Galaxy Zoo Talk

BIGGGGGG !!

  • p.draper by p.draper

    Analysis ! P Draper

    Posted

  • ElisabethB by ElisabethB moderator

    not so big ! ;-D
    just a bit too zoomed in

    enter image description here

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  • JeanTate by JeanTate in response to p.draper's comment.

    It's SDSS J033957.75+382548.1 (DR8 ObjId 1237661083201373439). It's in a field where no object has an SDSS spectrum; however, its PhotoZ is 0.065±0.034, which seems reasonable.

    fracDeV for all bands is 1, which means that its radial profile is (far) more consistent with a de Vaucouleurs than an elliptical one (i.e. n ~4 in a fitted Sérsic profileWP) - click on PhotoObj in the left panel of the SDSS Examine view for this, and other, photometric parameters; see the DR9 SkyServer Schema Browser pages for more on what each of these are and what they mean.

    In the same long list, the five DeVRad values range from 0.76" to 0.86". If you convert this to an estimate of the (projected) physical effective radius (for an n=4 Sérsic profile), in kpc, you'll see that this is a pretty small elliptical.

    Would you like to learn how to derive an estimate of physical size, from the redshift and an re of ~0.8"?

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  • fatha731 by fatha731 in response to JeanTate's comment.

    I would!! 😃

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  • JeanTate by JeanTate in response to fatha731's comment.

    OK, happy to oblige.

    The DeVRad values are the effective radii of the radial distribution of light derived from a 'fit' of a model of that light as ellipsoidal and following a Sérsic profile with n=4 (the details of how the SDSS photometric pipeline does this fitting? Don't know). The units are arcsecs.

    So you have an apparent angular distance (in ") and you want to convert it to a physical distance (in kiloparsecs, or kpc). If the universe had a flat, Euclidean geometry, it'd be very simple, especially as the angle is so small you can safely approximate it as a linear relationship.

    However, the geometry of the universe is better described by a ΛCDM model, which is based on General Relativity; pick your values of the parameters, put them into the relevant formula, turn the handle, and out pops the ratio you want: how many kpc does one arcsec correspond to, if the redshift is z?

    Fortunately, you don't have to work this out from scratch; Ned Wright has a very convenient online calculator, as well as a nice tutorial if you wish to learn how it's all done.

    Plugging in 0.065, and using the default values Ned supplies, I get "This gives a scale of 1.233 kpc/"."

    So, 0.8" is ... ?

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  • fatha731 by fatha731 in response to JeanTate's comment.

    Thanks for the explanation! 😃

    Posted