![]() ![]() This merging process differs from galaxy to galaxy. ![]() Key to understanding this is measuring the distribution of the halo component to ascertain the volume. Twelve billion years ago, successive merging of small galaxies confined by dark matter halos occurred. This is about 20 times larger than the distance between our solar system and the galaxy center. Thus, the team had finally observed the outermost edge of the galaxy. While this density generally decreases the further you go from the galactic center, the team discovered a sharp drop in density at around 520,000 light years away from the galactic center. Using the data from HSC-SSP, the team derived the spatial density of the BHB stars over the galaxy halo. The team carefully selected the BHB stars from the on-going survey program (SSP: Subaru Strategic Program) data against other contaminants having similar colors such as so-called blue straggler stars, white dwarfs, quasars and distant galaxies. It enabled them to capture remote, very faint halo tracers at the outer edge of the galaxy. The team of researchers led by Tohoku University graduate student Tetsuya Fukushima and his supervisor Masashi Chiba used the Hyper Suprime-Cam (HSC) digital camera on the 8.2 meter diameter Subaru Telescope. ![]() ![]() However, the galaxy is so large that it is impossible to identify the halo traces located at the outer boundary using 2.5 to four-meter telescopes. This is because they are naturally bright enough to determine the distance to and from them. So-called blue horizontal branch (BHB) stars as well as RR Lyr variables are ideal indicators for tracing the halo component. Investigating the extent of this halo component in the galaxy is similar to identifying the outer boundary of a forest from inside the forest and observing the trees. This suggests that the galaxy was quite large in its beginning before the later formation of the younger, disk component. The halo thus contains the remnants of long-lived stars and star clusters that formed in the first stage of the galaxy. The halo contains about 1 billion ancient stars and 150 globular clusters with ages as old as 12 billion years (Figure 1). The galaxy holds a broadly extended halo component, in addition to the bright Milky Way in the form of the stellar disk component. ![]()
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