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B Sloan digital sky survey

  1. Mar 7, 2018 #1
    What are the quasar observation criteria by GAIA and SDSS(what are the condition used by SDSS to define an observed object as a quasar), why some brilliant objects are observed by SDSS and not by GAIA?
  2. jcsd
  3. Mar 7, 2018 #2


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    GAIA is primarily intended to measure stars in our galaxy, not extragalactic objects. However, GAIA covers the whole sky, so any bright quasars should be seen. Do you have a reference that shows which objects you are talking about?
  4. Mar 7, 2018 #3
    The two projects do not have the same scope. They are not meant to find the same set of objects.

    GAIA: http://sci.esa.int/gaia/
    GAIA is producing a high resolution 3-d map of our galaxy. GAIA will flag some extra-galactic objects, but it is not designed to search for them.

    SDSS: http://www.sdss.org/
    SDSS is producing a 3-d map of our visible universe. It is designed to capture data on bright objects with higher red-shifts because those objects will tend to not be in our galaxy. This is why it finds so many quasars.

    SDSS identifies a possible quasar in their data by using a tool called "Photo" that identifies the spectra of an object and it's red-shift (among other things).

    Here is SDSS information on Quasars:

    And here is where you (and anyone) can search the SDSS Data for Quasars. They define a quasar as having a spectrum of "3" or "4" and a red-shift of greater than 0.35.


    Note the SQL query they offer is slightly wrong - you need to look for "Z" not "ZCONF" to get red-shifts greater than. 0.35. The corrected query you should cut and paste in is:

    Hope this helps you.
  5. Mar 7, 2018 #4
    I am talking about the LQAC (qusar catalogue which contain qusars from SDSS and GAIA, but not all the quasars observed by SDSS are observed by GAIA)
  6. Mar 7, 2018 #5


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    OK, so I think the answer is what rkolter and I said. GAIA is intended to measure the positions of stars in our galaxy. The only reason it observes quasars at all is that it uses them as fixed position reference points. The quasars are so far away that they are stationary for all practical purposes.
  7. Mar 7, 2018 #6
    So GAIA observe quasars to use them like reference points? but we know that quasar are variable. but why GAIA can detect some bright objects in a region of the sky and not others in different region? how GAIA detect the objects?
  8. Mar 7, 2018 #7


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    I don't really know, but I suspect GAIA has chosen quasars which are point sources with little variability so they can use them as reference points. You'd have to dig into it. This paper gives some details on the quasars used. Gaia uses a "drift scan" technique where objects generate star trails in the CCD sensors. It should see everything brighter than a certain brightness, so it is just a question of what they pay attention to. This paper describes some details on how it works.
  9. Mar 7, 2018 #8
    any information about SDSS??
  10. Mar 7, 2018 #9


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    Have you tried Google?
  11. Mar 7, 2018 #10
    What is that?
  12. Mar 7, 2018 #11
    1 - Quasars may be bright but they are distant and it takes an amount of time to get enough light from one to identify it; it takes far less time to get enough light to identify a star in our galaxy.

    2 - Not all regions of space are identical, particularly when you are trying to look for "nearby" things. If a part of the sky has a quasar in it, and also several bright stars, the quasar will be washed out.

    3 - They aren't looking for quasars. If they want to conserve their observational and computational time, they will exclude those objects that are known to not be from our galaxy from their data. Maybe there are more hints of quasars in GAIA's raw data that are simply being filtered out because they are not relevant to the GAIA project?
  13. Mar 14, 2018 #12
    Could it be because the g filter of gaia does not cover the SDSS filters (u,g,r,i,z)
  14. Mar 15, 2018 #13
    Gaia has a limiting magnitude of ~20, while SDSS has a limiting magnitude of ~22. So they are probing roughly the same brightness range and therefore Gaia will see tons of Quasars, and probably many new ones. The trick with quasars though is that they often have similar colours, and especially with Gaia, which has 2 massive photometric bands, it will be impossible to identify them as quasars and not just some star. Rather, what you need is spectroscopy. Now Gaia does have a high resolution spectrograph, but it looks at an extremely narrow wavelength range to measure radial velocities of Milky Way objects, making it non-ideal for Quasar confirmation (probably).

    Bottom line: Gaia will probably see a lot of quasars but probably won't be able to identify them as such without help.

    Caveat: Gaia uses this fancy dispersive photometry system so it doesn't just have 2 bands, it has more like a few hundred, they just haven't released this to the public yet (the public only gets the "merged" 2-band data right now). When they do what you will effectively have is very low resolution spectroscopy covering ~350-1050nm. I'm sure that someone will find a way to mine this data to identify quasars (combined with the fact that a quasar will show up as a non-moving, zero parallax point source).

    Hopefully that helps! I have limited experience with quasars, so these are my best guesses. You also might find this interesting: https://www.aanda.org/articles/aa/pdf/2015/02/aa23280-13.pdf
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