Galaxy Zoo Talk

My first classification

  • LluisHP by LluisHP

    Dear all. I am pretty new in Galaxy zoo and in my first attempt I have a doubt. In the begining I have classified this object as smooth (no disk) but now I think that it is also visible a hint of dark discontinuity around the periphery of the more intense central region. This recall me a spiral galaxy but with a very poor resolution. What do you think?

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  • FN-21-87 by FN-21-87

    So, I think it is a disk!

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  • Budgieye by Budgieye moderator

    Yes, I agree that it is not smooth. Yes, probably a spiral. Threre is no correct answer. If there are many different opinions, it means that the galaxy is difficult to classify, and that is important information in itself.

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  • FN-21-87 by FN-21-87

    I think we won't know with this picture...

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  • Budgieye by Budgieye moderator

    Agree, we'll have to wait for the next generation of telescope images. 😃

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  • JeanTate by JeanTate in response to FN-21-87's comment.

    Actually, although just eyeballing the DECaLS image alone may not give a definitive answer as to what sort of galaxy this is, when you use other sources of information I think it's clear that it's a (nearly) edge-on spiral (perhaps somewhat disturbed).

    For example, here's the DR9 SDSS image (the galaxy is named SDSS J151730.56+013606.7):

    enter image description here

    A DECaLS or SDSS image is, ultimately, just binary data, lots of 0s and 1s. And the SDSS photometric pipeline automatically analyzes the binary data, to produce oodles of numbers describing the way the intensity of the detected light varies over the whole galaxy. One set of numbers is particularly interesting; it goes by the esoteric name of "fracDeV_x", with "x" being u, g, r, i, and z. Generally, if the g and r numbers are less than 0.5, the galaxy is almost certainly not an elliptical; for your galaxy, they are 0.26 and 0.37, consistent with it being a spiral.

    Also, we are lucky that there's an SDSS spectrum of this galaxy (actually, of the central 3" only):

    enter image description here

    While the automated spectroscopic pipeline is not always correct, in this case calling the galaxy "starforming" seems accurate. Another indication that it's a spiral (elliptical galaxies almost never have "starforming" spectra).

    Hope this helps, and happy hunting! 😃

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