草榴社区

Kokoro Hosogi, a physics student at 草榴社区 (UAH), has achieved a rare honor for an undergraduate: her contributions are being recognized in . The researcher recently supported the X-Ray Imaging and Spectroscopy Mission (XRISM) studying celestial X-ray objects to help illuminate why gas at the core of the Centaurus galaxy cluster approximately 170 million light years away is not generating young new stars as rapidly as predicted, a discovery with important implications on the evolution of galaxy clusters.

XRISM is a cutting-edge X-ray telescope, a joint venture between NASA, the Japan Aerospace Exploration Agency (JAXA) and the European Space Agency (ESA), launched in Sept. 2023. The initiative focused on a region of space known as the Intracluster Medium, or ICM. This is an area of hot, diffuse plasma that fills the space between galaxies within a galaxy cluster, composed of ionized hydrogen, helium, heavy elements and electrons, detectable through its strong X-ray emission due to its high temperature.

Kokoro Hosogi, a UAH physics major and a student specialist with Dr. Ming Sun鈥檚 group.

Michael Mercier / UAH

鈥淭here is an old problem in galaxy clusters,鈥� explains Dr. Ming Sun, a professor of physics and astronomy at UAH, who is Hosogi鈥檚 mentor, as well as the only scientist from the state of Alabama to take part in the XRISM collaboration. Both Hosogi and Dr. Sun are co-authors of the Nature paper. 鈥淭he core of many clusters is very bright in X-rays, so you expect over time there should be a lot of gas cooled to form stars, but you see few young stars there.鈥欌��

This phenomenon is known as the 鈥渃ooling flow problem鈥� in astronomy, referring to the discrepancy between the predicted rate at which hot gas in the center of galaxy clusters should cool and the much lower observed rate of star formation in those regions, suggesting that most of the cooling gas is not actually forming stars. The discrepancy has been thought to be due to feedback mechanisms from active galactic nuclei (AGN) 鈥� a very bright, compact region at the center of a galaxy, powered by a supermassive black hole that is actively accreting matter, which in turn reheats the gas, preventing it from cooling too rapidly.

However, in the case of the Centaurus cluster, 鈥渢he new study shows the central dense X-ray core is not sitting still,鈥� Sun notes. 鈥淚nstead it can move, or 鈥榮losh,鈥� around the bottom of the gravitational potential well. This sloshing motion prevents excessive accumulation of cooled gas at the center. It may also redistribute the energy injected by the central AGN and bring in thermal energy from the surrounding ICM. The key breakthrough came through a new instrument on XRISM called Resolve that provides high-resolution X-ray spectroscopy to reveal the bulk motion of the hot gas, which was completely unknown before, as well as the turbulent motion of the hot gas.鈥�

Adding another shoulder to the wheel

鈥淏ulk motion鈥� refers to large-scale, organized flows of gas within a cluster, primarily caused by gravitational forces or large-scale processes like mergers. In contrast, turbulent motion is characterized by chaotic, irregular and smaller-scale movements, such as eddies within the gas, often driven by instabilities and energy dissipation.

The cluster study involved collecting observational data gathered by the Multi Unit Spectroscopic Explorer (MUSE) instrument, which is mounted on the Very Large Telescope (VLT) operated by the European Southern Observatory. The data provides a detailed spectroscopic image capturing both light intensity and wavelength information across a wide field of view, allowing astronomers to study the chemical composition and dynamics of distant astronomical objects with high spatial resolution in optical.

鈥淎s a guest scientist in the XRISM collaboration, I can bring in a student or a postdoc,鈥� Sun says. 鈥淜okoro has a near perfect GPA, and she has also developed the ability of problem solving and debugging without seeking help from more senior members. It is important for students to develop that trait to grow in confidence.

Dr. Ming Sun, a professor of physics and astronomy at UAH.

Michael Mercier / UAH

鈥淔or the Centaurus project, Kokoro reduced the VLT/MUSE data, and I analyzed the MUSE data further to provide important information, especially on the velocity of the central galaxy that is crucial to constraining the bulk motion of the hot gas detected by XRISM, as well as the velocity information of warm, ionized gas,鈥� Sun adds. 鈥淔or her contribution, I asked to include Kokoro on the paper. It had to go through the XRISM leadership team, but it was approved. Yes, it is a rare opportunity, but she made important contributions to the project.鈥�

鈥淚 am pursuing a bachelor鈥檚 degree in physics, with a concentration in astronomy and astrophysics, with the Data Science certificate program at UAH,鈥� Hosogi says, who is now working as a Research Assistant under Dr. Sun. 鈥淢y interest is in gravitational waves, pulsar timing arrays, cosmology, black holes and galaxies.鈥�

Originally from Japan, Hosogi chose to study at UAH at the recommendation of UAH alumnus Sakurako Kuba, who is also from Japan. She started to work with Sun in 2023 as a Research and Creative Experience for Undergraduates (RCEU) program student and has continued her work as a paid student specialist in Sun鈥檚 group since then. She graduated in December 2024 is currently applying for graduate schools in astronomy.

鈥淭hese large projects can have many pieces of detailed works, so people with different backgrounds and at different career levels can all make a contribution,鈥� Sun says. 鈥淚t can be rewarding to involve undergraduate students in cutting-edge research.鈥�