Radio Galaxy Zoo Talk

Red Shift?

  • Ptd by Ptd

    Hi RGZ Science Team

    http://radiotalk.galaxyzoo.org/#/subjects/ARG0003bq8

    In the above you mention red shift being diagnostic of the image. I know what red shift is, and I know why it is an indicator of distance/how far back in time we are looking.

    But I don't know much more than that, is the amount of red shift, data which is getatable by we lay users? (we'd at least then know how far back in time we're looking), how would we interpret that info and how and or should, we attempt to use it?

    Before posting this question I did a quick search in talk for red shift and couldn't find any other threads talking about it, maybe someone could give us a bit of a lesson?

    MT

    Posted

  • ivywong by ivywong scientist, admin

    Hello @Ptd,
    If you clicked on the SDSS link under each Subject's discussion page, it will bring you to SDSS's information page where you might be able to obtain a redshift (either photometric redshift or spectroscopic redshifts if they obtained a spectra). The caveat is that optical surveys such as SDSS typically is not sensitive to redshifts beyond 0.5. But if you can find a rough redshift value, you can use it to estimate the physical sizes of the jets. It's a fun exercise and provide more context to the images that you have been classifying.
    Does this help?

    cheers,
    Ivy

    Posted

  • Ptd by Ptd

    Hi @ivywong OK here goes, please tell me if I get this calculation correct or not, (after half an hour on Wikipedia, the GZ forums,the SDSS guide etc).

    SDSS z for this object ~ .13?
    this equals ~1.3Gly?
    Looking at the FIRST image, the angular size of the Radio Image ~ 1arcmin
    1min at 1.3Gly = 38 million light years

    Thus do I take it that ~ 400 times the diameter of the Milky Way is not a reasonable size for a Radio image like this one?

    MT
    Ptd

    Posted

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

    I'm a bit surprised no one has replied to your (excellent!) questions yet, Ptd. Never mind, here's my attempt at some answers:

    SDSS z for this object ~ .13? this equals ~1.3Gly?

    Using Ned Wright's CosmoCalculator, with the default values and a z of 0.13, I get an angular size distance of 1.57 Gly. That's pretty close to your 1.3 Gly, given the many uncertainties, so let's run with it.

    Looking at the FIRST image, the angular size of the Radio Image ~ 1arcmin 1min at 1.3Gly = 38 million light years

    The CosmoCalc gives the angular scale (at this z, for the default values) of 2.334 kpc/" 1 arcmin is 60", so 60 x 2.334 = 140 kpc. As I did not adjust the values in the calculator, I would not expect to get the same value as you did; however, it should be no more than ~20% off. And 38 million ly is very different from 140 kpc, so I think you may have not quite got this one right (or, alternatively, I got it wrong; quite possible, I often make mistakes).

    How did you get the value 38 million light years?

    Thus do I take it that ~ 400 times the diameter of the Milky Way is not a reasonable size for a Radio image like this one?

    ~40 million ly is ~400 times the diameter of the Milky Way; however, I think we should first work on deriving an estimate of the physical size of something that's at z=0.13, from its angular size.

    Hope this helps.

    Posted

  • ivywong by ivywong scientist, admin

    Apologies @Ptd. I must have accidentally missed this question when I do my daily check of RadioTalk. Thanks heaps @JeanTate for spotting this and helping out. The typical units used by astronomers are kiloparsecs or megaparsecs where 1 parsec is 3.26 lightyears. The cosmology calculator by Ned Wright is a very useful tool to work out the angular size of objects at different redshifts.

    Posted

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

    Hi JeanTate

    Many thanks for your response and for spotting my maths error, when I was typing into my calculator it seems I put two to many zeros in 1.3billion. The calc I used was, 1arcmin is (2pidistance/21600) = 380000ly which is 116kps.

    Which is about 4 times the size of the Milky Way?

    Not quite sure now where on the Internet I turned z=.13 into 1.3Gly, but Ned Wright's calculator is now in my favorites 😃
    I also found a tutorial on Ned Wright's Calculator page which I'll read through, it seemed quite extensive

    Hi Ivywong , no need too apologize for missing one of my questions as I know I've sprayed several around over the last week or so. My knowledge of astronomy is pretty much limited to what I've learned since joining the Zooniverse, and some of the terminology and concepts being discussed in RGZ are outside of my experience, but then we're hunting supermassive black holes, for real, which is pretty wild stuff 😃 . Maybe a School Room series of threads might help people like me?

    MT
    Ptd

    Posted

  • andymarrison by andymarrison in response to Ptd's comment.

    Hi Ptd
    A School Room is a great idea! I'd love to find out some more about the concepts and tools that the professionals use for interpreting what appears on all of these images.
    Andy

    Posted

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

    Hi MT Ptd,

    Happy I was able to be of some help.

    The calc I used was, 1arcmin is (2pidistance/21600) = 380000ly which is 116kps.

    One weird thing about General Relativity (GR) is that it uses a geometry that's not what we were taught in school! So in cosmological models - which are built on GR - intensity does not fall off as the square of the distance, angular size does not vary with distance as it does in Euclidean geometry, and so on. In fact, the whole concept of 'distance' becomes rather complicated, and there are now several different distances for the same object! 😮 It depends on how you measure 'distance'.

    In Ned Wright's CosmoCalc, once you put in the numbers you want (mostly redshift; unless it's important you probably won't change the default value for the Hubble constant, for example) and press a button, you'll get values for several of these distances. For the question you asked, the important one is "angular size distance", because you are measuring/estimating the physical size (well, projected size) of the object.

    And because you want to do a calculation, CosmoCalc includes the scale:

    This gives a scale of 2.334 kpc/". 
    

    So if you measure its angular size as 1' (one arcminute), then that corresponds to 60 * 2.334 kiloparsecs (kpc). Or 140 kpc. As you can see, at relatively small redshifts the 'Euclidean geometry' and 'GR-based cosmology geometry' values are similar, but not quite the same.

    Which is about 4 times the size of the Milky Way?

    Depends on what size you use as 'the size of the Milky Way', but yes, that's about right.

    I also found a tutorial on Ned Wright's Calculator page which I'll read through, it seemed quite extensive

    His Cosmology Tutorial is wonderful! 😃 However, it's a bit terse in places, and I find it can sometimes give me the feeling I've understood something but when I try to use it, I realize I didn't quite get it (and don't know where to turn).

    Maybe a School Room series of threads might help people like me?

    This sounds like a great idea! 😃

    How do you think it could work? I mean, a separate section here in Talk? Anyone can ask any question? We ask someone to write a short piece on redshift (say)?

    Posted

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

    Hi Andy,

    Are there particular things you'd like to know about? Or would you prefer someone - one of us, perhaps - to just write a few short pieces, on 'kiloparsecs', for example? (or both??)

    Posted

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

    Hi JeanTate
    Re School Room idea.

    Yes I think a separate bespoke section within talk would be a good idea, where questions can be asked about science, the current format of: Help, Science and Chat are a bit vague for example should I have posted this question in Help or Science?. I think it would need some structure and system to it, and very important if the science team aren't to be asked the same question many times, it would need to be searchable, maybe special hash tags e.g. #Qredshift and #ASTredshift.

    It would need subsections also I think. I suppose this thread would need to go in the 'Astrogeometry' section, there might also be a 'Internal Galactic Structure' section with things about spirals, ellipticals, jet science and black holes etc. Another section for 'Stars', I came across your thread about making overlays from various surveys which had alot of talk about Inferometery in it(didn't really understand much of that(yet)), maybe that could go in an 'Instrumentation' section? and even a 'How to Maths' section for those who think they can either handle it already or think they might be able to learn. I think a few zooites in various projects have written their own software(and even outside the zooniverse do that for a living) so maybe a 'Software Development' section for them might be in order.

    Maybe some kind of difficulty rating for the posts could be useful, if you do a search and find 150 posts on redshift you'd know where to start reading.

    Might need some control over who is allowed to use certain hash tags

    Also I don't see why the School Room would need to be project specific, knowing about red shift might be of interest to zooites from several projects.

    I don't think the threads should be sorted according to when they were started or when they were last posted in, maybe alphabetical within each section and the Science Team can change the header title once its been started?

    Once they've matured a thread might be condensed for its best science content?

    Posted

  • ivywong by ivywong scientist, admin

    Hi @Ptd, @andymarrison & @JeanTate,

    Why don't you put a schoolroom post in the Journal Club section? Since it was created as a source of reference to the latest papers, we can also put papers and resources there which could be used for a School Room.

    Each separate thread can be about topics such as Cosmology or Distances or Jet physics etc...

    What do you think?

    Posted

  • Budgieye by Budgieye

    Hi there are explanations about redshift in the Galaxy Zoo Talk (the other zoo, without the amazing jets)

    Here is a list of some in this index. The main links are there, but the index is still under construction, so more links will appear in the months to come.

    http://talk.galaxyzoo.org/#/boards/BGZ0000001/discussions/DGZ0000wrb?page=7

    There is a section on Radio zoo . Please pm me if I have missed something really important.

    6.6 radio data and Radio Galaxy Zoo http://talk.galaxyzoo.org/#/boards/BGZ0000001/discussions/DGZ0000wrb?page=6&comment_id=53d8bb14db90c7673f001035

    Posted

  • ivywong by ivywong scientist, admin

    Thanks heaps @Budgieye! This is a great repository. I'll cross link it to a thread in the JournalClub section.

    cheers,
    Ivy

    Posted