The Universe Revealed by Subaru Telescope Ⅲ
The Milky Way and its Group
Our solar system belongs to a large spiral-shaped group of stars called the Milky Way Galaxy, which is about 100,000 light years across. The next nearest large galactic neighbor is the Andromeda Galaxy (M31), located 2.5 million light-years away. It is spiral also and is similar size as the Milky Way Galaxy. Between Andromeda and the Milky Way galaxies there are dozens of smaller ones. The interactions between members of this galactic group provide us with important clues when we try to understand the mechanism of how galaxies are formed.
The World of Stars in the Andromeda Galaxy
The Andromeda Galaxy (M31) is just bright enough in our sky that it can be recognized with the naked eye. However, since the galaxy is so large and so close, it is difficult to observe the whole region with a large telescope. One of the best features of the Subaru Prime Focus Camera is its wide field of view. The camera succeeded in taking a very clear image of the southwest quadran of M31 (yellow square). From the upper left to the lower right in this image, the colors of the stars range from yellow to blue. What this means is that the older stars, which were formed about a few billion years ago, which characteristically shine with yellow light, are distributed more toward the center of the Andromeda Galaxy. The younger stars, which shine with blue light, are located more toward the outer reaches of the galaxy. This kind of observation allows us to understand more about how spiral galaxies formed.
The Structure of a Dwarf Galaxy
There are many small objects known as dwarf galaxies that populate our local galaxy group. Some of them lie relatively far away from the Milky Way and Andromeda galaxies, and astronomers made a theory that they evolved independently, without much interaction with each other, hence this structures are supposed to be simple. However, the analysis of star distribution in the dwarf galaxy Leo A, as seen in a Subaru Prime Focus Camera image, shows that it has a surprisingly complex structure. There is also a possibility that dwarf galaxies may form when even smaller ones merge together.
The First Stars in the Universe
A comparison between the optical spectra of the Sun versus a star HE1327-2326 with the least heavy elements. The top panel shows the low-dispersion spectrum while the bottom panel is the high dispersion spectrum captured by Subaru Telescope's HDS (High Dispersion Spectrograph). The solar spectrum has many absorption lines that are caused by the heavy elements at its surface. In contrast, the spectrum of HE1327-2326 shows almost no absorption lines due to the lack of heavy elements.
In the Milky Way Galaxy, there survives a type of star that was born very early in the history of the universe. How did it form? Just after the "Big Bang," the universe consisted of only hydrogen and helium. The first stars were thought to contain no heavy elements. Subaru Telescope has discovered such a primordial stars having lowest content of heavy elements known to date. Its chemical composition may hold information that could lead us to a better understanding of what the first stars were like.
Commentary from Dr. Nobuo Arimoto, Professor at NAOJ:
Subaru Telescope excels at extensive imaging, and succeeded in discovering the halo structure of a dwarf galaxy for the first time. Due to this discovery we can understand that even a dwarf galaxy has a complex history of formation. It is expected that in the future, further systematic research of this halo structure will show that it is an attribute of many galaxies that exist in our local group.