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The Forefront of Space Science

The Objective of Next-Generation Infrared Astronomical Satellite SPICA
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The drama of the birth of the galaxy

SPICA will target the greatest mystery. It will try to elucidate how galaxies were born and evolved in our universe.

The AKARI research provided us with the history of star formation in the universe tracing back to about nine billion years ago (Fig. 3). Surprisingly, the results showed that star formation activity then was 20 times more active than now. This indicates that the formation of galaxies and stars has been in the “decline phaseEfrom about nine billion years ago until now.

Figure 3
Figure 3. Star formation history in the universe
AKARI revealed that the present universe is in the decline phase of galaxy and star formation. SPICA will ascertain if the growth phase occurred during the period that AKARI was unable to trace. (Added author’s supposition to the results by Goto et al.)

AKARI also revealed that the types of galaxies have changed over time and that very bright infrared galaxies, or “ultra luminous infrared galaxies,Eplayed an important role nine billion years ago. Since they shine mainly in infrared, infrared observation is vital to know the activity level of star formation. In other words, without infrared observation, it would be impossible to learn the true history of star formation in the universe.

AKARI found that formation activity of galaxies and stars has been in decline for about nine billion years. This trend cannot go back to the onset of the universe, however, because no stars and galaxies existed then. Therefore, there must have been a “growth phaseEsome time between 13.7 billion years ago at the birth of universe and nine billion years ago traced back by AKARI, during which galaxies and stars must have started to form and become active. The growth phase is important because the current universe was shaped in that period.

The primary goal of SPICA is to examine whether the growth phase actually existed. By extending the observation range to 12 billion years ago, SPICA will directly observe the formation activity of galaxies and stars in the early universe.

Recipe of the planetary system

Another important mission of SPICA is to discover how our living planetary system was born. In particular, it will try to look at the sites of planetary system formation. We will explore planetary systems both within and outside the solar system, specifically those equivalent to the “past solar systemEwhere planets were just formed.

In the whole-sky survey data collected by AKARI’s mid-wave infrared observation, many stars with dust disks were discovered. These disks were presumably generated by collisions between celestial bodies as they grew to become planets. Most dust disks discovered before the AKARI observation were located far from their central stars, so we could not clearly examine the relation between the disks and the planet-formation activities. With AKARI, the presence of disks near the earth’s environment was confirmed. We are sure that they are strongly related to the existence of earth-like planets. Thus, it is thought that observation of stars with dust disks is equivalent to seeing the figure of the past solar system.

AKARI revealed the existence of dust disks, but unfortunately could not show their structure. Furthermore, we were unable to see the figures of planets themselves that might be born in the disks.

SPICA will clearly show the structure of the dust disks discovered by AKARI.

In the current solar system, there are two different types of planets: a “gaseous planetElike Jupiter or a “solid planetElike the earth. The “core accumulation modelEis today’s standard model for planet formation to explain the difference. The model presumes that gases and solids evolved in different courses to form their respective planet types. With its high-sensitivity infrared observation, SPICA can track back the evolution of both gases and solids simultaneously. We plan to verify the core accumulation model directly with the observation.

One challenging observation of SPICA is the direct observation of planets in planetary systems other than our solar system. In particular, the most promising, important capability of SPICA is to ascertain the atmospheric composition of planets by spectroscopic observation. There is a possibility that SPICA will find the existence of water, oxygen, and carbon dioxide, which are closely related to life. If their molecules are detected in the atmosphere on planets other than the earth, we should obtain valuable clues on the generation of planetary life.

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