Galaxies near and far from AKARI

Wednesday, September 5, 2007

This is a composite image of the spiral galaxy M101. The image shows the distribution of cold (blue) and warm (red) dust overlaid on the visible (green, showing distribution of stars) and far-ultraviolet (cyan, indicating the location of young stars) images of M101. Credits: JAXA

(click image for a larger version) The above panels show images of spiral galaxy M101 overlaid with information from FIS. The left panel shows the distribution of the cold dust in the galaxy. Whereas the right panel shows the distribution of warm dust in the spiral galaxy. Credits: JAXA

The images show the distant Universe seen at four different far-infrared wavelengths through the Galactic window near the South Ecliptic Pole in the sky.Many faint galaxies are seen in white over the wide area of 10 square degrees. Observation fields of other deep surveys at other wavelengths are overlaid. Differences in the brightness of individual galaxies in the different wavelength bands are seen. Credits: JAXA

FIS data was compared to an image of the galaxy in the visible and far-ultraviolet. It shows that the warm dust is distributed along the spiral arms, with many hot spots located along the outer edge of the galaxy. These spots correspond to giant star-forming regions. This is unusual because star formation is generally more active in the central parts of spiral galaxies.

The evidence points to M101 having experienced a close encounter with a companion galaxy in the past, dragging out gas from the hapless companion. The gas is now falling onto the outer edge of M101 at approximately 150 km/s, triggering the active star formation.

AKARI has also been observing galaxies in the far distant Universe to address one of the most important questions in modern astronomy: how did the galaxies evolve into their current form?

To help find the answer, Shuji Matsuura and Mai Shirahata, ISAS/JAXA, used AKARI to carry out one of the most extensive observations ever made in the far-infrared, detecting many faint galaxies in the distant Universe at FIRs four wavebands. This wavelength information is essential to investigate the mechanisms responsible for the emission of infrared light and to estimate the distances to the galaxies.

The white spots in these images are all faint galaxies of different brightnesses. They imply that ordinary galaxies seen in the present time shone much more brightly in the infrared when they were younger. In many cases this is due to explosive episodes of star birth during earlier times. Some galaxies appear to have a differing brightness at different wavelengths than others and it is suspected that this might be because these galaxies are brightened by the energy released from a black hole at their cores.

The AKARI data shows that the number of galaxies increases rapidly as they appear fainter and so indicates that the galaxies have merged. However, they do not seem to evolve as drastically as inferred by previous observations. As AKARIs are the most sensitive observations ever made at these wavelengths, this result suggests that a new galaxy evolution model may be necessary.

Source: ESA