Rikako Ishimoto (Doctoral Student, Department of Astronomy)

Nobunari Kashiwagawa, Professor, Department of Astronomy

Daichi Kashiwano, Project Assistant Professor, Nagoya University

Toru Misawa, Professor, Shinshu University

Katsuya Okoshi, Professor, Tokyo University of Science

Key points of the presentation

• We have clarified that fluctuations in the ultraviolet radiation field are the cause of the different progression of ionization in the early universe, which is called cosmic reionization, depending on the location.
• The study systematically investigated for the first time the regions of fast and slow reionization, and found for the first time a consistent relationship between the transparency of the universe and the density of galaxies.
• This research has brought us one step closer to elucidating cosmic reionization in the early universe. It is expected that further observations with next-generation telescopes and instruments, as well as the expansion of observation areas, will lead to a better understanding of the history of the universe.

Summary of the Presentation

Contents of Presentation

Approximately 13.8 billion years ago, the newly born universe was hot and all matter was ionized. Later, as the universe expanded, the temperature decreased and matter became neutral, but it is thought that the light emitted by the first astronomical objects that were born eventually ionized the matter again and the universe became the universe as we know it today. This process is called cosmic reionization, and is a major event in the early universe that depends on the properties and spatial distribution of intergalactic gas and ionizing sources. One way to investigate the extent of cosmic reionization is to use the spectra of bright objects called quasars ^{(Note 2)}and(Note 3) in the distant universe. Light departing from a quasar is scattered when it passes through neutral gas before reaching the Earth, so a portion of the quasar's spectrum is observed as darkened (Figure 1).

By measuring how much light is absorbed by a quasar, i.e., the transparency of the universe, we can determine the extent of the reionization progression between the earth and that quasar. In other words, the relationship is: darker (brighter) part of the quasar's spectrum = more (less) neutral gas that has passed through = slower (faster) reionization. By this method, the transparency of the universe at numerous locations has been measured. Surprisingly, the transparency varies greatly from place to place even at the same epoch, indicating that reionization did not progress uniformly throughout the universe. Furthermore, it has been pointed out that fluctuations in the density of gas in the universe alone cannot explain these differences in transparency from place to place, and the cause of the spatial inhomogeneity of reionization has remained a mystery.

Among the various possible causes of the inhomogeneity, fluctuations in the ultraviolet radiation field (Note 4) and fluctuations in the temperature of the intergalactic gas have been considered as two of the most promising ones. In the case of the UV radiation field fluctuation, it is predicted that the regions with more reionization progression have more galaxies, whereas in the case of the temperature fluctuation, on the contrary, it is predicted that the regions with more reionization progression have fewer galaxies. ^{(Note 5} ) Therefore, if we conduct a galaxy survey in a region of extreme reionization progression and determine whether there are more or fewer galaxies in that region, and which prediction is more consistent, we will be able to determine the cause of the reionization inhomogeneity. Prior to this study, galaxy distributions had been studied in some regions, but only two regions were covered, and the study was limited to regions with slow reionization progression, which could be interpreted in various ways and no firm conclusions could be drawn.

In this study, we investigated the transparency of the universe at a redshift ^{(Note 6) of} 5.7, about 12.8 billion years ago, using quasar spectra that have already been observed. We targeted three regions of the universe where the progression of reionization was extremely slow and fast, and made imaging observations ^{using the} Subaru Telescope Hyper Suprime-Cam ^{(Note 7)}. From the images obtained, they detected a race of galaxies called Lyman-alpha emission-line galaxies (Note 8), which exist in the same epoch as that in which the transparency was measured, and investigated their distribution. The density distribution clearly shows that there are more Lyman-alpha emitting line galaxies in the regions where the reionization progresses quickly and fewer galaxies in the regions where the reionization progresses slowly (Figure 2). This result is consistent with the model prediction that the density of galaxies is larger in regions with faster reionization progression, which is caused by fluctuations in the ultraviolet radiation field. For a more quantitative study, the relationship between cosmological transparency and galaxy density is examined in conjunction with the region studied in the previous study (Figure 3), and it is found that the relationship between galaxy density and cosmological transparency obtained in this study and the previous study is closer to the prediction of the model that attributes the non-uniformity to fluctuations in the UV radiation field than to the model that attributes it to gas temperature fluctuations. predictions. Therefore, it is plausible that the cause of the spatially inhomogeneous reionization progression is the fluctuation of the UV radiation field. This is the first study of galaxy density in a region of fast reionization progression, and this study is the first to investigate the relationship between reionization progression and galaxy density with reliable statistics.

Figure 2: Example of galaxy density distribution in the region observed by the Subaru Telescope Hyper Suprime-Cam. The brighter the color, the greater the density. The yellow rectangle in the center of the field of view shows the quasar line of sight passing through the region of fast reionization progression, and the white area shows areas where no data is available, such as around bright stars.

Figure 3: Relative density of galaxies near the quasar line of sight (average value of 1) versus the optical thickness of the intergalactic gas, a measure of the opacity of the universe. The red and blue lines are model predictions for the case caused by fluctuations in the UV radiation field and gas temperature, respectively, and the black and white dots are results obtained by this and previous studies, respectively.

The black and white dots are the results obtained by the present and previous studies, respectively. The present study clearly shows that the non-uniformity of cosmic reionization is caused by fluctuations in the UV radiation field. However, the results obtained so far are only for the redshift 5.7 epoch, and observations of older epochs are still needed. The possibility that the galaxies used in this study may be affected by the properties of Lyman-alpha emission line galaxies cannot be dismissed. In the future, we expect to further improve our understanding of the cosmic reionization by investigating the detailed spatial distribution of the galaxies found in this study, including their depth, using the Ultra-Wide Field Multi-Object Spectrograph, which is being constructed on the Subaru Telescope, and by observing fainter galaxies with the James Webb Space Telescope, which was newly launched last year. We can expect to make further progress in understanding the reionization of the Universe. Understanding how the early universe evolved will help to answer the question of how the galaxies we live in, and the universe itself, were formed.

This research was supported by Grants-in-Aid for Scientific Research (Grant-in-Aid for Scientific Research (KAKENHI) project numbers 21H0449, 21H01126, 21K13956) and by the Support for Pioneering Research Initiated by the Next Generation Program JPMJSP2108.

Journal

Journal name	Monthly Notices of the Royal Astronomical Society
Title of paper	The physical origin for spatially large scatter of IGM opacity at the end of reionization: the IGM Lyα opacity-galaxy density relation
Author(s)	Rikako Ishimoto*, Nobunari Kashikawa Daichi Kashino, Kei Ito, Yongming Liang, Zheng Cai, Takehiro Yoshioka, Katsuya Okoshi, Toru Misawa, Masafusa Onoue, Yoshihiro Takeda, and Hisakazu Uchiyama
DOI Number	10.1093/mnras/stac1972

Terminology

1 Ionization

The separation of electrons from the nucleus to separate states. ↑up

Note 2 Quasar

A celestial body that shines very brightly even at a distance due to the high temperature of matter falling into a black hole at its center. ↑up

Note 3 Spectrum

The intensity of light from a celestial object at each wavelength. ↑up

Note 4 Ultraviolet radiation field

Intensity of ultraviolet radiation in each region. It is determined by the sum of radiation from celestial objects in the vicinity of the region, and here we focus on the intensity of the ultraviolet radiation that ionizes hydrogen. ↑up

Note 5: Causes of Reionization Non-Uniformity

A model that attributes the cause to fluctuations in the ultraviolet radiation field predicts that in galaxy-rich regions, the transparency of the surrounding region increases due to the large number of ionizing sources, and thus the progression of reionization is enhanced. On the other hand, the model due to gas temperature fluctuations predicts that regions with more galaxies are ionized earlier than others, but cool down faster, and the lower temperature promotes the neutralization effect of ionized hydrogen, resulting in slower reionization progression. ↑up

Note 6 Redshift

Due to the expansion of the universe, light emitted from distant celestial objects is stretched before it reaches the earth and is observed with a wavelength longer than that of the original light. This is called redshift. The farther away the object is, the longer the wavelength becomes, so the degree of wavelength stretching is used as an indicator of distance. ↑up

Note 7: Subaru Telescope Hyper Suprime-Cam

Subaru Telescope is one of the largest Japanese telescopes on earth, built atop Mauna Kea on the Big Island of Hawaii. For this study, we used the Hyper Suprime-Cam camera installed on this telescope, which has the world's largest field of view. ↑up

Note 8 Lyman Alpha Emission Line Galaxies

The Lyman-alpha emission line is a light with a wavelength of 121.6 nm that is produced when an electron falls from the quantum number n=2 to the n=1 level in a hydrogen atom. Galaxies characterized by strong emission of this Lyman-alpha emission line are called Lyman-alpha emitting galaxies. Since the detection is made using a narrow-band filter with a narrow wavelength range, it is easy to select galaxies at a fixed distance from the Earth, i.e., of the same age, and thus was used in this study. ↑up