Galaxy Zoo Talk

in the absence of z

  • magslis by magslis

    Was wondering if there's any way to work out, at least roughly, how far an object might be when there's no redshift data on NED or SIMBAD. What, if anything, could be usefully concluded by amateurs from all those numbers in the galactic extinction box that's usually available when you click on the number next to a galaxy listed on NED? does galactic extinction have anything to do with distances or is it to do with the loss of light by dust, etc.?
    thanks

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

    An excellent question! 😃

    I rather doubt there's anything any of us ordinary zooites could do, using nothing but the information on a NED page, to estimate the likely redshift (even ±a very substantial percentage). However, if the object is an SDSS one, the relevant SDSS Explore page should have links to at least one 'photometric redshift' estimate (including 'error bars'). In my experience, these are generally pretty good; of course, there are also some catastrophic failures.

    Also, if you've been classifying SDSS galaxies for a long time, I reckon you have developed a very good sense for the likely redshift, based on the colors and shape, even if you are not aware of it (this would surely make for a very interesting citizen science project, but I doubt any professional astronomer is sufficiently interested, and no ordinary zooite has access to the tools to be able to initiate such a project, sadly).

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  • magslis by magslis

    Thank you for replying. I suspected it would be rather ambitious:-) yes, with practice it's possible to get a feel whether the galaxy in question is more or less distant but it's less than a guesstimate and some of the objects I've come across are quite extraordinary so it's just curiosity.
    Thanks again.

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  • Capella05 by Capella05 moderator in response to magslis's comment.

    As JeanTate said, you can get a rough approximation by looking at the colour 😃

    I seem to recall an old thread (I think it was on the GZ forum, but I could be wrong... ) where we had examples from the SDSS survey and their associated redshift - it was really useful.

    I will see if I can find it.

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  • magslis by magslis

    that would be great; thank you; I'll have a look around elsewhere, too, so will probably come back with more questions! 😃

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  • JeanTate by JeanTate in response to Capella05's comment.

    I think it's by Budgieye, and is, indeed, a really helpful set of objects ... I'll see if I can find it later today ...

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  • JeanTate by JeanTate in response to Capella05's comment.

    There are two, both by Budgieye:

    Enjoy! 😃

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  • magslis by magslis

    Thank you.

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  • magslis by magslis

    gosh! I'm just blown away - that's soooo helpful for a beginner like me! will keep me busy for a while:-) thanks a million to all the people involved!

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  • Capella05 by Capella05 moderator

    BudgieEye is a new moderator here on talk and I am hoping to convince her to do a thread on Spectra 😃

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  • magslis by magslis

    that would be wonderful; published papers in professional journals are mostly incomprehensible to a lay person - yes, have tried hard to follow but can only understand a tiny fraction:-) - so explanations of such complex problems in plain English would be so very helpful; I'm sure I'm not the only one who would benefit from a thread like that. So it's a big "Yes, please"😃

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  • KWillett by KWillett scientist, admin, translator

    Hi @magslis,

    That's an excellent question on Galactic extinction (and sorry I didn't see it for a long time). In this case, Galactic extinction refers to the loss of light caused by dust in our own galaxy, the Milky Way. When the word "Galaxy" or "Galactic" is capitalized, btw, astronomers are explicitly referring to the Milky Way; when it's in lowercase, they're talking about galaxies in general or external galaxies.

    So the figures for Galactic extinction really are just mapping the lines of sight based on our own position in the Milky Way. Looking at another galaxy in a particular direction, you'll get the same amount of extinction whether it's 1000 light-years away or 1 million light-years away. As a result, it can't be used to estimate the distances to galaxies. There is a tiny bit of extinction caused by the material in between galaxies (called the intergalactic medium, or IGM), but it's extremely small in optical bands, and measuring it accurately enough to use it as a distance measure isn't currently a workable technique, unfortunately.

    The most common way to get a redshift without a spectrum is through photometric redshift, in which we assume the galaxy has a particular color (and so we know what the ratio of light in the various bands should be). We then fit the light to a model and see which assumed distance gives us the best fit. There are other techniques that assume the brightness of the object is constant (such as Type 1a supernovae and BL Lac hosts), and then the apparent brightness can be used to infer the distance. These are called "standard candles" - they're tremendously useful, but also fairly rare and harder to calibrate. Spectra still remain the #1 best-choice for finding the distance to any galaxy, as long as you can get enough photons!

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