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| Photo: Dr. Masato Shirasaki (left) and Dr. Masaomi Tanaka (right). (Credit: NAOJ) |
Two researchers in the Division of Theoretical Astronomy, National Astronomical Observatory of Japan, received awards from the Inoue Foundation for Science. The objectives of these awards are to encourage science researchers and to support research grants. The awards ceremony was held on February 3, 2017.
(This article was posted on February 17, 2017.)
* Assistant Professor at Graduate University for Advanced Studies (SOKENDAI), Department of Astronomical Science
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| Photo: Dr. Masato Shirasaki (left) and Dr. Masaomi Tanaka (right). (Credit: NAOJ) |
Observational studies suggest that 96% of the total energy in the present Universe consists of the unidentified dark matter and dark energy. One effective method to determine their features is "gravitational lensing analysis" which statistically analyzes slight distortion of a celestial body’s image caused by gravitational lensing. Through this method, it is possible to reconstruct the matter distribution in the line of sight direction. The reconstructed matter distribution contains rich information about the dark matter and dark energy.
In Dr. Shirasaki's doctor thesis, which won the Inoue Research Award for Young Scientists, he investigated the usefulness and applicability of two different gravitational lens analysis methods with the aim of deepening our understanding of the dark matter and dark energy.
One method uses geometric information for the matter distribution of the Universe. Dr. Shirasaki developed a gravitational lens simulation that accurately includes the complex evolutionary processes of matter distribution. He developed a method to simultaneously include the three-dimensional position information of actually observed galaxies and the image distortion information. From this simulation result, he succeeded in modeling the accurate morphological information about the matter distribution and showed that the cosmological model can be constrained more than previous models (Figure 1).
The other method uses the relation between the matter distribution data obtained from gravitational lens observations and the data from gamma rays. It is thought that the gamma rays are emitted by "annihilation" which is a feature of elementary dark matter particles. So far, this was just a theoretically proposed method. However, Dr. Shirasaki applied actual observation data for the first time in the world. From this research project, he was able to establish new independent constraints on the possibility of dark matter annihilation (Figure 2).
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Figure 1 (left):
This figure shows the constraints (95% confidence) on the cosmological model obtained by applying this gravitational lensing analysis to the gravitational lens survey data using the Canada-France-Hawaii Telescope. Each location in the figure represents a different distribution of matter in the Universe. Blue shows the case based on the basic statistics used in the past. The red area shows the case taking into account the morphology of the matter distribution that Dr. Shirasaki focused on in his research. The green area shows the combined constraints on the cosmological model obtained from combining the previous model and Dr. Shirasaki’s new model. (©Masato Shirasaki) Figure 2 (right): This figure plots the upper limit (68% confidence) to the dark matter annihilation obtained by observation data from the gravitational lens survey with the Canada-France-Hawaii Telescope and analysis of the gamma ray observations by the Fermi Gamma-ray Space Telescope. The larger the dark matter annihilation cross section is, the more easily they annihilate. The red curve represents the upper limit of the reaction cross section when the dark matter is assumed to be tau / anti-tau particles, and the green curve is the reaction cross section when the dark matter is assumed to be bottom / anti-bottom quarks. The black dotted line is the dark matter annihilation cross section estimated from the current amount of dark matter obtained from the observations. (©Masato Shirasaki) |
Dr. Shirasaki’s research project is a new method to identify dark matter and dark energy, which are still not fully understood. By applying this method to data from large-scale observations currently underway, he expects to be able to elucidate the nature of our Universe.
Dr. Shirasaki said about the award, “I feel very honored to receive the Inoue Research Award for Young Scientists. I am deeply grateful for the teachers in the University of Tokyo, who supervised me when I was a doctoral student, and also for all the research collaborators. In particular, Professor Naoki Yoshida, my advisor when I was a doctoral student, introduced this attractive research theme to me. Furthermore, he taught me many things, such as physical studies of research subjects, analysis methods, and research directions. I would like to take this opportunity to express my gratitude to him.
This research started around five years ago looking forward to galaxy observations using Hyper Suprime-Cam (HSC) on the Subaru Telescope. Currently, I am applying the analysis method, which I have further improved since my doctoral thesis, and adapting it to the HSC data. This award encourages me to continue working hard on research projects and to elucidate the mysteries of the dark components that comprise most of our Universe.”
The Inoue Research Award for Young Scientists is awarded to researchers who submitted an excellent doctoral thesis within the past three years. Moreover, they have to be under the age of 37 and hold a doctor's degree in a field such as science, medicine, pharmacology, engineering, or agriculture.
In September 2015, the Advanced LIGO gravitational wave detectors in the United States of America captured the gravitational waves emitted by the merger of black holes for the first time in the world. This detection opened up the new era of gravitational wave astronomy. Detections of gravitational waves from other astronomical phenomena are also expected. Among them, the most anticipated is the merger of neutron stars. This phenomenon is thought to be a major source of heavy elements such as gold and platinum. Also, it is expected that "multi-messenger astronomy," which is a combination of gravitational wave observations and electromagnetic observations, will capture the moment when heavy elements are born.
Dr. Tanaka's research project focuses on the explosions in the Universe, such as the merger of neutron stars and the supernovae explosions that are caused by massive stars at the end of their lives. So far, Dr. Tanaka has clarified the optical characteristics of the supernovae explosions through simulations. Based on the results, he conducted a wide area survey to capture the moment of the explosions. Furthermore, Dr. Tanaka paid attention to the similarities between supernova explosions and neutron star coalescence events. By using numerical simulations, he derived the characteristics of the electromagnetic waves emitted by neutron star mergers. This result becomes an important reference for electromagnetic observation of neutron star coalescence events, which are expected to be detected in the future.
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Figure 3 (left):
Numerical simulation of electromagnetic emissions from neutron star mergers. It is thought that heavy elements are synthesized in the material ejected from neutron star mergers, and electromagnetic wave emission occurs due to the decay energy of radioactive elements. From this calculation, it was shown that the electromagnetic radiation from a neutron star merger shines mainly between red, in visible light, and the near infrared. (This simulation was performed using the CfCA/NAOJ supercomputer “ATERUI.” ©Masaomi Tanaka) Figure 4 (right): The locations of the two gravitational wave sources detected in 2015. Since it is not possible to determine an accurate position with only gravitational wave detectors, it is hoped that electromagnetic counterparts can be detected. Vigorous follow-up observations of both gravitational wave sources were conducted by the J-GEM group. (©LIGO/Axel Mellinger) |
Dr. Tanaka was awarded the Inoue Science Research Award because of the importance of his research results as the foundation for gravitational wave source surveys. Furthermore, his research is expected to make a significant leap in human knowledge.
Regarding the award, Dr. Tanaka said, "I am honored to receive the Inoue Science Research Award. I would like to thank all the collaborating researchers who worked together on the electromagnetic radiation of gravitational wave sources. Also, I would like to thank the J-GEM** group for advancing this very challenging observation theme to capture electromagnetic waves from gravitational wave sources. If we can identify the electromagnetic wave counterparts of gravitational wave sources, we may be able to get closer to solving the origin of heavy elements in the Universe. This will be very exciting. I want to understand more about the electromagnetic radiation from the gravitational wave sources through numerical simulations. Also, I want to promote the wide field survey of gravitational wave sources using the Subaru Telescope and other telescopes."
** Japanese collaboration for Gravitational-wave Electro-Magnetic follow-up
The Inoue Science Research Award aims to support the creativity and independence of young researchers who have displayed outstanding achievements in fundamental research in natural science, and aim to pioneer developments. It is awarded to researchers who received their doctor’s degrees within the last nine years.
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