This image shows the results of deep-survey observations at 15 micrometres by AKARI’s Near- and Mid-Infrared Camera (IRC). The white points in the picture are all thought to be distant galaxies. The image size is approximately 10 arcminutes squared in size.

Galaxies in which active star formation is taking place emit most of their energy as infrared radiation from the interstellar material heated by the light from the young hot stars. The light observed from the distant galaxies, has long left the galaxies and is stretched (or ‘red-shifted’) due to the expansion of the Universe. This is why it is observed at wavelengths longer than the original emitting wavelengths. Due to the redshift, observations with ESA’s Infrared Space Observatory (ISO) revealed that the number of faint, distant galaxies increased drastically when observed at 15 micrometres. The original emitting wavelength was 7 micrometres, approximately 6 thousand million years ago, and has been redshifted to 15 micrometres during its journey across the universe.

This emission feature around 7 micrometres, due to the characteristic emission from organic materials in the interstellar medium, is stronger in regions of active star formation. Thus, the ISO observations of a very small area of the sky containing only 24 galaxies already showed that active star formation took place at this epoch in the history of the Universe.

The area observed by AKARI is about three times wider than that of ISO. AKARI detected around 280 galaxies in this region, confirming the increase in the number of galaxies at 15 micrometres implied by the earlier ISO observations. Comparable numbers of even fainter galaxies have been discovered with AKARI, leading to the conclusion that the star formation activity was already intense even earlier than 6 thousand million years in the past. Similar deep surveys over the entire wavelength range (from 2 to 24 micrometres) are being carried out with AKARI. These data will provide a definitive description of the evolution of galaxies over the lifetime of the Universe.