Leo II: An Old Dwarf Galaxy with Juvenescent Heart (Forwarded)

National Astronomical Observatory of Japan
Tokyo, Japan

November 28, 2007

Leo II: An Old Dwarf Galaxy with Juvenescent Heart

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.

NOTES:

Note 1: Dwarf galaxies: small galaxies which are 10 times fainter than
the Milky Way galaxy.

Note 2: Local Group: a group of galaxies which extends about 4 million
light year centered on between the Milky Way and the Andromeda galaxy
(M31).

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.

Note 4: Red giant stars: a category in the evolutionary stage where stars
left the main sequence and are in the hydrogen shell burning phase.

IMAGE CAPTIONS:

[Figure1:
http://subarutelescope.org/Pressrele...28/fig01_l.jpg (701KB)]
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:
http://subarutelescope.org/Pressrele...1/28/fig02.gif (29KB)]
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.

Supplementary figu
[http://subarutelescope.org/Pressrele...g02_expl_e.gif
(25KB)]
Evolutionary track of a star overlaid on the color-magnitude diagram of a
Galactic globular cluster NGC5024.

[Figure 3:
http://subarutelescope.org/Pressrele...1/28/fig03.jpg (34KB)]
The brightness map of Leo II. The brightness becomes low (faint) as the
color goes white - red - yellow - green - blue - black. The area
where no stars belong to Leo II is found is represented as the
black-colored area. From this figure, very faint knotty structure (shown
in green to the east (left) of Leo II) was discovered. Note that the
brightness of the galaxy center is represented by the contour (from the
center to the periphery, 26.5, 27.5, 28.3, 29.0, 30.0 mag/arcsec2,
respectively). The dotted line represents the tidal radius.