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George
2006-Mar-13, 02:24 PM
I thought ionization created opacity for photons, disallowing light’s first release until recombination. However, I’ve read ionization is necessary to allow light to pass, so the “Dark Ages” existed until hydrogen was reionized by uv from vast numbers of early stars.

What simple thing am I missing? How can ionized hydrogen be both opaque and transparent? [I wish I could think of some pun to appear transparently opaque, too. ;)]

Ken G
2006-Mar-13, 03:37 PM
The Dark Ages refers not to the cosmic microwave background, which started out pretty darn bright but got redshifted away quickly, it refers to the lack of stars. If you look out into space now, you see stars, so the lack of them would be "the dark ages". Ionization never reduces the opacity except for ultraviolet light (and when the CMB was ultraviolet, there could not be much recombination anyway). If you read that ionization allows light to pass, I think you read something incorrect. Rather, it is the presence of light that causes ionization (witness "H II regions" around hot stars).

ngc3314
2006-Mar-13, 03:55 PM
I thought ionization created opacity for photons, disallowing light’s first release until recombination. However, I’ve read ionization is necessary to allow light to pass, so the “Dark Ages” existed until hydrogen was reionized by uv from vast numbers of early stars.

What simple thing am I missing? How can ionized hydrogen be both opaque and transparent? [I wish I could think of some pun to appear transparently opaque, too. ;)]

We have to consider a three-stage history for cosmic optical depth. Early on, everything was fully ionized. Free-free interactions between electrons and photons scatter at all frequencies, making the plasma opaque (although not dark, since the radiation rattles around having forgotten its initial direction). Then at recombination (z=1080+/-40 or so), the gas became predominantly neutral. Outside of narrow absorption lines and the ionization regime fpr energies above 13.6 eV, hydrogen is transparent.

However, neutral H/He is not what we find in the intergalactic medium today - what we find, with some difficulty, is in phases from 10^5-10^7 K. So it must have made another transition - reionization - of which we first found evidence for He II around z=2.7 (He II would take longer to recombine, being ionized by radiation so hard that H barely soaks it up at all) and finally for H for which the process was finishing around z=6.4. There is almost a consensus that the main energy source for reionization was early (but not as early as Pop III) stars in smallish galaxies; luminous quasars and really bright galaxies are not common enough to have provided enough energy.

Tim Thompson
2006-Mar-13, 04:28 PM
Outside of narrow absorption lines and the ionization regime fpr energies above 13.6 eV, hydrogen is transparent.
Correct, mostly. The neutral hydrogen will be locally transparent, but not cosmologically transparent; it will be cosmologically opaque. This is because the neutral hydrogen fills the available distribution of redshifts between the observer (us) and the source (a cosmologically distant quasar or galaxy). So the narrow line Lyman-alpha absorption (http://astron.berkeley.edu/~jcohn/lya.html) becomes a nearly continuum absorption. Observationally, this is what is responsible for the Gunn-Peterson Trough (http://cas.sdss.org/dr4/en/sdss/discoveries/discoveries.asp#gunn) in distant galaxy & quasar spectra. Once the universe reionizes, the Lyman-alpha absorption is gone and that mechanism disappears, so the universe once again appears transparent.

Transparency of the Universe after Reionization (http://imagine.gsfc.nasa.gov/docs/ask_astro/answers/041029a.html)
The Epoch of Reionization (http://www.lofar.org/science/urd100/Epoch_of_Reionization.html)
Reionization (http://www.ast.cam.ac.uk/~rtnigm/reion/RTNreionization.html)
Reionization of the Large Scale Structure (http://www.strw.leidenuniv.nl/~ritzervj/Research/Reionization.html)
The Reionization of the Universe by the First Stars and Quasars (http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=2001ARA%26A..39...19L&db_key=AST&high=4194d1543129359); Abraham Loeb & Rennan Barkana; Annual Review of Astronomy and Astrophysics, Vol. 39, p. 19-66 (2001); A comprehensive review of reionization in cosmology, a PDF version is available via this link.

George
2006-Mar-13, 04:50 PM
The Dark Ages refers not to the cosmic microwave background, which started out pretty darn bright but got redshifted away quickly, it refers to the lack of stars. If you look out into space now, you see stars, so the lack of them would be "the dark ages". Ionization never reduces the opacity except for ultraviolet light (and when the CMB was ultraviolet, there could not be much recombination anyway). If you read that ionization allows light to pass, I think you read something incorrect. Rather, it is the presence of light that causes ionization (witness "H II regions" around hot stars).
Thanks Ken. The "Dark Ages", according to the book I'm reading, refers to the restriction of light's passage caused by blockage from non-ionized hydrogen. At least, that is how it seems from what little I've read. It is a paradox for me, since it was recombination which released light to begin with. Or, are we only talking about u.v.? That would explain it, but I didn't read it that way since the topic was Pop III stars and early galaxies seen from Hubble's NICMOS and HUDF images. Certainly, u.v. would be a strong component if early massive stars came first, but that wouldn't warrant a "dark ages" view of light in general from that time and circumstance.

FWIW, since recombination was around 3,000K, there would not be much u.v. from the primordial material. The peak would be in the orange, yet I wouldn't call it an "orange flash". [Hmmm, maybe I would stubmle at the temptation, but the strength of the other wavelengths would alter the color appearance had we been there to see it. :) It might have been a little reddish, but wouldn't be too hard to determine since it is as close to a blackbody as anything known, I suppose. ]


We have to consider a three-stage history for cosmic optical depth. Early on, everything was fully ionized. Free-free interactions between electrons and photons scatter at all frequencies, making the plasma opaque (although not dark, since the radiation rattles around having forgotten its initial direction). Then at recombination (z=1080+/-40 or so), the gas became predominantly neutral. Outside of narrow absorption lines and the ionization regime fpr energies above 13.6 eV, hydrogen is transparent.

However, neutral H/He is not what we find in the intergalactic medium today - what we find, with some difficulty, is in phases from 10^5-10^7 K. So it must have made another transition - reionization - of which we first found evidence for He II around z=2.7 (He II would take longer to recombine, being ionized by radiation so hard that H barely soaks it up at all) and finally for H for which the process was finishing around z=6.4. There is almost a consensus that the main energy source for reionization was early (but not as early as Pop III) stars in smallish galaxies; luminous quasars and really bright galaxies are not common enough to have provided enough energy.

Thanks. Are you saying ionization energies determines wether hydorgen and helium are opaque to light? Opaque during its original ionization phase (prior to recombination), but transparent at subsequent lower ionization energies (from eventual u.v. penetration of the recombined soup)?

[Edit: Oops, I missed Tim Thompson's post. Let me cogitate. Everything is a little orange at the moment.]

George
2006-Mar-13, 06:16 PM
From Tim's Reionization (http://www.ast.cam.ac.uk/~rtnigm/reion/RTNreionization.html) link...

From observations of the microwave background, we know that the Universe recombined at a redshift of about a 1000. However, we also know the intergalatic medium (IGM) to be highly ionized below a redshift of six. If it were neutral, then scattering by neutral hydrogen would absorb all the light below the Lyman-alpha transition in the spectra of quasars. Since we observe flux, the IGM must be very highly ionized, to a level of 1:10000 or so. Hence:the universe has been reionized.
[my bold]
Is 1:10,000 a "very highly ionized" level, or is it 9,999:10,000? Assuming the former, is it a density issue then (consistent with the Lyman Alpha forest)?

Should I also assume recombination produced high energy states (at all levels producing a BB curve), yet, the neutral hydrogen quickly dropped to low energy states shortly after z ~ 1089?