Proc. Neutrino Astrophysics - MPP Theory Group

Helium Absorption and Cosmic Reionization
Craig J. Hogan
Departments of Physics and Astronomy, University of Washington
Box 351580, Seattle, WA 98195, USA
In my talk at Ringberg I emphasized the current concordance of light element abundances
with the Big Bang predictions and with the observed density of baryons. The substance of
most of these remarks together with further references can be found in recent papers [1, 2, 3].
Here I want to highlight one aspect of this puzzle which has not been commented on very
much, the interesting current state of observations on singly ionized helium at high redshift.
The gas in the emptier regions of the universe at the highest redshift observed (almost 5)
is already almost entirely ionized; the fraction of hydrogen which is neutral is less than 10 −4 .
It is small enough that over most of redshift space, the absorption from hydrogen has small
optical depth. HeII’s higher ionization potential (54.4 eV as opposed to 13.6 eV for hydrogen)
means that its ionization is delayed relative to hydrogen as such hard photons are relatively
rare. HeII is therefore more abundant in absolute terms than any other species which makes
it the best tool for studying matter in the emptier regions of space—the voids between the
density concentrations.
Currently there are two quasars where published high resolution HST/GHRS quasar absorption
line spectra permit the separation of the more diffuse component of the HeII resonance
absorption from the component in the concentrations of matter which appear as HI
Lyman-α forest clouds [4, 5]. Both datasets imply an upper bound on the diffuse matter
density at z ≈ 3 of less than 0.01h −1.5
70 , derived based on the maximum permitted mean HeII
Lyman-α optical depth allowed after subtracting the minimal contribution from the detected
HI Lyman-α forest clouds, while adopting the hardest ionizing spectrum allowed by the data.
This upper bound is important for constraining ideas about the evolution of the baryons and
especially the possibility of a large repository of baryons in the voids. Both datasets also agree
that there is HeII absorption at nearly all redshifts—that is, there is at least some matter
everywhere, even in the emptiest voids.
However the current published results do conflict in one important respect. Hogan et
al. [4] find in Q0302-003 that although the HeII optical depth is rather high (greater than 1.3
everywhere), there is also detectable quasar flux at all redshifts in between the identified HI
clouds, whereas Reimers et al. [5] find in another quasar (HE2347-4342) large regions where
the flux is consistent with zero. The first result suggests that there is rather little HeII and
that the helium is mostly doubly ionized HeIII already (unless the voids are swept implausibly
clean of gas), whereas the second result suggests that large volumes of space are still mostly
HeII and that helium ionization is not yet complete.
Since the ionization is expected to occur in patchy domains around the strongest sources
of ionizing photons (indeed near 0302-003 itself the sphere of influence is observed in the
spectrum to about 4000 km/sec in radius), it is possible that both interpretations are correct
and that the different directions are just different. This would be interesting because the
scale of the inhomogeneity in ionization history would be much larger than expected, with
likely consequences for large scale structure in the galaxy distribution. On the other hand
it could also be that one or both of the published spectra have an incorrect zero level—a
natural suspicion since they are taken with the one-dimensional GHRS for which background
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