Scientific Enigmas for XRISM to Investigate
A galaxy is a huge collection of stars, such as the Sun. We sometime call a galaxy an "island universe". If a galaxy is an island, a star is a "sand grain". Hundreds of billions of stars constitute a galaxy, while hundreds or thousands of galaxies forms a cluster of galaxies, and billions of clusters of galaxies are distributed throughout the universe. A cluster of galaxies is the largest observable structure of matter in the universe.
Assuming a galaxy as an island, the "sea water" surrounding islands is hot plasma. Plasma is hot and ionized (atoms and electrons) gas. We can see the hot plasma of clusters of galaxies by taking X-ray pictures. This hot plasma accounts for 80% of electromagnetically visible mass in clusters of galaxies; and is connected to the plasmas contained within the galaxies in a cluster. For these reasons, observing the "sea" of hot plasmas with X-rays is very important, so that we can know how galaxies and clusters of galaxies were formed throughout the history of the universe. Plasma flows and circulates among stars, like rivers, seas, clouds, and rain do on Earth. Observing hot plasmas is our way of seeing the flow of matter in the universe. XRISM is equipped with a high resolution X-ray spectrometer, an X-ray micro-calorimeter, which observes X-rays from hot plasmas to reveal their elements, temperature, density and velocity with unprecedented accuracy. XRISM will investigate various scientific enigmas regarding the formation of celestial objects in the universe.
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The Assembly of Clusters of Galaxies
A cluster of galaxies, the largest structure in the universe, is formed with a balance between the immense gravitational pull and the outward pressure of the hot plasma. X-ray observations before XRISM could only measure the temperature, and thus the thermal pressure, of the plasma, so we couldn't fully understand how the total gas pressure balances with the gravitational pull in our observational results. The new X-ray imaging spectrometer onboard XRISM is able to measure the plasma wind velocity to reveal its dynamical pressure. By measuring both the thermal and dynamical pressure of plasmas, XRISM will measure all aspects of this balancing act, revealing the "mechanical design drawing" of the formation of clusters of galaxies.
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The Chemical Makeup of the Universe
Until 300,000 years after the birth of the universe, only four elements from the periodic table existed. The universe was flavorless, as many of the indispensible elements for our life, like carbon, oxygen, or other metals were produced in the life of stars born after. These elements produced in stars were distributed amongst the galaxies, and flavored the stars, planets, and life among them.The history of enrichment is called the "chemical evolution" of the universe. The X-ray imaging spectrometer onboard XRISM detects these elements with unprecedented sensitivity and measures their velocity in space. Thus, XRISM reveals a kind of recipe for the flavor of the universe.
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The Extremes of Spacetime
There are places that no spaceship can ever take us. One is the distant universe running away from us due to the expansion of the universe, so fast we can never catch up. Another is the inside of a black hole. Black holes distort the surrounding spacetime and expands the wavelengths of X-ray photons from the nearby plasma. The X-ray imaging spectrometer onboard XRISM measures wavelength precisely, and will reveal these distortions of spacetime and the motion of material surrounding a black hole. This information enables us to investigate the spacetime around black holes, as well as the inflow and outflow of matter and energy from the black hole's surroundings.
