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Thread: Milky Way's thin disk

  1. #1 Milky Way's thin disk 
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    I remember when I first heard about the Milky Way having both a "thin disk" and a "thick disk", and that this thin disk is about 1,000 light years thick. I never really thought about this until I started playing with the wavelength slider at www.Chromoscope.net. Looking at that thin "line of fire" visible in the far infrared, I could see how thin the thin disk really is. With a span of 100,000 light years wide by 1,000 light years deep, its geometry is like a DVD... 10 cm across and 1 mm thick? Now that's a thin disk!

    Which got me wondering what to expect from missions like GAIA and APOGEE. Sure "1,000 light years thick" still means a lot of space, and at this scale a star really is like a point, but as we pan toward 0 degrees galactic latitude, when a few hundred solar disks and their coronae are almost lined up along each line of sight, is there some angle above or below the mid-plane where stars are packed so dense they form an opaque wall?

    For gravitational modelling, I'm not so interested in the well-studied region through SagA*. More interesting are regions (through latitude 0) near longitudes half way out to either side. What do astronomers see?

    thanks!


    Last edited by nnunn; February 7th, 2014 at 08:31 AM. Reason: added links
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  3. #2  
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    A lot of stars closer together in the telescopic view.


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  4. #3  
    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by nnunn View Post
    I remember when I first heard about the Milky Way having both a "thin disk" and a "thick disk", and that this thin disk is about 1,000 light years thick. I never really thought about this until I started playing with the wavelength slider at (chromoscope)(dot)(net). Looking at that thin "line of fire" visible in the far infrared, I could see how thin the thin disk really is. With a span of 100,000 light years wide by 1,000 light years deep, its geometry is like a DVD... 10 cm across and 1 mm thick? Now that's a thin disk!

    Which got me wondering what to expect from missions like GAIA and APOGEE. Sure "1,000 light years thick" still means a lot of space, and at this scale a star really is like a point, but as we pan toward 0 degrees galactic latitude, when a few hundred solar disks and their coronae are almost lined up along each line of sight, is there some angle above or below the mid-plane where stars are packed so dense they form an opaque wall?

    For gravitational modelling, I'm not so interested in the well-studied region through SagA*. More interesting are regions (through latitude 0) near longitudes half way out to either side. What do astronomers see?

    thanks!
    I found some pictures of the Milky Way that will help visualize what you are talking about.









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  5. #4  
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    Quote Originally Posted by Bad Robot View Post
    I found some pictures of the Milky Way that will help visualize what you are talking about.
    Thanks for the diagrams Bad Robot!

    More interesting is the real data collected at Chromoscope. Here are some samples:

    First, the Milky Way in visible light, the only view previous generations could see:



    But in the near infrared, we can see how thin our thin disk is:



    Even better in the far infrared:



    Going to high energy, here's home base as seen in the gamma range:



    Notice (in the IR and gamma views) how dense the stars are packed along 0 degrees latitude. So I'm wondering... if something was lying behind that line, beyond the Milky Way, co-planar with our disk, could it be seen? For example, imagine a neighboring grand spiral, perfectly co-planar with our thin disk, say 200,000 light years away. Would we notice it? Do you think the GAIA and APOGEE surveys will be able to untangle such an object from foreground (Milky Way) stars?


    Quote Originally Posted by nnunn View Post
    For gravitational modelling, I'm not so interested in the well-studied region through SagA*. More interesting are regions (through latitude 0) near longitudes half way out to either side.
    Reason for wondering is that even though our Milky Way is thought to be smaller than Andromeda's M31, our "little" Milky Way seems to have more gravitational effect. By measuring how dwarf satellites orbit both galaxies, studies [e.g. see ref 2] estimate the Milky Way could have up to twice (!) the mass of M31. Early ideas write this off as "dark matter", but why would the Milky Way have so much more dark matter than M31? Some recent work has tried to explain these tidal effects by adding (Standard Model) mass to the model, just beyond the Milky Way.

    I'm looking forward to surprises from GAIA and APOGEE. Remember the (gamma ray) Fermi Lobes? "Who ordered that?"

    [ref 2] The masses of the Milky Way and Andromeda galaxies
    Last edited by nnunn; February 25th, 2014 at 01:46 PM. Reason: added pictures
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  6. #5  
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    Updated previous post with some views of the thin disk taken from Chromoscope.

    PS: I still can't upload jpegs, so tried linking from a friend's website. If links don't work, please let me know.
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  7. #6  
    AI's Have More Fun Bad Robot's Avatar
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    Quote Originally Posted by nnunn View Post
    Updated previous post with some views of the thin disk taken from Chromoscope.

    PS: I still can't upload jpegs, so tried linking from a friend's website. If links don't work, please let me know.
    All your pictures are looking good from my POV. It seems like the galactic mass measurements might have some problems that need to be worked out, as Andromeda has more than twice as many stars as the Milky Way and a star disk that about twice that of the Milky Way's(100,000120,000) light years across.

    Next is the size of the central super massive black hole. It's believed that there is a direct correlation between the mass of the BH and the central core of stars around it. Is there a minimum and a maximum size to stars and black holes?

    The comparison size of the Milky Way BH and the Andromeda BH are considerable.

    The nearby Andromeda Galaxy, 2.5 million light-years away, contains a (1.12.3) 108 (110-230 million) solar mass central black hole, significantly larger than the Milky Way's. The largest supermassive black hole in the Milky Way's neighborhood appears to be that of M87, weighing in at (6.4 0.5) 109 (~6.4 billion) solar masses at a distance of 53.5 million light-years. On 5 December 2011 astronomers discovered the largest super massive black hole yet found to be that of NGC 4889, weighing in at 21 billion solar masses at a distance of 336 million light-years away in the Coma constellation.

    Supermassive black hole - Wikipedia, the free encyclopedia
    The Milky Way's BH is only about 4 million solar masses. I'd very much like to know how they can be saying that the Milky Way galaxy could have more mass than the Andromeda galaxy? This is not making much sense to me.
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  8. #7  
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    I am glad you told me about this one. It looks interesting. I like that I can browse the web version without installing it. However since it is free I might consider installing it locally after I have played with it a bit.
    I love these armchair astronomy tools and this one has a lot of frequencies to view the universe in.
    Thank you nunn for letting me know about it, and thanks to Cardiff University for funding it.

    Edit: I notice on some of the x-ray and microwave zooms the image in the window showed a bit of pixilation, but it is still a nice little program.
    Last edited by dan hunter; February 25th, 2014 at 04:47 PM.
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