When did the universe emerge from its 'dark age'? Spectrum of gamma-ray burst's afterglow indicates beginning of re-ionization process

Hydrogen is the main element of ordinary matter (5%) in the Universe, which is primarily composed of more abundant dark matter (27%) and dominant dark energy (68%). During a stage of high temperature right after the Universe's birth about 14 billion years ago, the hydrogen atom was ionized, i.e., split into a nucleon and electron. When the temperature dropped about 400,000 years after the Universe's birth, the nucleon and electron combined to make neutral hydrogen atoms. At that time, the Universe became transparent to radiation, and light-emitting objects became visible. In the present Universe, most hydrogen exists as diffuse, ionized intergalactic gas. To account for this difference between the past and present, there must have been a transition era of re-ionization after the dark age of the Universe, which was filled with neutral gas. However, scientists do not yet know when this transition occurred or understand how the process took place. One strong possibility is that the ultraviolet (UV) radiation from the first-generation galaxies in the one-billion-year-old Universe ionized the hydrogen gas. Therefore, it is very important to identify when the re-ionization occurred relative to the formation of the first generation of light-emitting objects.

When astronomical objects are far-distant, it means that they are also from the far-distant past. Recent observations from large, ground-based telescopes have enabled astronomers to study galaxies, quasars (Note 4), and GRBs during an era one billion years after the birth of the Universe. If an object already existed at a time re-ionization was occurring, it must have had some neutral hydrogen gas surrounding it. Although there have been many attempts to detect such neutral gas, no clear proof of it has emerged.

Previous observational approaches to discovering it have focused on galaxies or quasars. Observations of the galaxies are indirect in that a number-count decreases when neutral hydrogen gas obscures their light. Observations of quasars directly measure the absorption features in their spectra caused by neutral hydrogen. However quasars occur in the most developed regions of galaxy evolution, and their own radiation ionizes the surrounding material. These factors make it difficult to estimate the neutral gas in this environment. In contrast, GRBs allow the direct measurement of neutral hydrogen and also overcome the drawbacks of the quasar approach.

Although studies based on GRBs are highly desirable, the rare occurrence of GRBs bright enough to enable detailed analysis has been a challenge. The only previously available data about re-ionization was the current team's 2006 report of data from observations of GRB 050904 from the Subaru Telescope ("Universe Re-Ionized 900 Million Years After Birth"). This past data proved that the ionization rate was already high in that era, without any sign of intergalactic neutral hydrogen.

The current research team used Subaru Telescope's FOCAS to detect the afterglow of GRB 130606A on June 6, 2013 and studied its spectrum in great detail. Its afterglow was bright enough in the visible wavelength to allow for analysis, despite its great distance at a redshift of 5.913. Its distance situates the object at a time close to the presumed re-ionization era. The data from Subaru Telescope clearly show that intergalactic, neutral hydrogen gas accounts for the observed absorption feature. Further analysis led the team to conclude that more than 10% of the hydrogen gas was neutral relative to the total amount of hydrogen. This means that the Universe still had a high proportion of neutral hydrogen gas when it was one billion years old. This is the first time that a research team has made a quantitative measurement of such a high proportion of neutral gas during this era.

This finding marks a significant beginning for scientists to understand the era that preceded re-ionization. Next generation telescopes, whether in space or ground-based, such as the future Thirty-Meter-Telescope (TMT), will definitely show how the first generation galaxies formed in the primordial Universe and more clearly define the process of transition from an opaque, neutral-hydrogen-filled Universe to a transparent, re-ionized one.

Notes:

1. Spectrum and Spectroscopy Spectroscopic observation measures light split into a range of different wavelengths, i.e., its spectrum, which is like studying a rainbow of light dispersed by a prism. Since the light is split into small fractions in a narrow range of wavelengths, this kind of observation requires a very bright light source, long integration time, and a very sensitive instrument.
2. Gamma-Ray Burst A far-distant object brightens suddenly and is detected first in the gamma-ray wavelength. The burst lasts a few to several tens of seconds. Since it occurs in the very distant Universe, over ten billion light years away, the energy it expends is enormous. There are two classes of GRBs based on the duration of their explosions: a) shorter than 2 seconds, and b) longer than 2 seconds. GRB 130606A belongs to the longer duration class. It probably came from the supernova explosion of a supermassive star, and the observers see it face on, when the jet approaches the observer and is not tilted. This is no ordinary supernova; its energy is extremely high, but it occurs much less frequently.
3. Redshift The expansion of the Universe affects the wavelength of light from far-distant objects by causing their light to shift to a longer wavelength when observed, and it becomes redder; thus, the term "redshift." The amount of redshift (z) is determined by the apparent wavelength of the spectral lines from this object by (1+z) longer than that of the rest frame wavelength (i.e., when it is static object with respect to the observer). The object studied here has a redshift of z=5.913, i.e., the Lyman-alpha spectral line of hydrogen at 1216 angstrom (Å) at the rest frame is detected at 8406 Å, when the Universe was about one billion years old. This distant object shows a larger redshift, placing it at a time in the earlier Universe.
4. Quasar An active galactic nucleus derives its high energy from gas accreting on the center of a supermassive black hole. Its high luminosity allows researchers to detect it, even at a large distance, and it becomes a convenient tool for studying the distant Universe.