Galaxy Zoo Talk

Distance of objects

  • ermesy by ermesy

    Is there a quick and easy way to determine the distance of objects? Take this image - the main galaxy (centre) appears close to the galaxy at 7 o'clock. Yet, searching thru SDSS Skyserver, Ned and SIMBAD I am unable to find a distance from earth for these or any other galaxies. Occasionally I can find a redshift value (in SIMBAD) for objects in question which, as I understand things, can give a clue as to whether apparently interacting galaxies are, in fact, interacting (because a similar redshift value indicates a similar distance from earth). Maybe - probably(!) - I'm missing the blinding obvious but I would've thought that such a basic piece of information as distance would be easily found in amongst the masses of information these databases provide?

    Posted

  • Budgieye by Budgieye moderator

    We are using information used by astrophysicists, and they think in redshift, not in light-years or miles.

    How to do calculation to light years is described here.

    7.3 Measure distance https://talk.galaxyzoo.org/#/boards/BGZ0000001/discussions/DGZ0000wrb?page=7

    Redshift is not actually distance, though we use it a distance.

    The only accurate way to get distance is to take a spectral chart, and it is expensive. You can use Photoz, though it is not accurate, and has huge error bars, and is often not as good as an educated guess.

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  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    EDIT hmm typing while above answer was being given :p

    Hi ermesy,

    I'm not a scientist so hope to be corrected when wrong. But the basic answer is quite simple; a redshift value is a measurement, whereas a distance is based on assumptions and plugging in value(s). One of the main being the Hubble Constant which sometimes changes due to new insights. Making it even more difficult is the discovery of the accelerated expansion of the universe. That is why there are so many different methods next to redshift for getting a handle on distances and the relation with redshift.

    And then there is also the definitions of distance you can use; the distance from earth when the light of the galaxy was emitted, the distance 'now' taking into consideration how much time has passed since the light was emitted, or distance in time etc. etc.

    So it is much more convenient and independent of assumptions to use the unitless z value for redshift based on measurements. Or if its about very young galaxies the time it took the light to reach us to compare with the assumed age of the universe.

    As for getting redshifts, it is indeed possible that NED or SIMBAD has a z value when SDSS has not. BTW in SDSS press the Navigate button, then the button Objects With Spectra, every red square means spectra with redshift. It should give you the redshifts of both galaxies, which are quite similar.

    If there are no redshifts available just see if the galaxies are disturbed and look for tidal debris, these clues are often more than enough to deduce interaction 😃

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  • Budgieye by Budgieye moderator

    To my utter astonishment, these two galaxies are actually the same distance away. I would like to look for a mistake somewhere, but I don't have time right now, if someone else would like to do it, that would be nice.

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  • Ghost_Sheep_SWR by Ghost_Sheep_SWR in response to Budgieye's comment.

    enter image description here enter image description here
    http://skyserver.sdss.org/dr9/en/tools/explore/obj.asp?ra=183.3879&dec=14.48317

    enter image description here enter image description here http://skyserver.sdss.org/dr9/en/tools/explore/obj.asp?id=1237664289927725270

    I don't think there is a mistake, redshifts are pretty close.

    To me it seems that the top one is a 'little' galaxy not part of the big LSB-looking object. So that would make three galaxies in the image; two galaxies that have a little interaction going on and one big LSB without redshift value in the (pretty close?) background. If that's true it might be confusing things a bit.

    EDIT blue is in the FOREGROUND, see below. Thanks CeciliaB 😃

    Note to self: remember, accuracy over speed.

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  • CeciliaB by CeciliaB

    Of course you are right, Budgieye, the blue galaxy is the nearest. But that's not the one in the center and that's how the mistake was made. Partly behind the blue galaxy in the very center of the image is a small white dot. That's a galaxy, an AGN (in an AGN pair) with z=0.06380.

    The yellow galaxy further down is the other AGN in the pair with almost the same z=0.06350.

    The blue galaxy IC 3049 is the nearest galaxy with z=0.008146.

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  • Astro_Denny by Astro_Denny

    Hi everyone, in relation to @ermesy question about determining distances, I thought it would be useful to put here a link to a cosmology calculator, that I think is quite useful to calculate the distances:

    http://www.astro.ucla.edu/~wright/CosmoCalc.html

    The only thing you have to do is to put in the "z" field the redshift , and choose a flat universe., and on the right side you'll have the results.

    I hope it helps! Cheers!

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  • Budgieye by Budgieye moderator

    To learn to estimate distances by eye, look at

    PAGE 2 Galaxy Redshift Chart https://talk.galaxyzoo.org/#/boards/BGZ0000007/discussions/DGZ0000ulp?page=2

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  • ermesy by ermesy

    Thanks everyone - that's all very helpful. I'll follow up on the various links that have been posted here.

    Posted

  • Budgieye by Budgieye moderator

    Thank you Ghost Sheep and CeciliaB . I see now that the spectral chart is a combination of two galaxies, The blue rise at 5000 is due to the blue galaxy and the emission and absorption lines which give the software some good data to analyse is from the distant galaxy.

    http://skyserver.sdss.org/dr9/en/tools/explore/obj.asp?ra=183.3879&dec=14.48317

    I put this in the Index

    7.1 Spectral Charts, MaNGA https://talk.galaxyzoo.org/#/boards/BGZ0000001/discussions/DGZ0000wrb?page=7

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