Leo II: An Old Dwarf Galaxy with Juvenescent Heart
November 28, 2007
A team of 15 astronomers observed the dwarf spheroidal galaxy Leo II and found that the galaxy is more extended than previously thought and established a star-formation history of this galaxy.
Categorized as a dwarf galaxy (Note 1), Leo II is a small faint galaxy located relatively closely (760,000 light years away) and its mass amounts to only 1/20,000 of the Milky Way (10 millions of solar mass).About 10 such small and faint dwarf galaxies are found around the Milky Way, and 40 are found in the Local Group (Note 2). Figure 1 shows the image of Leo II obtained by the Subaru Prime Focus Camera (Suprime-Cam) on the Subaru Telescope.The prevailing scenario for galaxy formation reports that massive galaxies, such as our Milky Way, grow up by 'eating' small galaxies to attain their extensive size. In response, dwarf galaxies that have survived are important to study not only to understand how they themselves have formed and evolved but to comprehend the evolution of massive galaxies.
Since dwarf galaxies are faint and beyond the reach of small telescopes, the observing team chose Suprime-Cam because the camera covers a wide field of view extending beyond the tidal radius (Note 3) of Leo II. This extensive coverage enabled researchers to investigate how wide Leo II extends, and whether the properties of stars differ within the galaxy. In addition, since Leo II is located so close, we can measure the position and the brightness of each star in the galaxy very accurately. By comparing these measurements and the evolution theory of stars, we can investigate the evolution of the galaxy, as an assembly of stars in the galaxy, in detail.
The team first investigated the extent of Leo II by counting the number of red giant stars (Note 4), which were selected from the color-magnitude diagram of Leo II (Figure 2), as a function of the radius; over a dozen red giant stars were found to exist beyond the tidal radius. To investigate how these extra-tidal stars are distributed, the team made the surface brightness map determined by the stars that belong to Leo II (Figure 3). As seen in this picture, a knotty structure, whose surface brightness is as faint as about 31 mag/arcsec2, was discovered on the east (left) side of Leo II. The stars belonging to the structure are relatively old and their properties are similar to that of the stars located within the main body of the galaxy. The team suggested that this structure could be a small globular cluster being disrupted by the tidal force of the galaxy; however, further observation is required to give a definite answer.
The team also investigated distributions of red giant stars, horizontal branch stars and sub-giant branch stars located in the inner part of the galaxy. They showed that the property of the stars is different throughout the galaxy; the younger stars are found in the inner portions while old stars are found all through the galaxy. The team concluded that star-forming activity occurred more than 8 billion years ago throughout the galaxy, and the star-forming region gradually shrank from the outside toward the center. The formation of stars ceased approximately 4 billion years ago, except for the galactic center, where a small population younger than 4 billion years is present.
The dwarf spheroidal galaxy Leo II, which was thought to be a regular and simple system, turned out to be a complex system as it has an extended extra-tidal structure and a mixed star-formation history. Both further observation for other dwarf spheroidal galaxies and theoretical studies to investigate chemo-dynamical evolution of dwarf galaxies are expected in the future.
The results of this study were published in the August, 2007 issue of The Astronomical Journal. The team consists of 15 Japanese astronomers from the National Astronomical Observatory of Japan, the University of Tokyo, Japan Women's University, and the Space Telescope Science Institute in the USA.
The title and the author of the paper:"Wide-Field Survey around Local Group Dwarf Spheroidal Galaxy Leo II:Spatial Distribution of Stellar Content"Komiyama, Y., Doi, M., Furusawa, H., Hamabe, M., Imi, K.,Kimura, M., Miyazaki, S., Nakata, F., Okada, N., Okamura, S.,Ouchi, M., Sekiguchi, M., Shimasaku, K., Yagi, M., Yasuda, N.2007, The Astronomical Journal, Volume 134, Issue 2, pp. 835-845.
Note 3: Tidal Radius: in addition to the gravity of Leo II, stars belonging to Leo II are affected by tidal force caused by the gravitational potential of the Milky Way. Therefore, stars which are located beyond a fixed radius are disrupted; such a radius is called the tidal radius.
Figure1: False-color image of Leo II (composed from V and Ic band images). The field of view is 26.67 by 26.67 square arcminutes. Integration times are 3000 sec in V band and 2400 sec in Ic band, respectively. North is up, east is left.
Figure 2: Color-magnitude diagram of Leo II. The horizontal axis is the color (V-Ic) and the vertical axis is the V-band magnitude of stars. As explained in the supplementary figure, stars trace the evolutionary track (main sequence -> sub-giant branch ->red giant branch -> horizontal branch) as they evolve and, hence, age of stars are estimated from the color-magnitude diagram. The color-magnitude diagram of Leo II resembles those of old Galactic globular clusters, suggesting that the majority of stars of Leo II are as old as those in the Galactic globular clusters.