Constraining the reionization history from Lyman alpha emitting galaxies
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.