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Thread: How close must a galaxy like our own be to clearly detect?

  1. #61
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    Quote Originally Posted by TooMany View Post
    But still I wonder if you can answer the specific question I asked about a visual spectrum image with HST of M31 at z~1.2. The paper I referenced (that you pointed out) had a picture of what seems to be a face on spiral at that distance so I wonder what would M31 look like. Would it show up as a spiral in a similar image? Or is the galaxy pictured much brighter and/or larger allowing an image to be made that would not be possible for M31.
    Quote Originally Posted by Nereid View Post
    General readers may be interested in some images, to get an idea of what spiral galaxies look like.

    Many BAUT members are - I hope! - aware of the Zooniverse, and its first Citizen Science project, Galaxy Zoo. Further, I hope that you have all signed up, and spent many happy hours as citizen scientists, classifying galaxies, studying galaxy mergers, or solar storms, discovering supernovae, finding new exoplanets, or plotting bubbles in space.

    I certainly have, and I also spend an enjoyable few minutes (or more) every day reading some of the blogs or posts on the forums associated with each of these projects. Directly relevant to this thread is the galaxy zoo forum, and one post in particular: two classical spirals; one near, one far*.

    From only the SDSS image of this z=0.239 spiral galaxy, you'd be hard pressed to say if this was a Milky Way look-alike or not. However, from the Hubble image you can instantly see that, while it's a spiral, it's certainly not at all like the Milky Way**!

    At greater distances what do spiral galaxies look like? Well, even with the Hubble, not many look like spirals, but many are clearly disk galaxies. For example, look at the postage-stamp images in this recent paper: CANDELS: Correlations of SEDs and Morphologies with Star-formation Status for Massive Galaxies at z ~ 2.

    * I tried to post the two images, but got a strange error message, so you'll have to click on the link to see how dramatically different the two images, of the same galaxy, are.
    ** it is obvious to you, isn't it?
    Here's part I of my promised answer to TooMany's question.

    Using the assumptions I used in the Bias effects in galaxy detection thread, together with values appropriate to SDSS J141822.30+524306.1, per the links, I calculate that this galaxy has an absolute magnitude of ~-20 in a waveband centred on 450 nm (and with comparable width to the SDSS r' band, suitably blueshifted), and ~-19.8 in one centred on 660 nm (ditto).

    The "z=0" radius of this galaxy is ~9.5 kpc (Petrosian radius, measured at 450 nm, rest frame) or ~13.8 kpc (Kron radius, measured at 660 nm, rest frame).

    In comparison, M31's diameter is ~45 kpc (assuming 190' as its apparent diameter, equal to the angular length of its major axis, and a distance of ~780 kpc). -20 is a commonly quoted value for its V-band absolute magnitude (the central wavelength of the V band is ~550 nm).

  2. #62
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    Part II.

    At z=1.2, M31's ~23 kpc radius would subtend an angle of ~2.8", so ignoring projection effects, it'd appear comparable in size to SDSS J141822.30+524306.1 (making the same assumptions in part I). When observed in the appropriately redshifted V-band, of course (central wavelength ~1.2).

    Its integrated magnitude, in the band, would be ~24.5, making it invisible in SDSS, but easily detectable with the Hubble's current cameras. How far - from the nucleus - the disk could be traced would depend on what technique was used; converted to a monochrome 2D image, with an appropriate softening function for the faint bits, I would guess that it'd look about half the size of SDSS J141822.30+524306.1.

  3. #63
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    Quote Originally Posted by Nereid View Post
    Part II.

    At z=1.2, M31's ~23 kpc radius would subtend an angle of ~2.8", so ignoring projection effects, it'd appear comparable in size to SDSS J141822.30+524306.1 (making the same assumptions in part I). When observed in the appropriately redshifted V-band, of course (central wavelength ~1.2).

    Its integrated magnitude, in the band, would be ~24.5, making it invisible in SDSS, but easily detectable with the Hubble's current cameras. How far - from the nucleus - the disk could be traced would depend on what technique was used; converted to a monochrome 2D image, with an appropriate softening function for the faint bits, I would guess that it'd look about half the size of SDSS J141822.30+524306.1.
    Thanks for that info. Next I'd like to get some idea what it would look like to the HST using 2.8" and mag 24.5.

  4. #64
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    Quote Originally Posted by TooMany View Post
    Thanks for that info.
    You're welcome.
    Next I'd like to get some idea what it would look like to the HST using 2.8" and mag 24.5.


    I don't follow; didn't I already answer that? Change the degree of inclination (to match that of M31), bulk up the nucleus a bit, replace every ~40 electrons in each CCD pixel in the HST SDSS J141822.30+524306.1 image with 1 (but leave the 'sky' unchanged), and you've got some idea of what a 2D direct image would look like (of course, you'd have to use a different HST camera than the one used to image SDSS J141822.30+524306.1, the angular resolution would be correspondingly worse, and ...).

    Could you clarify your question please?

  5. #65
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    Here are some examples of the various kinds of radii, derived from analysis of the (digital) data that an image comprises.

    They are from 2MASS, but the principles are the same, when applied to data from instruments such as the cameras on the HST. In particular, note how small the "half-light" radius is, compared with the various other radii. If you measure the radii of the 'obvious' part of the SDSS and HST images of SDSS J141822.30+524306.1 (see my earlier post), you'll find they are considerably smaller than the Petrosian and Kron radii, estimated by the automated pipelines.

    For comparison, how big does M31 appear, when you go out and look at it with your unaided eyes, on an appropriate clear, dark, moonless night (assuming you live sufficiently far north that it's high in the sky)? Quite a lot smaller than the 190' x 60' (a LOT bigger than the Moon) you'll often see quoted as its size, that's for sure.

  6. #66
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    Quote Originally Posted by Nereid View Post
    You're welcome.



    I don't follow; didn't I already answer that? Change the degree of inclination (to match that of M31), bulk up the nucleus a bit, replace every ~40 electrons in each CCD pixel in the HST SDSS J141822.30+524306.1 image with 1 (but leave the 'sky' unchanged), and you've got some idea of what a 2D direct image would look like (of course, you'd have to use a different HST camera than the one used to image SDSS J141822.30+524306.1, the angular resolution would be correspondingly worse, and ...).

    Could you clarify your question please?
    I'm sorry, but I was having trouble interpreting your answer. So it would be invisible in the SDSS survey. It would be 40 times dimmer than the HST photo of that galaxy using the same camera. I guess to that camera it might appear as a small dim blob and with any HST photo no structure would be apparent. Is that about right?
    Last edited by TooMany; 2012-May-04 at 11:17 PM. Reason: Missunderstanding

  7. #67
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    Quote Originally Posted by Nereid View Post
    Here are some examples of the various kinds of radii, derived from analysis of the (digital) data that an image comprises.

    They are from 2MASS, but the principles are the same, when applied to data from instruments such as the cameras on the HST. In particular, note how small the "half-light" radius is, compared with the various other radii. If you measure the radii of the 'obvious' part of the SDSS and HST images of SDSS J141822.30+524306.1 (see my earlier post), you'll find they are considerably smaller than the Petrosian and Kron radii, estimated by the automated pipelines.

    For comparison, how big does M31 appear, when you go out and look at it with your unaided eyes, on an appropriate clear, dark, moonless night (assuming you live sufficiently far north that it's high in the sky)? Quite a lot smaller than the 190' x 60' (a LOT bigger than the Moon) you'll often see quoted as its size, that's for sure.
    Indeed. Even with small binoculars, it's just a fuzzy spot which I suppose is the bulge.

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