen, oxygen, and nitrogen. Broad hydrogen emission lines come from the densest environment, and tell us that the gas clouds responsible are moving at thousands of miles a second, vastly more than they would do anywhere else in the galaxy, in a region within light-weeks of the nucleus itself. Such high velocities indicate the presence of an enormous, yet condensed mass that could only be a supermassive black hole. Narrower emission lines indicate much lower velocities from a more extensive region, even hundreds of light-years in size.
  Quasars and Seyfert 1 galaxies show both broad and narrow lines. Seyfert 2 galaxies and Liners have only narrow lines, while BL lac objects have no emission lines at all, but show variations on time scales as short as hours. (The curious name "BL Lacertae" was given to the prototype object, when it, too, was mistaken as a foreground star.)


  Recent years have seen a model emerge that unifies these different types. The key element of this model is that the accretion disk close to the nucleus forms an obscuring torus - same shape as a ring doughnut. If you happen to view this torus side-on, the nucleus and its immediate surroundings will be hidden.
  To see a quasar or Seyfert 1 galaxy, you need to see the torus flat-on, so the nuclear regions are not obscured.
  As the line of sight moves from flat-on toward head-on, the increasing obscuration makes the object look like a Seyfert 2 galaxy and then a Liner.
  If the active nucleus is emitting jets, and a jet is seen end-on, the outcome is a BL Lac object.
  There are not as many quasars as there used to be. When we look deep into space - and therefore back in time to when the universe was about a third of its present age - we see a much higher population of bright quasars. But the galaxies then were much closer to one another and interactions far more common. As with the giant elliptical radio galaxies, it is interactions that seem to feed material into the supermassive black holes, so turning on quasar activity. Possibly then, all giant spiral galaxies have the potential to have periods of quasar activity. Although our own galaxy has only a modest supermassive black hole, perhaps its nucleus once shone brightly, though from Earth it would have largely been obscured by the dust clouds of the Milky Way.

Hubble images of two Quasars


Left: HE0450-2958 (about 5 billion light-years away) does not have a massive host galaxy.
Right: HE1239-2426 (about 1.5 billion light years away) has a normal host galaxy.

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