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Mysteries of Active Galaxies and Their Types

written by: •edited by: RC Davison•updated: 7/23/2011

In the Universe there are galaxies whose central core—their nuclei—are exploding. Not in a destructive blast, but in a continuous stream of various kinds of radiation sent into space over millions of years. Some is visible, some in other parts of the spectrum. These are known as active galaxies.

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    Galaxies in Action

    In the latter part of the 20th century, astronomers, as they peered ever further into the Universe, found some strange objects at the edge of the known Universe, at the time about 12 billion lys distant. They were unbelievably bright, brighter than any galaxy, but they seemed to be about the size of a large star. And as astronomers probed these phenomena, they found they emitted incredible levels of gamma rays, 100 million electron volts (MeV) and more, some up to several trillion eV (TeV—one eV is the energy gained by an electron accelerated through a potential of one volt).

    They called them quasi-stellar objects, or Quasars, because they looked like stars, but couldn’t be.

    They weren’t.

    As more powerful telescopes, radio telescopes, and the Hubble Space Telescope (HST) were turned on these incredibly distant objects, their true character began to take shape.

    They were galaxies, formed near the early stages of the beginnings of the Universe. Looking back through time to less than 2 billion years after the Big Bang, astronomers were seeing galaxies also in early stages of development. They had appeared smaller when first seen because we were seeing only the nuclei—the central core—of the galaxy. That’s where all the activity was taking place.

    The supermassive black hole at the center of the galaxy was obviously, 12 billion years ago, swallowing up stars around it. This was causing the explosion of radiation from the nuclei.

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    An Explanation for Other Mysteries

    In science, often the solution for one mystery leads to another mystery. But sometimes—sometimes—it leads to a solution for others.

    Astronomers had known of strange galaxies much closer to us that exhibited strong radiation emissions, both in gamma rays and visible light, as well as the RF (radio frequencies). Since 1943, astronomers have known of galaxies that emit low energy gamma rays and x-rays in a directed beam. They were discovered by astronomer Carl Seyfert and are called Seyfert galaxies. Seyferts emit low energy gamma rays up to about 100 keV.

    With the mystery of Quasars solved, it was obvious that the Seyfert emissions were the result of a supermassive black hole wrecking havoc on surrounding stars. Radiation Jets From an Active Galaxy 

    Also closer to us is another type of active galaxy. And this one is the gamma ray champ. It is called a Blazer. Blazars all emit gamma rays exceeding 100 MeV—and beyond. They also emit over the widest range of frequencies, from gamma rays, x-rays, visible light and RF. And they exhibit some strange behavior. Both in gamma ray emission and visible light, they are variable. Their emissions vary over days or weeks. Astronomers have not been able to explain this phenomenon yet.

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    An Einsteinian Viewpoint

    So what we have are three active galaxy types—Quasars, Seyferts and Blazars. All are produced by the supermassive black hole at the center pulling in surrounding stars like a starving hyena. As the stars fall through the event horizon that is the demarcation line between normal space and the black hole’s space, tremendous amounts of radiation, in various parts of the spectrum, are produced. These shoot out from the center of the black hole in a jet traveling very near the speed of light. The jet extends many light-years into space.

    But if all three types are produced by the same mechanism, why are they different?

    It seems they may not be.

    In all of Einstein’s relativity laws, everything is based on the viewpoint of an observer. An occurrence is relative to the observer. Astronomers think that is the case with the three types of active galaxies.

    All active galaxy types are the same. They have supermassive black holes at their center devouring the central stars and producing those relativistic jets of radiation spewing into space. We see three different types depending on their orientation towards Earth.

    We are seeing Quasars with the jet pointed directly at us, so we see the very high energy radiation. We are looking at Seyferts from the side, so we don’t see the high energy radiation in the jet.

    Blazars also have their jet pointed directly at us allowing us to detect the high energy radiation. But as we’ve said, we have yet to explain their variability.

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    More Than Meets the Eye?

    At this writing some 709 AGs have been found. The most distant is Quasar ULAS J1120+0641 at 12.9 billion lys. That’s just 770 million years after the Big Bang. These findings were just announced June 30 of this year by a team from the European Space Organization (ESO).

    Why are there so few compared to the billions and billions of galaxies in the Universe?

    Some astronomers think all galaxies at one time, in their early stages, were active. Then as they became populated with stars they settled down to become normal galaxies like our Milky Way. The central black hole absorbed all the nearby stars and had nothing left to feed on.

    But just how ‘normal’ is the Milky Way?

    The Fermi Gamma Ray Telescope was launched in 2008 and discovered something strange about our home galaxy. At the galactic core, 25,000 lys away, gamma ray bubbles are ballooning out into intergalactic space above and below the galactic center. These bubbles extend out 25,000 lys on each side of the galaxy.Milky Way's Gamma Ray Lobes 

    The leftovers of an active stage—or the beginnings of one?

    In 2006, our sister galaxy, Andromeda, M31, recently evidenced even stranger behavior. It's core flared to 100 times its previous brightness. It has since calmed down—to 10 times its previous activity. Is our neighbor about to go active again?M31 Center Before and After Flareup 

    For the same astronomers who think all galaxies started life as AGs think the ‘normal’ stage is just that, a stage in the evolution of galaxies. After billions of years as a calm hatchery of stars, they think all galaxies will again become active, when enough central stars come within striking distance of the central supermassive black hole.

    Then another Quasar, Seyfert or Blazar will be born. depending on where you're looking at it from, of course.

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    Sources and Credits

    Sources:

    Active galaxies: http://imagine.gsfc.nasa.gov/docs/science/know_l1/active_galaxies.html

    Types of AGs: http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html

    Milky Way lobes: NASA http://www.nasa.gov/mission_pages/GLAST/news/new-structure.html

    Active galaxies: NASA http://www.nasa.gov/mission_pages/GLAST/news/galaxy_surprise.html

    Active galaxies: University of Texas http://www.as.utexas.edu/astronomy/research/group_extragal.html?a=61

    Black holes: University of Texas http://blackholes.stardate.org/resources/articles/article.php?p=birth-of-black-holes

    Supermasssive black holes: University of Texas http://blackholes.stardate.org/

    Credits:

    Radiation jets: NASA http://imagine.gsfc.nasa.gov/docs/science/know_l2/active_galaxies.html

    Milky Way Lobes: NASA http://www.nasa.gov/images/content/498884main_DF3_Fermi_bubble_art_labels.jpg

    M31 flareup: University of Texas http://blackholes.stardate.org/news/m31-gets-frisky.php