Cosmological Reionization by Early Galaxies

Brant E. Robertson, Richard S. Ellis, James S. Dunlop, Ross J. McLure&Daniel P. Stark, 'Early star-forming galaxies and the reionization of the Universe', Nature November 4, 2010 v 468 doi:10.1038/nature09527

Alex Fry journal club talk November 5th 2042.

Big Bang

inflation

matter and anti-matter annihilate, quarks and what not form protons, and Helium nuclei

370,000 years later electrons and protons get together forming nuetral hydrogen and photons free stream (cmbr) away.

electrons and photons from hot Big Bang rattle around until…

dark ages

first stars and galaxies

something causes reionization

Milky Way forms, you smile and nod at nice story

Highlights

• The physics of reionization, AMAZING!

• Observational probes of the reionization epoch, AWESOME!.

• The discovery and study of high redshift (z>7) galaxies, OUT OF THIS WORLD!

Outstanding problems

• Alternate sources of reionizing photons:

Active galactic Nuclei

Decaying elementary particles*

The cosmos will conspire to reionize depending upon the physics:

• Co-moving star formation rate.• Number of ionizing photons (λ < 91.2

nm or energy > 13.7 eV) emitted and the recombination rate.

• Fraction of photons that escape the host galaxy.

The cosmos will conspire to reionize depending upon the physics:

• The first generation of stars created complicated feedback to suppress/enhance further star formation.

• The recombination rate depends upon local density through atomic physics and cosmological clumping.

• The escape fraction is a mess.

• Ultimately the Universe will be ionized and remain ionized when the integrated star formation history has produced sufficient UV photons.

S. Baek, B. Semelin, P. Di Matteo, Y. Revaz, F. Combes, 2010, Reionization by UV or X-ray sources

Observational probes…

• Early galaxies (z~7 or 800 Myr after Big Bang) provide detailed constraints on the amount of ultraviolet radiation available.

• Quasar Lyman α forest measures the line of site amount of Hydrogen.

• Lyman-α line emission from a galaxy indicates that neutral gas outside the galaxy is not present.

• Radio interferometry at 21-cm will ultimately map the epoch of reionization large scale structure.

The method:

• Photometric drop out technique

• These are Lyman-break galaxies

1) Even small amounts of intervening neutral hydrogen absorb photons blueward of the Lyman-limit through ionoization.2) Very little flux it emitted blueward of the Lyman-limit because ‘few’ stars are hot enough3) Left over photons absorbed by hydrogen clouds along line of sight.

5

600 800 1000 1200 1400 1600 λ (nm)

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29.0

28.5

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8.0

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F160WF125WF105W

F850W

Photometry with the Hubble Space Telescope’s Wide Field Camera 3 in the infrared

Z=2Z=5Z=7Z=9

Z=7 drop out

R. J. McLure et al.

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HThe models (in green) for star formation rate density are consistent with data (in black) and match models of reionization (in orange) completing at z~5 to 8.

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Enough galaxies have been measured determine the galaxy luminosity function and galaxies could reionize the universe and more data could extend these results to higher z and fainter magnitudes…

Confounding Factors:

Contamintation

...our test indicates that at least > 50% of them are very likely genuine candidates at z 10. We point out that the recent criticism that the majority of our J125-dropouts are implausibly too close to “bright foreground” objects is not justified. While there are a few cases that our dropouts are close to a neighbor, the excess fraction is 30%... could be genunine z 10 galaxies that are gravitationally lensed by their foreground neighbors, and that the seemingly high rate could be explained by their intrinsically very steep LF and the magnification bias R. J. McLure et al

Confounding Factors:

Escape Fraction

• UV photons may not leave their host galaxy/star and may simply ionize local hydrogen in situ which then recombines.

• Measured by extrapolating from low redshift (z~3) spectroscopic measurements or by inferring a nebular/stellar contribution model based on the slope of flux.

• Tentative measurements at high redshift (z~7) indicate dust-free, metal-poor systems with high escape fractions.

Conclusion

• The UV photons from the first galaxies is plausibly sufficient to reionize the universe by z~6 which is consistent with other measurements and theory.

• Need more data (aside: arguably the HST is so inefficient at this

survey in the IR, 100 orbits per galaxy, that observations which can only be done with Hubble’s unique bluer observation window in space should take priority. Unless JWST blows up, which it may, it would be able to do these same observations much more efficiently).

The escape fraction of ionizing photons

• Only a fraction of the uv photons emitted by a galaxy may escape, fesc<<1.

• f(λ) = λβ increasing beta indicates reder.

• This makes things confusing!

The escape fraction of ionizing photons

• Xi is the number of hydrogen ionizing photons per second per unit star formation rate

• Fesc is the fraction of photons that escpae the galaxy