Koki Kakiichi

My research is centred on trying to understand the nature of the Epoch of Reionization and Cosmic Dawn, which is thought to happen during the first billon years in the cosmic history. During this era, the very first stars and galaxies are born, marking the end of the Dark Ages when the universe had no luminous object before. These sources or their descendants (we don't know yet!) drove the cosmic phase transition, the process called 'Reionization', as the intergalactic medium has transformed from neutral to ionized gas. This drastic event transformed the infant universe, adding colours in our world picture with galaxies and black holes, which then changed their environments to the present-day state of the universe around us.

I think the essence of this exciting field of the ultra-deep universe exploration is captured by this single image -- Hubble eXtreme Deep Field -- taken by the Hubble Space Telescope. This image contains thousands of galaxies ranging from high-redshift (z>6, presumably in the heart of reionization era) to low-redshift (present-day) objects. Our aim is to construct a complete story of cosmic history from these kind of deep imaging and spesctroscopic observations, with aid of supercomputer simulations based on the physical principles. Along this line, my research is broadly divided into three catelogries: Lyman alpha emitting galaxies, intergalactic medium, 21 cm cosmology.

Lyman alpha emitting galaxies & QSO absorption spectroscopy

I am very enthusiastic about using and understanding the nature of Lyman-alpha emitting galaxies, which provide an excellent platform to explore the nature of reionization, galaxy formation, and cosmology. Lyman alpha line, 2P → 1S transition in atomic hydrogen, is my primary tool to understand the states of high-redshift galaxies and the intergalactic medium. Together with the absorption spectroscopy of quasars and gamma-ray bursts, surveys of Lyman-alpha emitting galaxies offer a great observational window to probe the cosmic interplay between galaxies and the intergalactic medium from the Epoch of Reionization to the present day. Recently, we have launched a specotroscopic galaxy survey in the foregrounds of high-redshift (z≥6) quasars to probe the state of galaxies and the intergalactic medium at the reionization epoch using the two of my favourite tools together: 'Lyman-alpha emission and absorption'!

intergalactic medium

The diffuse matter between galaxies, called the intergalactic medium (IGM), contains the majority of baryonic matter in the universe. The ionizing background, thermal states, and chemical enrichment of the IGM retain valuable information about the role of early galaxies and black holes on the reionization process of hydrogen and helium. To make a progress in this avenue, radiative transfer/radiation hydrodynamical simulations are the key tool to interpret many observational data, especially Lyman-alpha forest probed by the deep spectroscopy of high-redshift quasars.

21 cm Cosmology

We can use the redshifted 21-cm line of atomic hydrogen, the spin-flip transition between the two hyperfine levels, to probe directly the state of the early universe. This 21 cm signal from the neutral intergalactic hydrogen is arguably the most direct probe of reionization; however, the interpretation of data, both power spectra and tomography, requires a new generation of data analysis techniques. Successful 21 cm experiments with the first and future generation of low frequency radio interferometers such as LOFAR and SKA, together with deep galaxy surveys, should give us a complete picture of cosmic reionization in the future.