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Thread: Observing hydrogen through hydrgen?

  1. #1
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    Observing hydrogen through hydrgen?

    Some of the biggest discoveries over last few years relate to observing hydrogen (hot gas halos, bridges between galaxies, upper atmospheres, Hubble deep field emissions etc.)

    What tricks/techniques do astronomers use to see hydrogen when viewing it through hundreds of light years of mixed hydrogen?

    For example I can't find any numbers or metric to put against:
    * H II regions are visible because the produce powerful broad emission lines that aren't absorbed easily by warm interstellar medium.
    * Supersonic flows in molecular clouds & high velocity clouds above the galaxy can be seen because the emission and absorption lines are doppler shift relative to the gas we are viewing through.

    What can or can't be seen by modern instruments?
    See: 'The God Kit' -- 'The Brigadier And The Pit' -- Carl N Graham -- Sci-fi blog: The Alien Reporter

  2. #2
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    Except for stellar atmospheres (where radiative transfer calculations are Big and Complicated Things), we are often saved by the fact that neutral hydrogen in interstellar and intergalactic space spends almost all of its time in the lowest-energy (ground state) - which is to say, the mean time between excitation events is much longer than the decay timescale of an excited state. So, in H II regions, all the optical lines of hydrogen (the familiar Balmer series of H-alpha, H-beta...) escape unhindered by surrounding hydrogen because these transitions occur between excited states (and similarly for the infrared Brackett, Paschen... transitions). Only in the very densest nebular regions might we see so-called self-absorption in these lines. (There are a couple of carbon excited states that last long enough to appear in UV absorption from gas in galaxy halos even when only the cosmic microwave background radiation is considered, which is a way used to trace the history of the CMBR temperature with redshift).

    Lyman alpha, the strongest resonance transition from n=1 to n=0 or the other way, is a glaring exception. What we see in this (emitted deep-UV) transition is strongly affected by the surroundings, both in density and redshift (Doppler or cosmological). For the intergalactic medium, it is often the case that cosmological redshift separates interfering clouds by enough in wavelength not to be an issue, but for escape of Lyman alpha photons from a galaxy, they may be scattered many times before escaping (and there, the outcome of the scattering depends on velocity structure and direction). A recent example of this is in the escape of Lyman alpha from galaxies, where the photons they escape most easily are from the far side of a galactic wind (favored both by Doppler shift putting them in a lower-absorption wavelength and then backscattering giving a larger wavelength shift). This slice of spectrum from Hubble data on the starburst galaxy Mkn 357 shows how Lyman alpha mostly comes from wavelengths significantly redshifted from the system mean as defined by other emission lines. Another outcome of this resonant trapping process is that gas around a quasar would continue to be seen in Lyman-alpha for much longer if the quasar turns off than in other emission lines, as the photons find their way outward via random walk.
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  3. #3
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    Thanks.
    I think I need to do a bit more background reading to understand your answer fully.
    See: 'The God Kit' -- 'The Brigadier And The Pit' -- Carl N Graham -- Sci-fi blog: The Alien Reporter

  4. #4
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    Quote Originally Posted by PetTastic View Post
    Some of the biggest discoveries over last few years relate to observing hydrogen (hot gas halos, bridges between galaxies, upper atmospheres, Hubble deep field emissions etc.)

    What tricks/techniques do astronomers use to see hydrogen when viewing it through hundreds of light years of mixed hydrogen?

    For example I can't find any numbers or metric to put against:
    * H II regions are visible because the produce powerful broad emission lines that aren't absorbed easily by warm interstellar medium.
    * Supersonic flows in molecular clouds & high velocity clouds above the galaxy can be seen because the emission and absorption lines are doppler shift relative to the gas we are viewing through.

    What can or can't be seen by modern instruments?
    This may be beyond the scope of your questions, but the “tricks” for UV blueward of the Lyman limit may be interesting. Neutral H will absorb this, liberating the electron and leaving a bare proton. And there’s enough neutral H near us, in the MW, that the EUVE (?) could see little beyond our solar system, at these extreme UV wavelengths. Just a few, local, white dwarfs.

    But neutral H becomes less and less efficient at absorbing UV as its wavelength decreases, until, at the low energy end of the soft x-ray region, some extragalactic sources begin to appear (mostly AGN).

    Then there’s the “dropouts”; high redshift sources that are invisible blueward of the (redshifted to optical or IR) Lyman limit. Back then, the IGM was not ionized at all, the “Era of Reionization” had yet to start.

    And perhaps the coolest new dawn in astronomy is the SKA’s ability to probe this era, by observing redshifted 21cm H emission. Bonus: do you why 21cm absorption is never (?) seen?

  5. #5
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    Quote Originally Posted by Jean Tate View Post
    Bonus: do you why 21cm absorption is never (?) seen?
    It is seen. For example, look here. You just need a radio source with an intervening cloud of hydrogen.
    Conserve energy. Commute with the Hamiltonian.

  6. #6
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    The biggest bit bit I was missing is that atomic hydrogen with its electron in ground state can't absorb Balmer series etc etc.

    Hydrogen maser type effects looks fascinating. How do you know if the you a looking at a stimulated emission when the radiation does not have many properties seen from a laser cavity type device?
    What do we see if the maser is not pointing at us?

    Also Hydrogen UV lasers around O type stars in HII regions?
    Last edited by PetTastic; 2018-Nov-10 at 10:32 PM. Reason: Only interested in hydrogen
    See: 'The God Kit' -- 'The Brigadier And The Pit' -- Carl N Graham -- Sci-fi blog: The Alien Reporter

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