Polycyclic-aromatic hydrocarbon in outer space
There is a distinct series of band radiations among infrared spectra in outer space emitted from interstellar matter. These were discovered as “unidentified infrared bands” in a planetary nebula in our Galactic system in the 1970s. Later, it was revealed that these radiations were widely and universally seen in various places in space, namely in ionized hydrogen regions, around reflection nebulae and fully evolved stars, in diffusion light emitted from interstellar matter drifting between stars in our Galactic system, and in other galaxies. As a result of research in the laboratory and study based on the quantum chemical calculation, “polycyclic-aromatic hydrocarbon” was identified as a probable candidate contributing to these band radiations. Polycyclic-aromatic hydrocarbon dust contains molecules that have a number of benzene-ring structures. They are excited by absorbing a single ultraviolet photon energy, mainly from stars, and releases the energy by lattice vibration of carbon-carbon or carbon-hydrogen in the molecules. The energy of each vibration corresponds to photonic energy with infrared wavelengths of 3.3μm, 6.2μm, 7.7μm, 8.6μm, 11.2μm and so on. As a result, it is believed that polycyclic-aromatic hydrocarbon existing in outer space shows the series of the band radiations in those wavelength. The strength ratio, shape, etc., of these band radiations varies according to the physical state of radiators. Thus, by scrutinizing the characteristics of the series of observed band radiations, we can identify the radiatorís physical state and further, based on such information, estimate the physical environment surrounding the radiators.
In the mid-infrared spectrum of the nearby galaxy NGC6946 observed by AKARI, we can see clearly infrared band radiations that have probably been emitted from polycyclic-aromatic hydrocarbon. In the course of close study of the data, we discovered that the strength ratio and spectral shape of the radiation bands from a galaxy-arm region with many star-formation regions differed from those of a relatively calm region with no star-formation activity. Moreover, as a result of our investigation into the cause of said difference, based on laboratory data and comparative research with the results of quantum chemical calculation for the infrared radiation of polycyclic-aromatic hydrocarbon, the following scenario was suggested: polycyclic-aromatic hydrocarbon, that exists in a cluster due to a kind of van der Waals attraction, is dispersed when located around star-formation regions with strong radiation fields; and it is transformed to a state of ionization because its electrons are removed by ultraviolet radiation.
A number of mysteries remain on the evolution of polycyclic-aromatic hydrocarbon in outer space. Nonetheless, by clarifying the mysteries in the process one by one, we may close in step by step on a new picture on how the current universe was formed, with its materially and chemically rich environment. This is because polycyclic-aromatic hydrocarbon is believed to be an important substance that may have a close relation with organic matter such as amino acid via several chemical processes. From this viewpoint, an attempt to explore the evolution of polycyclic-aromatic hydrocarbon in outer space is a significant theme because it bridges interstellar chemistry and exobiology. Therefore, research using the latest observation data from AKARI and others is a crucial challenge that should be addressed.