Probing Galaxy Formation and Evolution with the Nancy Grace Roman Space Telescope
Feb. 26, 2026 | Aerospace Project Research Associate, ISAS people
DAIKUHARA Kazuki / Dept. of Space Astronomy and Astrophysics
Research Summary
Understanding how galaxies formed and evolved is one of the major unresolved problems in modern astrophysics. Galaxies have been influenced by their surrounding environments within the large-scale structure of the Universe, and their properties have changed over time. I aim to clarify when, where, and how the environment affects galaxy formation and evolution. For this purpose, I investigate protoclusters, which represent the early evolutionary phase of present-day galaxy clusters (Figure 1). In the present Universe, it is well known that star formation is suppressed in high-density environments. However, in the distant Universe about 11 billion years ago, the situation is different. Recent observations have shown that galaxies in protoclusters tend to have higher star formation rates than field galaxies at the same epoch. The reason for this trend is still not clear, and several possibilities — such as efficient gas accretion, galaxy–galaxy interactions, and mergers — are currently under active discussion.
To understand galaxy growth from the perspective of environment and to uncover the underlying physical processes, it is essential to combine wide-field coverage, high sensitivity, and high spatial resolution observation. Many galaxies in the protocluster outskirts are expected to accrete onto the forming cluster through infall and eventually become bound members. A key question is whether environmental effects on galaxies arise primarily after (proto)cluster infall, or whether they already begin via pre-processing in infalling groups and filaments before accretion. Past observations have been limited in the field of view of the instrument and have not fully captured the surrounding large-scale structures or the populations of galaxies that will eventually accrete onto forming clusters. A more comprehensive and systematic survey is therefore required to reveal how galaxies grow within the large-scale structure. Furthermore, to elucidate the physical processes driven by environment, detailed spatially resolved observations of individual galaxies are also required.
The key to this effort will be the Nancy Grace Roman Space Telescope (Roman), scheduled for launch in 2026 (Figure 2). Roman is NASA’s next flagship mission, with ISAS/JAXA participating as an international partner. With its Wide Field Instrument (WFI), Roman will deliver Hubble Space Telescope*1-class angular resolution over a >200× larger field of view with high sensitivity. Roman will therefore allow us not only to investigate protocluster cores but also their surrounding regions, and to capture the internal structures of individual galaxies efficiently. This will provide an unprecedented data set for statistically testing the impact of (proto)cluster environments on galaxy evolution.
The primary scientific objective of the Roman project is to precisely measure the accelerated expansion*2 of the Universe and the growth of cosmic structure to probe the nature of dark energy*3. As a by-product of the wide-field imaging and spectroscopic surveys designed for these dark-energy studies, Roman will also provide an unprecedentedly large dataset of galaxy distributions, offering new insights into the formation and evolution of cosmic structures over a wide range of cosmic history and into how the environment affects galaxy growth. The mission will also conduct a statistical survey of exoplanets and demonstrate advanced high-contrast imaging technologies with its Coronagraph Instrument (CGI) *4. CGI will be the first coronagraph on a space telescope to employ active wavefront control*5, and is expected to serve as a key technology pathfinder toward the direct imaging of Earth-like exoplanets with future large space telescopes.
Roman’s unprecedented wide-field near-infrared imaging, combined with high sensitivity and high spatial resolution, will enable the detection of galaxies across a wide mass range and the discovery of young protocluster candidates that have previously been missed. In addition to mapping large-scale structure in the distant Universe, Roman will also carry out dark energy studies through baryon acoustic oscillations, weak gravitational lensing, and Type Ia supernovae, conduct exoplanet searches via gravitational microlensing, and perform technological demonstrations with its coronagraph instrument.
One of Roman’s most remarkable features is its ability to obtain Hubble Space Telescope–class spatial resolution over a field of view more than 200 times larger. This unprecedented capability will generate “high-resolution panoramic images of the Universe,” producing as much as 20 PB of data over its five-year mission. To ensure reliable reception of this enormous data volume, JAXA is installing a new Ka-band (26 GHz) high-rate receiving system on the 54-m antenna at the Misasa Deep Space Station in Nagano Prefecture as part of the science data downlink for Roman.
The Misasa Deep Space Station happens to be located in my hometown. Being able to contribute to Roman’s science through a world-leading piece of infrastructure situated in the place where I grew up is a source of great pride and motivation for me. As a project researcher for the Roman mission, I will contribute to both the advancement of its scientific and operational goals and the synergistic development of my own research on galaxy formation and evolution using the unprecedented data from Roman.
Terminologies
- *1 Hubble Space Telescope (HST): A space-based observatory with a 2.4-meter reflecting telescope covering ultraviolet to near-infrared wavelengths. It is a benchmark facility for high angular-resolution studies.
- *2 Accelerated expansion: The phenomenon that the cosmic scale factor is increasing at an accelerating rate at late times. It was first inferred from the Hubble diagram of distant Type Ia supernovae and is now supported by multiple independent probes, including baryon acoustic oscillations and weak gravitational lensing.
- *3 Dark energy: A physically unknown component that dominates the present cosmic energy budget (≈70% in the standard cosmological model) and is invoked to explain the observed late-time acceleration of the cosmic expansion. Its properties are constrained indirectly via cosmological observables such as Type Ia supernovae and the large-scale distribution of galaxies.
- *4 Coronagraph Instrument (CGI; Coronagraph Instrument): A high-contrast imaging instrument that suppresses starlight to enable direct detection and characterization of faint companions (e.g., exoplanets) at small angular separations from bright host stars.
- *5 Active wavefront control is a technique that corrects distortions in the light wavefront caused by small aberrations or temporal variations in the optical system, thereby sharpening images and improving high-contrast performance. In coronagraphic observations, it is essential for reducing the impact of extremely bright starlight and for distinguishing very faint objects such as exoplanets. With Roman/CGI, this wavefront control will be demonstrated in space for the first time, serving as a key technology toward future direct-imaging missions of Earth-size planets.
- *6 James Webb Space Telescope (JWST): A 6.5-m infrared space telescope jointly operated by NASA, ESA, and CSA. JWST is the most complex and powerful space telescope built to date. It is equipped with instruments covering the near- to mid-infrared wavelength range, enabling detailed studies of the physical properties of a wide variety of astronomical objects, including galaxies, stars, and planetary systems.