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Quasar Features

Before we can think hunting for quasars, we need to know their features that can help us to identify them. By features we mean observable properties that separates them from the rest of the objects in the sky. Well, I must admit that this is going to be nontrivial. The only confirmatory test for a quasar is its spectra. Astronomers have picked up the spectra of thousands of quasars, averaged them and have prepared what is called a template spectra. By a process called template matching, an object may be confirmed to be a quasar or not. Sounds simple, but the process is really complicated.

As we mentioned before, quasars are far in the deep sky. The brightest quasar 3C 273 is at a distance of 2 to 2.5 billion light years from us. We measure this distance from their redshifts. In the Hubble model of the universe, the universe is expanding like points on the surface of a balloon that is being inflated. This makes every object to appear receding from each other and hence there will be a doppler shift in their spectra towards the red colour. The farther is the object, greater will be the speed of recession and hence greater will be the redshift. Comparison of the spectra of 3C 273 with that of a standard spectra shows that it is redshifted by about 16% than normal. This means that the object is receding from us at about 16% the speed of light. By comparing with the Hubble's empirical rule for the expansion of the universe, this could mean that the object was at about 2 to 2.5 billion light years from us when light that we detect today left the quasar.

There are many catches here. First of all, we assumed that the universe is expanding at a uniform rate.How true is this? We have no clue. Recent observations on Type 1a supernova has made cosmologists conclude that the expansion of the universe is in an accelerating phase. Even if we decide to assume that the universe is expanding at a constant rate, there is lot of uncertainty in the exact value of the Hubble constant. Hubble computed it to be about 500 km/s/Mpc which was soon shown to be an exaggerated value. Its value oscillated between 50 and 90 km/s/Mpc all through the 20th century and is now estimated to be close to 70 km/s/Mpc based on recent WMAP(2003) and Chandra(2006) observations.

The second catch is in identifying the redshifted spectral lines correctly so that the estimate of the redshift may be correct. Each star and each galaxy is unique in the sky and so does their spectra. What astronomers do to determine the redshift by hand is to look for a set of prominent emission or absorption lines in the spectra. Assuming that one such line is found, they compute the redshift from it. If the redshift is the correct estimate, all the remaining lines should fall back in their proper places when the redshift correction is applied to them. If they don't they repeat the computations assuming that the line has a different rest frame wavelength. A slight misjudgment can produce high variations in the redshift computation. An alternate method is to build templates and shift it through different redshifts while making a statistical comparison with the observed spectra of the object. The redshift that has maximum statistical correlation could be assumed to be the correct estimate. This could be automated to handle large amounts of data in a given time. However the procedure again is not fool proof as the template for each type (stars, galaxies,quasars) of object could be totally different complicating the whole process. Nevertheless this is the best available tool to-date.

The third catch and the most severe one is that estimating the spectra of all the objects in the sky is impossible even with latest equipment and dedicated sky survey projects. This is because most of these objects are very faint and prolonged exposure is required to obtain a spectra of each of them. Recent deepsky space surveys such as the SDSS use optical fibers to simultaneously measure the spectra of a bunch of selected candidates but are still leaving out over 90% of the objects without taking their spectra.

This brings us to the conclusion that an alternate technology has to be evolved to hunt for quasars, or to that matter, any specified kind of object in the deep sky.


next up previous
Next: Candidate selection process Up: On a Hunt for Previous: What are Quasars?
Ninan Sajeeth Philip 2007-05-31