The neutral hydrogen number density at a redshift of z=7 in slices of the simulations for different reionization models with large-scale ionized regions (bubble model), small-scale structures (web model), and both combined (web-bubble model). © MPA
This
plot shows the cosmological fraction of neutral hydrogen (HI) in the
diffuse intergalactic medium at various redshifts z. For earlier cosmic
times (higher z), the universe is increasingly neutral. Constraints from
previous work are shown with different symbols, the constraints from
this work are shown as the orange regions. A late and rapid reionisation
is clearly favoured. The solid line indicates the evolution of
ionisation according to the models. © MPA
In cosmology, one of the major challenges in next decades will be
probing the epoch of reionization in the early universe. Scientists at
MPA, the University of Oslo, and INAF have now used cosmological
hydrodynamical, radiative transfer simulations to understand the impact
of the complex distribution of neutral gas in the intergalactic medium
on distant galaxies. Combining the simulations with observations of
so-called Lyman alpha emitting galaxies they find that despite the
uncertainty, the current simulation-calibrated measurements favour a
late and rapid reionization history. The study also emphasizes that both
the large-scale distribution of ionised gas regions and the small-scale
structures of the intergalactic gas around galaxies must be understood
to derive more robust constraints on the reionization epoch.
The epoch of reionization, when early
galaxies or black holes drastically transformed the global state of the
universe from neutral to an ionized plasma, is one of the major unsolved
mysteries in modern extragalactic astronomy. Big questions remain
unanswered: What was the history of reonization? Which sources were
responsible for driving it?
One possibility to probe the physical
state of the universe at very early times is by observing distant,
high-redshift 'Lyman-alpha emitting galaxies'. These galaxies are
emitting a strong Lyman alpha line, i.e. radiation from hydrogen gas in
their interstellar medium. This strong emission line enables astronomers
to observe these objects out to very far distances, at redshifts as
high as 10. By now, hundreds of Lyman alpha galaxies have been found
beyond redshift 6.
Observations show that the apparent
demographics of Lyman alpha emitting galaxies changes over cosmic
history. Beyond redshift 6, i.e. when the universe was less than 1
billion years old, the observed population of galaxies with Lyman alpha
emission suddenly decreases. This is difficult to explain with galaxy
formation alone. From medium to high distances (redshift 2 to 6), the
fraction of star-forming galaxies that show a strong Lyman alpha
emission increases, which is partly caused by less dust in these
galaxies. Therefore, the sudden drop at very hight distances, beyond
redshift 6, seems to indicate that something is blocking this kind of
light. This drop is often interpreted as evidence of the gas in the
universe being increasingly neutral at earlier cosmic times – this means
the drop marks the time of reionization.
The idea to use Lyman alpha emitting
galaxies as a probe of reionization is based on a simple idea. With more
neutral gas along the line-of-sight to the galaxies, less Lyman alpha
flux reaches the observer. The difference between the expected flux from
a galaxy and the observed flux then tells us how much neutral gas
exists along the line-of-sight.
Kakiichi and collaborators have used this
method to infer the neutral hydrogen content of the universe at
redshift 7. They used cosmological hydrodynamical, radiative transfer
simulations of reionization (see Figure 1) to interpret observations of
Lyman-alpha emitting galaxies. The observations are then compared with
theoretical models of the apparent population of Lyman-alpha emitting
galaxies. In this way, the neutral gas fraction can be inferred from the
models that best fit the observations.
The new constraint this provides for the
reionization history is shown in Figure 2, which shows that the universe
is still very neutral at redshift 7. The present analysis therefore
seems to suggest that reionization occurred late and rapidly around
redshift 6 to 8.
This study also highlights an important
uncertainty in this simulation-calibrated measurement of the neutral
fraction. Figure 3 shows that completely different values of the neutral
fraction combined with other 'topologies' of reionization work equally
well in explaining the observed luminosity function (Figure 3). In fact,
this leads to a systematic uncertainty in the inferred neutral fraction
as high as an order of magnitude. Knowledge about the topology of
reionization, namely both the large-scale distribution of ionized
bubbles and the properties of small-scale self-shielded gas around
galaxies, is crucial to robustly infer the reionization history. Only
models containing both large and small-scale structures are able to
coherently explain the observations of the Lyman alpha forest and Lyman
alpha emitting galaxies from the reionization epoch to the
post-reionized universe.
This difficultly, however, does not limit
the scope of using Lyman alpha emitting galaxies as a probe of
reionization. The uncertainties can be reduced by simultaneously using
multiple statistics such as the luminosity function and the fraction of
strong Lyman alpha line in Lyman Break Galaxies in surveys of Lyman
alpha galaxies. New survey strategies search for early galaxies in the
foreground of quasars at the reionization epoch, which will drastically
increase the scope of this method because it allows astronomers to
directly study both the state of the intergalactic gas and the
properties of Lyman alpha emitting galaxies.
Together with the increasing capability
of radiative transfer simulations, Lyman alpha emitting galaxies serve
as important beacons to probe the state of the infant universe.
Authors
Kakiichi, Koki
PhD student
Phone:
2034
Ciardi, Benedetta
Scientific Staff
Lyman-Alpha Emitting Galaxies as a Probe of Reionization: Large-Scale Bubble Morphology and Small-Scale Absorbers
submitted to MNRAS
Source: http://arxiv.org/abs/1510.05647
