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The Forefront of Space Science

Exploring Fine Solid Particles Distributed in Vast Outer Space
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It is known that a great number of fine solid particles smaller than 1-micron float in interstellar space together with gaseous matter called “interstellar gas.” These dust particles in the universe are called “interstellar dust” or simply “dust.”
Since the dust scatters, absorbs and blocks light from stars, it is very troublesome for astronomical observations in the ultraviolet or visible light wavelengths. To understand accurately the nature of far-distant stars and galaxies, therefore, it is very important to know the nature and amount of dust floating in interstellar space. Further, the dust plays a significant role in the energy balance in interstellar space and a crucial role in the material evolution of interstellar space through, for example, catalysis reactions on its surface. New-born galaxies might be embedded in dust. It is thought that the earth we live on was also originally made of an uncountable accumulation of dust.

Dust is closely related to the “origin” of ourselves and is almost always involved in various phenomena in the universe. Thus, dust is an important existence. To understand many events in interstellar space, an understanding of the real nature of dust is significant. Fortunately or unfortunately, spectra features of dust, which would reveal the nature or properties of dust, are rarely within the visible wavelength. Dust’s important spectral bands are centered in the ultraviolet or infrared regions, which we cannot observe from the ground. For this reason, most information on dust has been retrieved from observational data outside the atmosphere. In particular, dust absorbs light from stars and emits in the infrared region as thermal radiation. Since thermal radiation light spreads across the whole sky and shines weakly, we cannot observe it with telescopes on the ground because radiation from the atmosphere is too strong. “Cooled” telescopes in space that suppress background radiation are indispensable for observation. To study dust, observation in space plays a big role.

It is believed from a combination of elements abundant in interstellar space that tend to become solid that there are two main constituents of dust: carbonaceous substances like coal; and silicate substances like stones. As of today, however, we have no accurate information on what actual substances exist and to what degree. To reveal the real nature of dust is one of the biggest themes for modern astronomy. In particular, the exploration of where the varieties of dust were born in the universe and how they evolved is a major challenge for the future space observation.

Ultra-fine particle dust discovered by infrared satellites

If we suppose that dust like coal or stone exists, we can expect that its temperature would become around 20K in interstellar space on the basis of its energy balance. Accordingly, the peak of thermal radiation is in the far-infrared region longer than 100-micron wavelength. Launched in 1983, the world’s first infrared astronomical satellite, IRAS, detected a very large excess component of the diffuse emission in the mid-infrared region in addition to the expected thermal-emission component in the far-infrared region. This fact was also confirmed by observation of the COBE satellite launched in 1989 (Fig. 1). This surprising fact was actually, to some degree, predicted by studies prior to the observation. That is, if we assume dust of 10 nanometers or smaller, it is found by simple calculation that its heat capacity is far lower than the absorbed energy of a photon. In such tiny dust, the temperature surges every time the dust absorbs a single photon and the absorbed energy radiates in shorter infrared wavelengths. This can explain the observed excess emission. What we were unable to predict was that there existed the huge amount of such “ultra” fine particle dust.

Figure 1
Figure 1. A spectrum of diffuse infrared radiation from the Galactic plane. The black circles are data from the COBE satellite. The dashed line shows the expected thermal radiation from submicron dust. The dotted line from 10 to 60 microns shows the excess emission from ultra-fine particles. The spectrum from 5 to 12 microns is that observed by Infrared Telescope in Space onboard a satellite. The band structure from a substance with a PAH structure is visible. The radiation in wavelengths shorter than 5 microns is mainly the light from faint stars.

Following the above satellites, spectroscopic observations were made by the Japanese Infrared Telescope in Space (IRTS), on board a satellite launched in 1995, and the European Infrared Space Observatory (ISO). These observations revealed that the excess emission from 6 to 12 microns was not smooth in its spectrum but had band structures at 6.2, 7.7, 8.6 and 11.3 microns (Fig. 1). These band structures are a common feature for the group of molecules having a number of benzene rings called the Poly-cyclic Aromatic Hydrocarbon (known as PAH), which suggests that there is a great amount of small hydrocarbon dust in space. PAH is just a collective designation of a group of molecules and, of course, the spectrum of each molecule has very minute band structures. The observed band, however, did not show the minute structures seen in each molecule. One possible interpretation is that radiation of various molecules overlapped and was visible as one. The band structures observed in our Galaxy, however, showed little variation. While suggesting that the bands are emitted from more stable substances, this adds a question mark to the many-molecule overlap interpretation. More-stable hydrocarbon substances with PAH structures may exist in the universe.

Recent studies that scrutinized the IRTS data clarified the variations in the band structures and the correlation between the band intensity and far-infrared emission in the diffuse Galactic radiation for the first time. These findings are valuable data to explore the birth of the dust and the cause of its death. The currently active Spitzer Space Telescope has detected bands between 16 to 18 microns, which was first confirmed by ISO, in many celestial objects. We expect that the telescope will elucidate in detail the nature of the dust. Further, it is known that very different band structures are observed in special (?) environments such as elliptical galaxies with many old stars. With the ASTRO-F satellite planned for launch in 2006, we expect that, in addition to the IRTS’s results, we will be able to observe and clarify variations in radiation from the dust and then reveal the dust’s origins, while covering various regions in our Galaxy and nearby galaxies.

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