Galaxy Zoo Talk

Size approximation

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Please feel free to correct me if i'm wrong

    Redshift z to lightyears.
    http://www.astro.ucla.edu/~wright/CosmoCalc.html

    Lightyears + arcseconds to size in lightyears
    http://chandra.harvard.edu/photo/scale_distance.html

    Z=0.912 = 10.197 Gly, or 10.200.000.000 lightyears

    and

    Size 1.5" (arcseconds)

    Gives 73,440 lightyears (22.5 kpc) across

    Or according to Ned Wright's calculator appr. 8kpc/1", which gives

    8kpc/1" x 1.5"= 12 kpc = 39,000 lightyears

    Quite a difference, might have to with the large redshift?

    Posted

  • Budgieye by Budgieye moderator

    I'm not sure, you haven't shown all your workings.

    Don't forget the size of the Universe changes as you look back in time.

    Thurs Sept 27, 2012 Measuring the size of distant galaxies http://www.galaxyzooforum.org/index.php?topic=280432.msg616716#msg616716

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Oh thanks, i used this one from the links in your redshift / size measurement pages here in the new talk;
    http://www.galaxyzooforum.org/index.php?topic=277034.msg416504#msg416504
    (BTW WolframAlpha's redshift to distance is now probably behind a 'Pro' version paywall)

    Papier-and-pencil method yields 15 kpc., the fast way 😃
    http://arxiv.org/abs/1303.5961

    Thanks!

    Posted

  • Budgieye by Budgieye moderator

    This is about measuring while the universe has been expanding. I'll let Dr Brooke explain.

    The Universe is smaller at higher redshift, and because of this the
    way the apparent size of a galaxy changes as a function of redshift is
    a little non-intuitive. Galaxies of the same intrinsic physical size
    appear smallest at about redshift 1-2, and then start to appear larger
    as you go to really high redshift. It's a little jargony but the
    Wikipedia description of angular diameter distance may help.

    http://talk.galaxyzoo.org/#/boards/BGZ0000005/discussions/DGZ0001f7c?page=1&comment_id=55479fdea77a1b1f33000a6e

    So at higher redshifts z greater than 1 the problem gets difficult.and you are getting close at at 0.9

    Posted

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

    I understand the concept / problem there, not the formulas unfortunately.

    According to the NED page of DEEP2-GRS 33039828 the Cosmology Corrected Scale at z=0.910786 is about 7.626 kpc/arcsec, so an estimated size of 1,5" gets around 11.5 kpc or 37,000 lightyears.

    Going from the NED page of DEEP2-GRS 33039828

    http://ned.ipac.caltech.edu/cgi-bin/objsearch?search_type=Obj_id&objid=9860943&objname=2&img_stamp=YES&hconst=73.0&omegam=0.27&omegav=0.73&corr_z=1

    to SDSS DR9 page of object

    http://cas.sdss.org/dr9/en/tools/explore/obj.asp?ra=353.169340&dec=0.259112

    Gets the same object as suspected 'star', not the blue patch next to it.

    enter image description here

    'Partial' spectrum from NED i believe used to determine redshift in the DEEP2 Redshift Surveys

    enter image description here

    I hope you can get the Hubble images

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    So if i understand correctly, redshift is actually a 2-component effect?

    1 - the initial speed difference between two galaxies as fraction of the lightspeed at the moment the light left the galaxy ( 'Doppler' part)

    2 - the expansion of spacetime in between the time the light left the galaxy and reaches us ( 'Stretching' part)

    Like a person on a long treadmill that is slowly accelerating, initialy that person could be walking towards us(?). That is, the farther that person is, the faster the treadmill is accelerating. Blueshift + expansion might be seen as redshift?

    How to discern between the two parts?

    Posted

  • Budgieye by Budgieye moderator

    I don't think you can separate the two, but blueshift is only a small part of the vector. Just about all of the redshift of galaxies is due to the expansion of the Universe. Only things that go very fast such as relativisitic jets or the whirlpool of matter around black holes can contribute effectively to redshift.

    Posted

  • vrooje by vrooje admin, scientist

    As @Budgieye says, for all but the most local galaxies, the doppler shift observed is dominated by the expansion term. The velocity of the galaxy itself (which is often referred to as the "peculiar velocity") causes scatter around the expected velocity just due to expansion. One of the most pronounced of these effects is seen in galaxy clusters, where the velocities of galaxies in the cluster cause it to look stretched in redshift space compared to field galaxies, because –– although the whole cluster is expanding away from us –– individual galaxies are moving toward us and away from us relative to the expansion. It looks pretty cool on a redshift-space plot of galaxies.

    More info here (for example): http://w.astro.berkeley.edu/~louis/astro228/redshift.html

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Thanks, interesting document you posted there!

    A bit of nitpicking maybe, but theoretically it would be possible for two galaxies with the same redshift to have very different velocities away (towards) from us / have different distances, and no absolute way to tell the difference from the spectral chart?

    Posted

  • Budgieye by Budgieye moderator

    yes, 1.000 redshift -0.002 blueshift = 0.998

    would be the same as 0.998 redshift ( I would think, but I'm not an expert)

    Forum thread: Even closer? z less than 0.000 The blueshift galaxy thread http://www.galaxyzooforum.org/index.php?topic=279890.0

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    So no intrinsic difference in the cause of the redshift in a spectrum, Hubble vs. Einstein so to say.

    But i think there are possibilities to distinguish between the two and the part each of them is contributing to the total redshift.

    The first method would be to compare spectra of one galaxy taken at different times. When the first redshift is established you get an apparent distance, and then also know at which rate the redshift should increase over time. If the rate at which the redshift changes over time is different from the supposed rate i'd think you can calculate the peculiar velocity of the galaxy. (And proper distance with corrected z)

    The second method would be to image a galaxy at such high resolution that you can see individual stars. Preferably a spiral seen from 'above' and not a too high redshift.

    The size of these stars can than be used as 'rulers' to measure the real size of the galaxy in kpc or lightyears. When comparing the real size of the galaxy with the apparent size in arcseconds ( -minutes) you can calculate the real distance of this galaxy without using it's redshift.

    The difference between this distance and the distance calculated from it's redshift should give the peculiar velocity of the galaxy. This method could also be a sort of litmus test for redshift in general.

    Am i making any sense here? Haven't seen any other method of establishing galaxy distances than redshift so far.

    Posted

  • Budgieye by Budgieye moderator

    The calculation of velocity of galaxies is being done for some more local galaxies, for example to estimate the pull of "The Great Attractor" I wonder how they do the calculations.

    Here is some info about galaxy spin and z

    SDSS Mapping Nearby Galaxies at APO (MaNGA) http://www.sdss.org/surveys/manga/ "Unlike previous SDSS surveys which measured spectra only at the centers of target galaxies, MaNGA bundles sets of optical fibers into tightly-packed arrays, enabling spectral measurements across the face of each of ~10,000 nearby galaxies. MaNGA’s goal is to understand the “life cycle” of present day galaxies from imprinted clues of their birth and assembly, through their ongoing growth via star formation and merging, to their death from quenching at late times."

    Redshift / Blueshift can be detected in rotating galaxies, but it takes more sensitive equipment than this to do a good job.. See this post in the forum.
    http://www.galaxyzooforum.org/index.php?topic=277867.0 Saturday, 12 June 2010: Hydrogen on the radio by EigenState

    Author Topic: Can the z's indicate galactic spin? (Read 1258 times) http://www.galaxyzooforum.org/index.php?topic=9436.msg95553;topicseen#msg95553

    Short answer - yes! Orbital speeds in big spirals can reach about 300
    km/s, which is a change in z of 0.001, so multiple SDSS spectra should
    easily distinguish between approaching and receding sides of nearly
    edge-on disks. You can get at least one more significant figure on z
    by clicking on the spectrum plot and looking at the bottom.

    This approach, carried out all along a slit lined up with the
    galaxies' major axes, gave some of the first widely-convincing
    evidence of the issue of dark matter in galaxies, since the orbital
    velocity does not decline with radius as Newtonian gravity predicts if
    the mass is distributed at all like the starlight.

    But I would think that technology is not advanced enough to make these kinds of measurements you suggest.

    Compare spectra over time: Spectra are fuzzy and it would take a million years to detect changes in them to make the measurements you suggest.

    Size of stars: We can't "see" individual stars, except in the closest galaxies to our own, such as the Magellanic clouds and the larger stars of the Andromeda galaxy. The stars that we see in our images are fuzzy, the actual size of a star would be much less than a pixel.

    https://en.wikipedia.org/wiki/Peculiar_velocity

    https://en.wikipedia.org/wiki/Great_Attractor

    Posted

  • Budgieye by Budgieye moderator

    Someone with a newer computer, put 33039828 into this website, and see if you can get a galaxy image come up at the bottom on the page.

    http://tkserver.keck.hawaii.edu/egs/dataAccess/query/egs_query_deep2.php

    Posted

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

    I'd say measuring the spin of a galaxy is based on differences in z within a galaxy, and still based on the redshift of a spectral chart. All i see is z

    This approach, carried out all along a slit lined up with the galaxies' major axes, gave some of the first widely-convincing evidence of the issue of dark matter in galaxies, since the orbital velocity does not decline with radius as Newtonian gravity >predicts if the mass is distributed at all like the starlight.

    I'll get back on this issue later 😃

    But I would think that technology is not advanced enough to make these kinds of measurements you suggest.

    Where would we be without great scientific and technological challenges?

    Compare spectra over time: Spectra are fuzzy and it would take a million years to detect changes in them to make the measurements you suggest.

    I was afraid this might be the case, hence method 2

    Size of stars: We can't "see" individual stars, except in the closest galaxies to our own, such as the Magellanic clouds and the larger stars of the Andromeda galaxy. The stars that we see in our images are fuzzy, the actual size of a star would be much less than a pixel.

    This might be the case right now, although i'm not sure about the resolution of Hubble in combination with a long exposure time focusing on a galaxy with 'relative' small redshift.

    This might not be a problem in the future with new-generation telescopes.

    In any case, current technological impossibilities do not discard this method theoretically do they? Usually measuring things another way or using other instruments yield surprising results and new insights, or if nothing else deepens and verifies current knowledge.

    I'll quantify a recipe later

    Thanks a lot!

    Posted

  • vrooje by vrooje admin, scientist

    Just a quick note: Hubble being able to resolve individual stars in a galaxy isn't a matter of exposure time; it's just the distance to the galaxy (plus the size of the telescope mirror and wavelength of light you're observing, but I'm assuming that's fixed in this example). Hubble can resolve individual stars in M31 (Andromeda), but even for many other galaxies "nearby" it can't quite get there.

    There are many other ways to measure distance independent of redshift, most of which involve stars -- not so much the sizes of them, but rather their luminous properties such as variability.

    Posted

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

    @Hubble; Ah well that's 'resolved' then

    Ah yes ofcourse luminosity + variability. Cepheids, RR Lyrae and such.. Any other methods besides spectral redshift and luminosity of variable stars? ( supernovae no doubt)

    Below method already considered before?

    And are there theoretical obstacles / flaws i'm missing for it to work? ( besides obvious technological obstacles ofcourse)

    Recipe for 'Distance Dish Deluxe'

    Warning: do not attempt without the supervision of an experienced chef-scientist!

    Preparation time: 1-20 years

    Ingredients

    • Very sharp telescope

    • Spiral galaxy, seen face-on, z below 0.5, but depending on sharpness of telescope

    • Redshift from spectrum of the spiral and associated size and distance

    • Processing machine for cutting
    (Can also be done manually)

    Directions

    1. Use the telescope to image the spiral in the smallest possible parts, at least star-sized or smaller

    2. Use the cutting machine to measure these small pieces and use this to establish the real size of the galaxy in kiloparsecs or lightyears

    3. Now compare the real size of the galaxy with it's apparent size in arcseconds and calculate it's distance from us

    4. Compare this distance and size with the distance and size following from it's redshift

    Result

    • Peculiar velocity if the galaxy

    • Percentage the peculiar velocity is contributing to the total spectral redshift

    • Method of determing galactic distances and sizes based purely on trigonometry, next to existing methods based on spectral redshift and luminosity among others

    • Surprising aftertaste!

    Results may vary, but might be surprising if you've only had the usual bland redshift / Cepheid distance dishes 😃 .

    And thanks for the feedback

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Still hoping for some constructive feedback.

    Or maybe of interest for scientists in this particular field?

    Posted

  • Budgieye by Budgieye moderator

    I don't have the technical expertise to comment more, except that you might change your preparation time to 1 million years. These images will change very slowly.

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Not based on change, but the future possibility of imaging individual star / star sizes of galaxies to determine galaxy size + distance.

    Thnx

    Posted

  • Ghost_Sheep_SWR by Ghost_Sheep_SWR

    Oh...

    Well here's a tidy list of standard candels and rulers 😮

    https://ned.ipac.caltech.edu/Library/Distances/

    Posted