The problem is the synthesis of silicon and sulfur
The problem is that the composition ratio of oxygen is less than half that of silicon and iron. In fact, standard models of element synthesis by supernova explosions cannot reproduce such composition ratios. We discussed and examined vagueness in identifying elemental composition from the observation data, but our examination results endorsed our conclusion that the composition ratio of oxygen is small.
It was thought that supernova originating from light stars produced mainly iron. Now, however, we think that supernova from light-stars must also synthesize a fair amount of silicon. Most stars in the elliptical galaxy are old. It is known that the brightness of a supernova originating from light-star depends on the age of its stellar system. As supernova brightness is related to element synthesis, we think that element synthesis by supernova from light-star may be systematically different between old and young stellar systems. Further, we discovered that some stars in the galactic system, which have many elements and must have been produced recently, might contain more silicon than oxygen. These results suggest that, in both the galactic system and elliptical galaxies, supernova originating from light-stars synthesize more silicon as the stellar system ages.
Another problem is that, moving away from the center, the composition ratio of sulfur deviates from that of silicon. The ratio of sulfur and silicon in the outmost part of the observed region is half compared to that of the center. The ratio of oxygen and iron proves that the outmost region is strongly affected by the contribution of supernova originating from heavy-star. It was thought that silicon and sulfur were synthesized in equal amounts. To explain the observed findings, however, we have to accept the fact that less sulfur is synthesized than silicon in supernova from heavy-star.
Our previous interpretation was that silicon is synthesized mainly by supernova explosion originating from heavy-star. If supernova from light-star synthesize silicon as well, our interpretation on silicon might be incorrect. The composition ratios of oxygen and magnesium, which are synthesized only by supernova from heavy-star, become important. Generally, however, the observation of these composition ratios is difficult. As the Virgo Galaxy Cluster is near the earth, we were fortunately able to observe oxygen and magnesium. If we can suppose that less sulfur is synthesized than silicon in supernova from heavy-star, the ratio of sulfur and silicon could be a good indicator to demonstrate the contribution of supernova originating from heavy-star. This is of great help to our research since there is little vagueness in the observation of sulfur/silicon ratios.
Expectations for ASTRO-EII
ASTRO-EII, a Japanese X-ray astronomical satellite, is slated for launch after 2005. The detector onboard ASTRO-EII can measure X-ray energy more accurately than ever before. The detector is especially powerful with extended objects such as high-temperature gases in the galaxy cluster. With this capability, we will be able to identify accurately the intensity of even weak bright lines. It also allows us to limit infiniteness when converting from intensities to elemental-composition ratios. Therefore, we will be able to obtain accurate composition ratios for various elements in various galaxies and elliptical galaxies. With ASTRO-EII, we expect to see advances in the study of element synthesis, the history of element synthesis, and the history of star formation.
Reference: Matsushita K., Finoguenov A., Boehringer H., 2003, Astronomy and Astrophysics, vol. 401, p443