Cosmological Reionization by
Early Galaxies
Brant E. Robertson, Richard S. Ellis, James S.
Dunlop, Ross J. McLure&Daniel P. Stark, 'Early starforming 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 antimatter annihilate,
quarks and what
not form protons,
and Helium nuclei
electrons and
photons from hot
Big Bang rattle
around until…
dark ages
370,000 years
later electrons
and protons get
together forming
nuetral hydrogen
and photons free
stream (cmbr)
away.
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
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
enough
3) Left over photons absorbed by hydrogen
clouds along line of sight.
• These are Lyman-break galaxies
5
Photometry with the Hubble
Space Telescope’s Wide Field
Camera 3 in the infrared
Mab
28.0
27.5
27.0
Z=9
Z=7
Z=2
Z=5
F125W
F160W
F850W
29.0
28.5
F105W
600
800
1000
1200
λ (nm)
1400
1600
Z=7 drop
out
R. J. McLure
et al.
Volume fraction of ionized H
z
z
The 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.
Galaxy Luminosity Function
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