Author Topic: QUASARS. What they are and how to find them.  (Read 13901 times)

Thomas J

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QUASARS. What they are and how to find them.
« on: September 30, 2008, 06:45:21 pm »
   Quasars appear to be a popular staple of the GZ forum and can be quite rewarding to find, particularly at high redshift. With this in mind, I thought it may be helpful to discuss them a little more and attempt to provide some useful information along with examples. If you are new to Quasar hunting you may find the spectrum examples helpful.

For any questions you have regarding spectra or anything else covered below, please PM me and I will attempt to provide you with an answer.


So why ‘quasar’?

    Quasar is a term derived from the words Quasi-stellar radio source and is a contraction of

    Originally discovered in the late 1950’s, they were detected as very distant radio sources, many of which had no corresponding visual image. Advances in technology and observational techniques allowed many more of these deep sky objects to be detected, visually.
    Visual detection became very important as it transpired that only around ten percent of quasars were actually ‘radio loud’.
To compensate for the other ninety percent that were ‘radio quiet’, a new term was introduced, ‘QSO’ or Quasi-Stellar Object. This is the term we are all now familiar with.                                        

What is a QSO?

    As an example, we can use a distant galaxy containing, in its centre, a super massive black hole that is feeding or 'active'.

    As vast amounts of material are drawn in towards the galactic nucleus a huge spinning disc of dust, gas and stars known as an 'accretion disc', forms.  

    This is now, what is commonly known as, an ‘active galactic nucleus’ or AGN

    It is the accreting material that produces the high energy emissions detectable at great distances. When this energy gets sufficiently powerful, the AGN becomes so bright it actually outshines the entire host galaxy. At its greatest luminosity and at the correct angle it becomes a quasar.

Redshift (Z)

    Quasars were more common in the distant past, shining at luminosities of around one trillion suns.
        All recorded quasars have redshift values between 0.06 and 6.4. This places them at distances between 780 million and 28 billion light years away.

Spectral data.

    Following, are two spectral charts of galaxies found at random with the third and fourth being charts for qso.

Typical galaxy spectra (above) and QSO spectra (below)

    There are a number of things to look for in the chart, to determine whether, or not, you have found a Quasar and the following will be helpful;

    Check the linear distance between the lines of high energy emissions, they should be sufficiently broad.
    These emissions come from the ‘broad line region’ around the black hole.

    If the emission lines are narrow and thin, then it is, probably, not the spectrum of a Quasar.

    These lines may also have a tendency to rise towards the blue end (left side) of the chart

    Check the redshift (Z) value at the bottom of the chart which should be somewhere between 0.06 and 6.4, as mentioned above.

    Next to the redshift value is the object type which will read; ‘galaxy’, ‘star’, 'star late’ etc. This should read 'QSO' or in some cases 'HizQSO' if the redshift is particularly high.

    Lyman Alpha Line

    In cases where there is a significant redshift the symbol Lya enters the chart at the top left (see last chart above). Remember that the elements move further along, left to right with the increase in redshift. The 'Lyman Alpha line' represents neutral hydrogen and forms part of the 'Lyman series' which goes far off the chart into the ultraviolet (extreme left).

    An evident Lyman Alpha Line is, therefore, a good marker for deciding whether, or not, you have a QSO, once all the above mentioned pointers have been taken into account

    Here might be a good time to see the redshift in action.

    You will notice the Lyman Alpha Line on the chart above lies at 4200 Angstroms and has a redshift value of z= 2.4
    The line on the chart below has been 'shifted' to the red (right) and lies at around 4800 Angstroms with a redshift of z= 2.9

    Lyman Alpha Forest

    Below is another spectral chart showing the Lyman Alpha Line redshifted even further, to approx. 5900 Angstroms giving a redshift value of       z= 3.9

    This particular chart is important as it clearly shows us something unique to extremely distant objects, namely, the Lyman Alpha Forest.

    At first glance you would be forgiven for thinking that something is wrong with the spectrum to the left of the Lya line but this is quite correct. The Lyman Alpha Forest, put very simply, shows that light from the distant object has been absorbed by Intergalactic Medium on its journey through the Universe and each line can tell astronomers the composition of this medium. A chart showing the Lyman Alpha Forest will usually be marked as HiZQSO.  

(remember; emission lines go up and absorption lines go down)

Here is a useful link to a post by dthomas which clearly shows the colour difference between Quasars at different redshift values



    Here are the links to

    'quasi-stellar radio source'


    'AGN' thread

    I hope this is helpful to all the new Quasar hunters out there.

    Many thanks, Tom

« Last Edit: September 03, 2009, 09:55:37 pm by Thomas J »
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