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The Dark Ages… First Stars… QSOs… and Reionization of the Universe

The Dark Ages… First Stars… QSOs… and Reionization of the Universe. Karl Isensee Dan Polsgrove 5 Dec 2006. Pre-Presentation Quiz. True or false. When we observe the Universe today, we find that the majority of Hydrogen is in which form? Molecular (H 2 ) Atomic, a.k.a. Neutral (HI)

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The Dark Ages… First Stars… QSOs… and Reionization of the Universe

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  1. The Dark Ages…First Stars…QSOs…and Reionization of the Universe Karl Isensee Dan Polsgrove 5 Dec 2006

  2. Pre-Presentation Quiz • True or false. When we observe the Universe today, we find that the majority of Hydrogen is in which form? • Molecular (H2) • Atomic, a.k.a. Neutral (HI) • Ionized (HII)

  3. Goal #1: Pass along a few interesting tidbits about things going on in the universe between 400,000 and 1 billion years after Big Bang. • Part I: The Dark Ages… • Start: ~400,000 years after BB (z ~ 1100) • “End”: ~ 100 million years after BB (z ~ 20) • Why we care: The seeds of all “structure” we see today were planted in the Dark Ages • Part II: The Epoch of Reionization… • “Start”: end of Dark Ages (z ~ 20) • End: ~ 1 billion years after BB (z ~ 6) • Why we care: Because most ordinary matter in the Universe is still ionized today; how did that happen?

  4. Goal #2: Introduce you to (or remind you of) a few terms/concepts you might see again… • Ionization • Absorption cross-section • Lyman-alpha forest • Gunn-Peterson trough • 21-cm line • QSO/quasar • Stromgren sphere • WMAP, LOFAR, MWA, PaST

  5. Cosmic Timeline Part II Part I

  6. Part I The Dark Ages • Begins at z ~ 1100 with ______________. • So as we’ve learned in this class, there were already “dark matter halos” and “potential wells” and “density perturbations” in place… eventually leading to galaxies/clusters/stars/planets of today • But if it was the “Dark Ages”, how can we possibly hope to directly observe or “see” anything from this era? There is ONE thing neutral Hydrogen hooks us up with…

  7. Part I The Dark Ages – 21cm line I don’t quite get conditions for emission vs absorption, or maybe it’s just magnitude of emission? and then how that can lead us to knowledge of H density distribution…

  8. Diagram of H distro we can get from 21cm line, different densities

  9. Part I Investigating the Dark Ages…Instruments coming up soon • So far we’ve only looked back to z ~ 6.5… need to look further to directly observe Dark Ages… • MWA (pics? Stats? Just talk about it?) • LOFAR • PaST

  10. Part II Part II: The epoch of reionization IIa: Ionization 101 IIb: The Source (or is it sources?) - Massive stars? - Black hole accretion? (aka QSOs, quasars) IIc: The “Beginning” of reionization - When? How do we know? IId: The “End” - When? How do we know?

  11. Part II IIa: Ionization 101 • Neutral H atom will ionize (lose its electron) if something gives it 13.6 eV (or more… extra will just go into the newly freed electron’s kinetic energy). • 2 most common “donors” of 13.6 eV: • Collisions - A particle has mass & speed such that KEparticle = (1/2)mv2 > 13.6 eV - Particle rams into neutral H atom - Neutral H atom absorbs 13.6+ eV, electron escapes • Photoionization - Incoming photon has E > or = 13.6 eV (λ < or = 938 Å) - Photon rams into neutral H atom - Neutral H atom absorbs 13.6+ eV, electron escapes

  12. Part II IIb: The Source(s) of reionization…Who’s the energy donor? • Consensus: UV photons • (average Universe temp not hot enough for collisional ionization to be much of a factor… need T >105 K) • From where? • Option #1 = massive stars • Option #2 = black hole accretion • Option #3 = combination of #1 and #2 • Which is it? Astronomers can’t come to total agreement yet, but massive stars is the favored theory… • Madau claims it has to be massive stars, mostly because we don’t see many bright QSOs at z > 3

  13. Part II Reionization: The story Objective: Start with overdense dark matter halos, end up with massive stars, ionize all the Hydrogen in the Universe. • Dark matter halo gets more dense throughout Dark Ages (baryons, freed from photons, start flowing into potential wells)… • Temp. & density have to be just right for baryons (Hydrogen) to actually collapse all the way down to make a star (Jeans criteria)… • But the Hydrogen must “cool” on its way down, releasing half the energy provided by gravitational “work” of contraction (virial theorem)… • How does it release energy? Has to be radiating away photons. Only option is rotational transitions within H2. Not very efficient. And H2 isn’t too plentiful. So it takes a long time. But it eventually works. • Theory says a DM halo with T ~ 2,0000 K and mass ~ 106 Msun could lead to a core of 100-1000Msun that will form a star…

  14. Part II • 100-1000Msun?! That’s huge. Won’t last long. But while it’s burning, it’s crazy hot and expels most of its energy as short wavelength (UV) photons (Wien’s law)… • Just what we needed to reionize neutral Hydrogen – UV photons! Stromgren sphere expands around the star, creating a “bubble” of HII that keeps getting bigger. • As time goes on, • Baryons in more and more dark matter halos reach Jeans criteria • Therefore, more massive stars are formed • Therefore, more Stromgren spheres are bubbling up & expanding • Therefore, eventually all the bubbles overlap each other • Therefore, all the Hydrogen in the Universe is eventually reionized Roll simulation. Everyone is happy. Well almost…

  15. Part II Reionization: The REST of the story 1. Rogue photons – harassing the neighbors • Once all the H in massive star’s local halo is ionized, high energy photons can then fly right through unimpeded (nothing’s there any more to absorb them). Those with E ~ 11 eV could escape, meet up with a neighboring halo that hasn’t made massive stars yet, and short-circuit the cooling process by dissociating the H2 that was helping cool the collapsing baryons in that halo. Possible result: 1st generation of massive stars self-destruct the reionization process by disrupting the very mechanism by which they themselves were able to form. Must be more to the story since most H is ionized today. 2. Supernovae – massive stars won’t live forever - When they blow, they have major “impact” on the neighborhood. Literally. Each SN puts ~ 1051 erg of energy into local environment; 10 of them is enough to blast away everything in a 106Msun halo (speed it up to escape velocity). Not necessarily a bad thing: now you have heavy elements created in supernova streaming out into the IGM, ready to merge with other halos. And now there’s heavy elements available to radiate away energy during baryon collapse into a 2nd generation of stars… and these could have various masses (not all huge)… and you could also get QSOs/quasars forming at the center of infant galaxies.

  16. Part II IIc: The Beginning When did it happen? z = 48? z = 20? z = 10? • It obviously wasn’t instantaneous. But we can get some semblance of a quantitative and directly observed answer from… CMB polarization! • Anyone recognize this guy?

  17. Part II Summary of the preceding gibberish: • The astronomical community currently believes it’s appropriate to say that the reionization epoch was in full swing at z ~ 11 because: 1. Smart people on the WMAP team cranked through theories & equations that predicted what the CMB polarization power spectrum should look like if reionization of the Universe “happened” at z = 100, 99, 98,… 8, 7, 6. 2. They then built WMAP, launched it, and took a bunch of data 3. They analyzed the data (looked painful). 4. They concluded that the appearance of the CMB polarization power spectrum looks like what they predicted it would look like if reionization was going strong around z ~ 11.

  18. Part II IId: The End of Reionization… Any Questions?

  19. Part II Gunn-Peterson troughLyman-alpha forest LAF = Lyman Alpha Forest See a Trough? If answer is No: Photons with λe < 1216 Å made it through. Univ. is ionized between source and us. If answer is Yes: Photons with λe < 1216 Å did NOT make it through. So Univ. between source and us is not ionized. LAF tree = Neutral H region tree Gunn-Peterson trough Standard Belief: reionization “complete” At z ~ 6.

  20. Part II Gunn-Peterson trough QSO HII (Hydrogen “fully” ionized) QSO HI z 9 8 7 6 5 4 3 2 1 0

  21. Lyman-alpha forest E = 10.2 eV λ = 1216 Å J Cohn, UC-Berkeley astro.berkeley.edu/~jcohn/lya.html

  22. References • 1. Loeb, A. “The Dark Ages of the Universe”, Scientific American, Nov 2006. • Miralda-Escude, J. “The Dark Age of the Universe”, Science, Jun 2003. • Madau, P. “The Era of Reionization”, Galaxy Evolution: Theory and • Observations, 2002. • Cohn, J. http://astro.berkeley.edu/~jcohn/lya.html. • Ritzerveld, J. www.strw.leidenuniv.nl/Research/Reionization.html • Komatsu, E. http://gyudon.as.utexas.edu/~komatsu/talks.html

  23. Return to Ionization 101… tying up loose endsPhotoionization cross-section, av 7 H0 6 5 Ionization Cross-section (10-18 cm2) 4 3 2 1 0 1 2 0.32 Photon frequency (1016 Hz) Boardwork?

  24. Part II Return to ionization 101…a photon’s journey from source to you HI HI HI star Photon, E=10 eV

  25. Part II Ionization illustrated Comment on how cosmo Redshift ensures photon E Will “look” like 13.6 eV at Some point in travel? HI HI HI star Photon, E=14 eV Define this, Distinguish from Quasar (radio emission)

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