Daily paper summaries

Stuck in neutral: how did the Universe become reionized?

Often, the most profound questions in physics are those you wouldn’t think to ask.  A perfect example is that leading to the discovery of the Higgs boson: why do particles have mass?  Today’s paper is on a question in this category in astronomy: why is the Universe ionized and not neutral?  After all, the background temperature today, 2.73 K, is far lower than the ~10,000 K needed to ionize hydrogen.  Well, what keeps us, and the Universe, warm?  Stars.  In particular, the Universe became ionized (for the 2nd time) because of starlight heating up the neutral hydrogen and popping electrons off their nuclei.

 

When this occurred, and how quickly, is a tremendous open question in astrophysics.  If you know the answer, please write us and we will be glad to co-author the paper and share your Nobel prize.  Nonetheless, we do have some constraints, which Robertson et al, assemble. They particularly focus on what new things we learned from the latest Ultra-Deep-Field Hubble Space Telescope observations, which go to z~8 and limiting magnitude -17.  Redshift z is a proxy for distance, while magnitude is a proxy for how faint the minimally detecting objects are. Ultra-Deep-Field just means it goes to high-z (very deep out into space).

Image of Hubble Ultra-Deep-Field---deep because it goes out super-far into the past, to redshift 8 (and remember, farther into the past is also farther away!) From http://atomictoasters.com/2011/08/the-hubble-ultra-deep-field-2/

Image of Hubble Ultra-Deep-Field—deep because it goes out super-far into the past, to redshift 8 (and remember, farther into the past is also farther away!) From http://atomictoasters.com/2011/08/the-hubble-ultra-deep-field-2/

 

Constraints on reionization.  Horizontal axis is a proxy for time while vertical axis is fraction of ionized hydrogen (1-neutral hydrogen fraction).  The yellow oval highlights the pentagons, which are constraints from the Lyman alpha absorption.  The straight dashed lines are constraints from the kSZ effect: this effect only gives a constraint on the total time for reionization, so the start and endpoint of the lines are arbitrary and just chosen by the authors.  From the paper.

Constraints on reionization. Horizontal axis is a proxy for time while vertical axis is fraction of ionized hydrogen (1-neutral hydrogen fraction). The yellow oval highlights the pentagons, which are constraints from the Lyman alpha absorption. The straight dashed lines are constraints from the kSZ effect: this effect only gives a constraint on the total time for reionization, so the start and endpoint of the lines are arbitrary and just chosen by the authors. From the Robertson paper.

The constraints are:

  1. Number of stars and luminosity. Stars produce the light to ionize hydrogen—so the more stars, the more ionization, and the hotter and brighter these stars, the more ionization.  So measuring how numerous and how luminous and hot stellar populations were at different times in the Universe’s history (this can be derived, with some additional assumptions, from stellar mass density over time) gives you a budget of photons to spend reionizing.
  2. Thomson optical depth.  Electrons scatter photons in a process called Thomson scattering.  The more ionized the Universe, the more electrons are out there scattering photons, and the less far back into the past/ far out from us you can see, as photons from farther away have higher chance of being scattered.
  3. Lyman-alpha emission from galaxies passing through the inter-galactic medium (IGM).  Lyman alpha emission is just when a photon leaves a hydrogen atom be causes its electron drops from n>1 to n=1.  Galaxies emit Lyma-alpha photons and these are then absorbed by neutral hydrogen in the IGM.  Thus, the more galaxies we can see, the less neutral hydrogen in the IGM we can infer.  This constrains the history of neutral hydrogen in the IGM.
  4. The kinetic Sunyaev-Zeldovich (kSZ) effect.  Whew, what a mouthful.  Cool facts: I have met Sunyaev (and got his autograph!), and Zeldovich basically did everything in cosmology after retiring from heading the Soviet nuclear weapons program.  The kSZ effect has to do with the motions of electrons during reionization.  In general, photons from the cosmic microwave background (CMB) scatter off of electrons in between it and us. Normally, the electrons’ motion is completely random so the scattering averages out to zero.  But reionization can make the electron’s motion coherent in different patches, and so net on net the scatterings no longer cancel out.  We can detect this and use it to constrain the total length of time reionization could have gone on.

 

Robertson et al. also include a few other constraints omitted for brevity here. Overall they conclude that the galaxy population measured in the Hubble UDF program is not enough to reionize the Universe by the time we know it is (z~6) and simultaneously get the right Thomson optical depth (see 2) above).  So they do the simplest thing they know how to: extrapolate from the known properties of galaxies in the sample and assume that galaxies continue to exist with the same properties out to z~12-15 and limiting magnitude -13.  In other words, that there are more galaxies out there, basically the same as those we see, but too far away and too faint to show up in Hubble’s UDF.

Ionized hydrogen in the Universe versus redshift (a proxy for time---farther back into past is farther to right on plot.)  The black dashed curve is the real results, the mustard curve those with extrapolation fainter and farther as explained in text.  From the paper.

Ionized hydrogen in the Universe versus redshift (a proxy for time—farther back into past is farther to right on plot.) The black dashed curve is the real results, the mustard curve those with extrapolation fainter and farther as explained in text. The dashed line is too low to satisfy all the constrains in Figure 2, but the mustard line just makes it.  From the Robertson paper.

 

Reionization is an incredibly complex area of research that is only just being attacked with both observations and numerical simulations, so we should expect many more exciting constraints in the near future. After all, how many papers do you read a day that have a plot showing the history of the element hydrogen over the last 10 billion years?

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Zachary Slepian

I’m a 2nd year grad student in Astronomy at Harvard, working with Daniel Eisenstein on the effect of relative velocities between regular and dark matter on the baryon acoustic oscillations. I did my undergrad at Princeton, where I worked with Rich Gott on dark energy, Jeremy Goodman on dark matter, and Roman Rafikov on planetesimals. I also spent a year at Oxford getting a master’s in philosophy of physics, which remains an interest.

Discussion

4 Responses to “Stuck in neutral: how did the Universe become reionized?”

  1. Thank you for this fascinating contribution, I so enjoy this. Under point 3 you mention the Lyman-Alpha absorption. I have been puzzled by the associated Gunn-Peterson effect (or should I say absence of) in some high red-shift quasars with z>6. One would expect complete absorption of the Lyman-alpha forest for all high red-shift quasars, yet when I look at the literature this does not seem to always be the case. What is the reason for this, is this some an-isotropic ionization?

    Posted by Christian Sasse | October 25, 2013, 2:40 pm
    • Thanks very much for the positive feedback. I don’t know the answer to this question, but maybe astrobiter Evan Schneider, one of the co-authors of this paper, does, so I’ve passed it on. Yuan-Sen Ting recently did a great interactive website on the Gunn-Peterson effect and Lyman alpha forest so that may let you play around and get a feel for how sensitive these effects are to changes in the parameters: https://www.cfa.harvard.edu/~yuan-sen.ting/lyman_alpha.html

      Posted by zslepian | October 25, 2013, 7:42 pm
    • Hi Christian – Indeed, you have answered your own question. It takes very little neutral hydrogen to create the absorption seen in the Lyman-alpha forest quasars, so for most quasars above z ~ 6, even though over 99% of the universe is ionized, we see some hydrogen absorption; for for most higher redshift quasars, we observe the Gunn-Peterson trough. However, re-ionization is expected to proceed in a patchy manner, as early galaxies ionize neutral hydrogen in bubbles around them, and then those bubbles overlap. The quasars that happen to be along sight lines where all of the ionized bubbles have overlapped will not exhibit the hydrogen absorption seen in other directions – it is, as you said, an an-isotropic phenomenon.

      Posted by Evan Schneider | October 28, 2013, 2:40 pm
  2. Thank you very much Evan, now it is clear

    Posted by Christian Sasse | October 30, 2013, 11:34 am

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