Subaru Tackles Galaxy Evolution in Distant Universe
~The Discovery of Galaxy Groups 8 Billion Light Years Away~
October 30, 2008
As scientists study the interconnectedness of environmental systems on Earth, astronomers are looking at environmental forces in the Universe to see how they affect the evolution of galaxies. These forces can change the shape, color, and size of galaxies, and do so in ways that are hard to pinpoint -- until now.
Galaxies are dynamic star systems that have enormous numbers (millions to billions) of stars in wide varieties of colors and shapes. Some galaxies have blue beautifully knotted spiral arms, while others are red diffuse light without arms (Note 1). As most celestial objects, galaxies are NOT uniformly distributed throughout the Universe. They tend to flock together and form a “cosmic web”, which represents the large-scale structure of the Universe. At intersections within this webwork, astronomers observe massive concentrations of galaxies called Galaxy Clusters.
Astronomers have observed that the colors and shapes of galaxies change depending on their location. In clusters, where so many galaxies live together, galaxies tend to be red and elliptical. On the other hand, galaxies living alone are predisposed to be blue and spiral shaped. Astronomers do not yet have a clear idea why galaxies change their colors and shapes contingent on their location and their surrounding environments. Nonetheless, one way to explain this phenomenon is to observe galaxies at great distances that represent periods long ago in the past. Simply stated, the Universe is like a time machine - as we look deeper into the Universe, we go further back in time. Using the Subaru Telescope, astronomers explored answering the question, “How do galaxies change their colors and shapes over time?”
Recently, an international team of astronomers used Subaru to observe two huge groups of galaxies, galaxy clusters, in the distant Universe. One cluster (RDCS0910) was 8 billion light years away while another (RDCS1252) was 8.5 billion light years distance. (8 billion light years equals about 50,000,000,000,000,000,000,000 miles). These two clusters are among the most distant clusters known to date. Galaxies at such a long distance appear very faint, but Subaru is sensitive enough to collect the dim light from such great distances (Note 2).
Galaxy shape, color, and distribution in these structures are keys to solving the riddles of galactic evolution in the distant reaches of Universe. Using Suprime-Cam, the astronomers discovered a lot of galaxies within the clusters and found that the clusters are surrounded by small groups of galaxies (Note 3). Not all these small groups were confirmed to lie at exactly the same distance as the clusters, but this discovery of prominent large-scale structures in the distant Universe is a seed to understanding overall galactic evolution.
Small galaxy groups at this distance (in the astronomy distance scale redshift ~ 1.2) have previously not been studied in detail, and, as such, are very interesting objects for astronomers. The small galaxy groups may get closer to the nearby larger cluster through gravity and eventually will collide a few hundred million to a few billion years from now. In fact, collisions and mergers are considered primary factors in galactic cluster growth. That is, clusters grow by accreting small galaxy groups around them. It should not be surprising if colors and shapes of galaxies in small groups change during these large-scale collisions.
Looking at the small groups of galaxies in Figures 1 and 2, the first thing noted is the fantastic scale. The large zoom-in boxes are 3 million light years across (20,000,000,000,000,000,000 miles), indicating that the galaxy groups are huge. Looking closely at the boxes, many reddish (well, orange-ish) galaxies are observed. These galaxies are so far away from us that it is impossible to spatially resolve their shapes. However, it is clearly visible that the small groups of galaxies are already dominated by red galaxies at least 8 billion years ago.
The fundamental question for this study was “What do these observations mean?” As mentioned earlier, galaxies living alone tend to have blue colors, while galaxies in clusters have red colors. Galaxies likely change their colors from blue to red during the course of relocation and structural growth. Nearby galaxies pulled together by gravity, form a group of galaxies, and eventually collide with other groups and form large clusters (Note 4). At some point in this structural growth, blue galaxies become red. The team found in their study that small groups are already dominated by red galaxies before they collide with big clusters. Astronomers have long believed that large clusters are the key environment changing colors and shapes of galaxies, but the findings at Subaru now point astronomers at small groups. Galaxies become red in groups before they get together in large clusters - small groups are the key environment for these changes in shape and color.
Astronomers know several physical processes that can affect colors and shapes of galaxies. Among them, collisions and mergers between galaxies (not between groups) are the ones that work very efficiently. Because groups have many galaxies in them, galaxies can attract each other by gravity and eventually collide and merge. In fact, a computer simulation has shown that a collision between two blue galaxies leaves a single red galaxy (Note 5).
The Universe 8 billion years ago shows us that collisions with other galaxies might be the reason why galaxies change their colors and shapes depending on environment around them. While the astronomers observed just a tiny fraction of the Universe, galaxy collisions alone cannot explain everything and some mysteries remain. Subaru will continue to observe at great distances and at various wavelengths to get a better understanding of galactic transformation.
This work is published in two papers, the first one in Monthly Notices of Royal Astronomical Society in May 2007 (Tanaka et al., 2007, Vol. 377, pp. 1206-1214), and the second one will be published in Astronomy & Astrophysics in Oct. 2008 (Tanaka et al., 2008, Vol. 489, pp. 571-581).
Note 3: One difference between Galaxy Groups and Galaxy Cluster is their mass. Roughly speaking, Clusters are more massive than 200,000,000,000,000,000,000,000,000,000,000,000,000,000 metric tons while Groups are less than that. Groups have less galaxies than Clusters.
Note 4: Galaxies actively forming stars appear blue, while those that have not formed stars for a long time appear red. Young hot stars shine blue because of their high temperature, while older cold stars shine red. The color change from blue to red means that galaxies were forming stars in the past, but stopped at some point and transitioned to red as hot stars die. In general, galaxies are huge assemblies of stars, and once they completely stop forming stars, they will not grow any more. Some astronomers call red galaxies “dead” galaxies.
Note 5: Extremely intense star births (starbursts) can happen during the course of galaxy collisions. These starbursts consume a huge amount of gas and dust. Because gas and dust are critical ingredients for stars, galaxies may not be able to actively form stars after starbursts.