The birth of galaxies occurred more than ten billion years ago in the ancient Universe. Assembled under their own gravity, early galaxies formed into big clusters or small groups. During the process of assemblage, their properties changed in relation to their surrounding environments, just as human traits change in the contexts of their lives. For example, galaxies grouped in high-density environments such as clusters tend to be elliptical or lenticular while solitary ones tend to be spiral galaxies. How galaxies form and evolve is one of the biggest mysteries in recent extragalactic astronomy.
When and how did patterns of galactic formation become established and evolve? To address this question, many astronomers are investigating distant clusters of galaxies where assemblage of galaxies occurred in the early universe. A research team led by Dr. Yusei Koyama used the Subaru Prime Focus Camera (Suprime-Cam) to carry out a panoramic observation targeting a relatively well-known rich cluster, CL0939+4713, located four billion light years away from Earth (see Fig. 1). The team used a special filter that can detect the hydrogen-alpha (Hα) line emitted by ionized hydrogen four billion years ago (see Note 1). Koyama's team members carefully compared the images taken with and without the special filter and then identified more than 400 galaxies showing an excess of Hα in the special filter images (see Fig. 2). Such narrow-band "excess" galaxies are likely to be star-forming galaxies. Surprisingly, Koyama's team found that an unexpectedly large number of star-forming galaxies had red colors. Even more interesting was the location of these red-burning galaxies; they reside primarily in the group-scale environments located far away from the main body of the cluster (see red symbols in Fig. 2 and also Note 2).
These findings raise some intriguing questions. What is the physical origin of these red-burning galaxies? Why are they concentrated in groups and not in clusters? No one, including the research team members, knows the answer yet. At a minimum, the strong Hα emissions clearly show that the red-burning galaxies are actively forming new stars. Therefore, their red colors are likely to be produced by dust rather than by old stellar populations (see Note 3). The researchers expect that the strong gravity of the main cluster will cause the groups where the red-burning galaxies are most numerous to merge with it. The most significant result of this research is that the properties of galaxies are indeed changing in relatively sparse environments before they are finally absorbed into a very rich cluster.
The research team noticed that the number of old galaxies, without active star-formation, appeared to be increasing in the group environments, exactly where the red-burning galaxies are most numerous. This suggests that the red-burning galaxies are related to the increase in old galaxies, and that they are likely to be in a transitional phase from a younger to an older generation. The finding that such transitional galaxies are located most frequently within group environments shows that galaxy groups are the key environments for understanding how environment shapes the evolution of galaxies.
The research team emphasized the important contribution of the unique wide-field capability of the Subaru Telescope for accomplishing this research, because its panoramic imaging revealed the location of the transitional galaxies. The same research team now plans to conduct a new observation to identify the physical origin of the red-burning galaxies discovered in this study. This should be an important and exciting step toward a more complete understanding of the environments shaping the galaxies in the present-day universe.
The results presented here were published in the paper, "Red Star-Forming Galaxies and Their Environment at z=0.4 Revealed by Panoramic Hα imaging" (Koyama, Y. et al., 2011, The Astrophysical Journal, vol. 734, pp. 66-78).
Yusei Koyama (University of Tokyo/National Astronomical Observatory of Japan, JSPS research fellow)
Tadayuki Kodama (National Astronomical Observatory of Japan/Subaru Telescope)
Fumiaki Nakata (National Astronomical Observatory of Japan/Subaru Telescope)
Kazuhiro Shimasaku (University of Tokyo)
Sadanori Okamura (University of Tokyo)
1) The research team used a narrow-band filter called NB921, which transmits light within a very limited wavelength range, around 918 nm. The Hα line (656 nm in the rest-frame and an excellent indicator of star formation) emitted four billion years ago is expected to fall in this narrow-band filter due to the redshift effect. Therefore, a galaxy showing an excess emission in the narrow-band image is a strong candidate for being a star-forming galaxy associated with the targeted cluster. Note that galaxies having emissions in this band but at different redshifts may sometimes confuse the identification of narrow-band excess galaxies. However, this can be easily clarified by using broad-band color information.
2) A star-forming galaxy is dominated by blue, short-lived O- or B-type stars and generally appears blue. However, once star formation has ceased, the blue bright stars soon die and then the colors of galaxies change to red. Therefore, interpretation of the red-burning galaxies discovered in this research is challenging, because these galaxies show red colors, which suggest weak or no star formation, as well as strong Hα emissions, which suggest active star formation, at the same time.
3) If a galaxy contains a large amount of dust, the color of the galaxy becomes redder. This "dust reddening" occurs because the dust in galaxies tends to absorb bluer light, which has a shorter wavelength, more effectively than redder light, which has a longer wavelength. Therefore, it is likely that the red-burning galaxies identified in this study are such dusty galaxies. A firm conclusion awaits mid- or far-infrared observations, which can directly identify the dust emissions.