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Homework Help: Active galaxies

  1. Apr 21, 2015 #1
    1. The problem statement, all variables and given/known data

    Describe what optical astronomers would consider the properties of an active galaxy and also describe what radio astronomers would describe as the properties of an active galaxy and explain how two sets of astronomers came up with these differing conclusions.

    2. Relevant equations


    3. The attempt at a solution

    Ok. So optical astornomers would see a bright AGN and strong, broad emission lines suggesting high velocites for the gas within the galaxy. With the example of Centaurus A, optical astronomers would see an eliptical galaxy whereas radio astronomers would see jets of radiation feeding two lobes either side of the galaxy.

    What other properties would they be asking for? And it asks to describe in about 100 words how they could come to differeing conclusions. Well what more can I say other than one is viewing the galaxies at optical wavelengths whilst the other at radio wavelengths?

    Any ideas?

  2. jcsd
  3. Apr 23, 2015 #2


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    The mechanisms responsible for radio emissions is not generally the same as those that emit in the optical regions.
  4. Apr 28, 2015 #3
    It doesn't matter how many times I read the chapter on Active galaxies in the book, it doesn't seem to be sinking in.
  5. Apr 29, 2015 #4


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    Well, maybe it will help if you discuss it here.
    You may experience this problem because you get lost during some point in the text.
    That is something is unclear and then the rest becames a blank.
    So try and identify where in the text you don't know what is going on.
    Last edited: Apr 29, 2015
  6. Apr 29, 2015 #5
    The main differences I'm getting, is that optical astronomers would have missed things like the jests and lobes in radio galaxies. And I think Quasars were first thought to be stars in our own galaxy until it as realised they were in another very distant galaxy, but I'm not sure if this discovery was made by radio astronomers.

    I think also that optical astronomers would see strong, broad emission lines which would indictae high velocities of gas. But wouldn't radio astronomers see this too?
  7. Apr 29, 2015 #6


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    Last edited: Apr 29, 2015
  8. Apr 30, 2015 #7


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    You firstly need to identify the physical processes responsible for the
    emissions in the visible and the radio wavelength regions in active galaxies.
  9. Apr 30, 2015 #8
    I've spent far too long on this question now.

    Thanks again for your time :-)

    `This is what I ended up with....


    Seyfert galaxies are extremely luminous with an excess of radiation. Optically, this radiation is seen to be variable, implying that the light is coming from a very small region in comparison to the rest of the galaxy which is known to be the AGN. It is so luminous in fact that the nucleus of an active galaxy could completely wash out the rest of the galaxy.

    Narrow and broad forbidden lines are evident in optical spectra. And strong and broad emission lines would imply large velocities of a hot gas similar to HII regions. The velocities of these gasses are calculated from the broadening of the emission lines and are in excess of thousands of km –s.


    At radio wavelengths, radio galaxies can be seen to have jets radiating away from the AGN, feeding two very large lobes on opposite sides of the galaxy.

    Quasars often emit at radio wavelengths. Because of their vast distances and their huge redshift, some emission lines would not be seen in the optical spectrum, but at different wavelengths.


    Observing a galaxy such as Centaurus A, optical astronomers would have seen a large elliptical galaxy, whereas radio astronomers would see entirely different features, such as the lobes and jets feeding them and a very luminous nucleus.

    Originally Quasars where thought to be a type of star with strange emission lines until it was realised that they were in fact very distant galaxies and their emission lines were just highly redshifted.

    These differences seen by the different astronomers arise from observing the galaxies at different wavelengths.

    Another reason astronomers were seeing the same kind of galaxies very differently was because of the viewing angle. Depending on the angle the galaxy is seen from, it could be classified as a Quasar, Blazar, Type I Seyfert or Type II Seyfert.
  10. Apr 30, 2015 #9


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    spiral galaxy with radio-emitting jet.jpg I think you did an excellent job with this description. Think it is worth 90% or more...
    As far as my knowledge goes the same mechanism is responsible for all active
    galaxies. A supermassive object, most likely a black hole, is gobbling up the rest
    of the galaxy (self-cannibalism) or other nearby galaxies (cannibalism). As the
    matter is pulled in it is compressed by the strong gravitational field and pulled
    into a rotating accretion disk around the black hole. Syncrotron radiation results
    as the matter accelerates into the bh. Some of the matter gets ejected in jets at
    relativistic velocities from this process. These jets feeds the lobes as you remarked.
    The syncrotron radiation occurs at virtually all wavelengths with the same intensity.
    Problem is the shorter wavelengths can be absorbed by dust and gas in between
    the source and the observer (that is why the centre of our galaxy, also thought to
    be a bh can only be observed at radio wavelengths).

    In the case of Centaurus A the radio features is much larger than the optical galaxy - about 100x.
    Quasars are thought to be the nuclei of active galaxies. Their galactic structure can only be seen
    with special techniques due to the large intensity of the nuclei. Also wort mentioning is the fact that radio telescopes are much larger than optical telescopes and can thus pick up much fainter sources.
    Last edited: Apr 30, 2015
  11. Apr 30, 2015 #10
    Thanks very much for this. The whole question was worth 15 marks and I just couldn't see what I'd wrote picking up that many, but I'll be happy with about 10 I guess.

    I had read somewhere about synchrotron radiation, but after going back through the chapter I can't see anything about that, and as you say it's seen at all wavelengths, I probably don't need to mention it.

    Well my deadline was today. Need to submit it now. The rest of the assignment is mostly calculations. It's strange as I hate the maths involved, for someone who's not studied in a long time it's very tough. But I think I prefer the maths questions as at least you should know if you're getting it right, whereas with this question I could still be way off.

    Thanks again bud :-)
  12. Apr 30, 2015 #11


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    The photo from the Hubble site demonstrates it well what the question wants you to discuss - at optical wavelengths it looks like a normal galaxy, but at radiowavelengths it looks active - a jet shooting out of its nucleus and radio lobes present. Yes astronomy is quite a challenge - all of the physics processes is used to explain what is going on - electromagnetism, general relativity, nuclear physics, classical mechanics.
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