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Thread: 21-cm hydrogen

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    Default 21-cm hydrogen



    Hey!

    I understand that it (21-cm radiation) is important for astronomers but I really don't understand why? What is it that makes this particular radiation so important for mapping the galaxy? Is the 21-cm wavelength extra good for not being blocked by dust and so?

    And why is this radiation more important that other radiation from hydrogen? I understand it is emitted when the proton flips but can't we get information from hydrogen even when the proton spins the same way as the electron?

    Thx, and sry if the question is stupid.

    Edit: I guess that with the dopplershift it is possible to see how the hydrogen moves, and if the wavelength is stretched u can see how old the hydrogen is i.e. the radiation line is stetched due to the expansion of the universe.

    So is this 21-cm radiation line the ONLY way to see NEUTRAL hydrogen in the universe? Is that why it is so important?
    Last edited by Nightslash; 08-24-2019 at 01:28 PM. Reason: Adding some stuff

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    Default Re: 21-cm hydrogen

    Cold neutral hydrogen is in its ground state. In the ground state the electron and proton have anti parallel spins. Slightly above the ground state there is an energy level of slightly higher energy such that the electron and proton spins are parallel. This is the only line available for cold neutral atomic hydrogen.

    Since the excited state is very long lived, the line is extremely sharp. So it can be measured very precisely giving excellent Doppler shift velocities.

    There's no obscuration from dust, true but TV and man made interference are big problems.
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    Default Re: 21-cm hydrogen

    Quote Originally Posted by not_Fritz_Argelander View Post
    Cold neutral hydrogen is in its ground state. In the ground state the electron and proton have anti parallel spins. Slightly above the ground state there is an energy level of slightly higher energy such that the electron and proton spins are parallel. This is the only line available for cold neutral atomic hydrogen.

    Since the excited state is very long lived, the line is extremely sharp. So it can be measured very precisely giving excellent Doppler shift velocities.

    There's no obscuration from dust, true but TV and man made interference are big problems.
    So this is basically what make it so special (bold text)?

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    Default Re: 21-cm hydrogen

    That it's the only game in town certainly makes it unique. But I wouldn't underestimate the importance of the line being intrinsically very, very sharp either. That makes it a very powerful tool for exploring galactic structure.

    If the line were broader due to shorter lifetime (Heisenberg uncertainty principle) correcting the observed Doppler shifted line profiles to get a true density map would be a more hazardous and messy undertaking.
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    Default Re: 21-cm hydrogen

    Ok thanks Frits! It's clear now.
    Thank you very much for the informative answers!

 

 

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