Andrew Yee[_1_]
January 20th 08, 02:48 AM
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/Pressrelease/2007/11/28/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/Pressrelease/2007/11/28/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 figure:
[http://subarutelescope.org/Pressrelease/2007/11/28/fig02_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/Pressrelease/2007/11/28/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.
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/Pressrelease/2007/11/28/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/Pressrelease/2007/11/28/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 figure:
[http://subarutelescope.org/Pressrelease/2007/11/28/fig02_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/Pressrelease/2007/11/28/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.