(The image is 4 arcminutes acros)
Like the lost little puppy that wanders too far from home, astronomers
have found an unusually small and distant group of stars that seems
oddly out of place. The cluster, made of only a handful of stars, is
located far away, in the Milky Way’s “suburbs.” It is located where
astronomers have never spotted such a small cluster of stars before.
The new star cluster was discovered by Dongwon Kim, a PhD student at the
Australian National University (ANU), together with a team of
astronomers (Helmut Jerjen, Antonino Milone, Dougal Mackey, and Gary Da
Costa) who are conducting the Stromlo Milky Way Satellite Survey* at ANU.
“This cluster is faint, very faint, and truly in the suburbs of our
Milky Way,” said Kim. “In fact, this group of stars is about ten times
more distant than the average globular star cluster in the halo of our
galaxy -- it's a lost puppy,” Mackey adds. Globular clusters are
spherical cities of stars that form a vast, extended halo around the
core of our galaxy, the brightest of which are easily seen in amateur
telescopes or even binoculars. However, this new discovery required one
of the world’s largest telescopes to confirm, “it’s definitely a
diminutive oddball,” says Milone.
The oddly small, far-flung, cluster was discovered using the Dark Energy
Camera (DECam) on the 4-meter Blanco Telescope at the Cerro Tololo
Inter-American Observatory (CTIO) in Chile. “This discovery sheds new
light on the formation and evolution of the Milky Way,” said Daniel
Evans, National Science Foundation program director for Gemini
Observatory. “It's great to see so many telescopes come together to
produce this result, not the least being Gemini Observatory with its
incredible light-gathering power.”
The team’s first evidence of the unusually remote star cluster came when
they ran detection algorithms on a 500 square-degree imaging data field
obtained with DECam. “Such objects are too faint and optically elusive
to be seen by eye. The cluster stars are sprinkled so thinly over the
image, you look right through them without noticing (see image on
electronic release, URL above). They are hiding in the sea of stars
from the Milky Way. Sophisticated computer programs are our tools to
find them,” said Jerjen.
Because it is so faint, ultra-deep follow-up observations using the
Gemini Multi-Object Spectrograph (in imaging mode) confirmed that the
new globular cluster is among the faintest Milky Way globular clusters
ever found. Seven out of 150 known Milky Way globular clusters are
comparably faint but none are located as far out toward the edge of the
Milky Way. This new globular cluster has 10-20 times fewer stars than
any of the other outer halo globular clusters. Also, its star density is
less than half of that of other Milky Way globular clusters in the same
luminosity (brightness) range.
The new star cluster, named Kim 2, also shows evidence of significant
mass loss over its history. Computer simulations predict that, as a
consequence of their evolution over many billions of years, including
the slow loss of member stars due to the gravitational pull of the Milky
Way, star clusters ought to be arranged such that their more massive
stars are concentrated toward their centers. “This ‘mass segregation’
has been difficult to observe, particularly in low mass clusters, but
the excellent Gemini data reveal that Kim 2 appears to be mass
segregated and has therefore likely lost much of its original mass,”
said Da Costa. The finding suggests that a substantial number of
low-luminosity globular clusters must have existed in the halo when the
Milky Way was younger, but most of them might have evaporated due to
internal dynamical processes.
The observed properties of the new star cluster also raise the question
about how such a low luminosity system could have survived until today.
One possible scenario is that Kim 2 is not actually a genuine member of
the Milky Way globular cluster family, but a star cluster originally
located in a satellite dwarf galaxy and was accreted into the Milky
Way’s halo. This picture is also supported by the fact that the stars in
Kim 2 appear to be more chemically enriched with heavier elements than
the other outer halo globular clusters and are young relative to the
oldest globular clusters in the Milky Way. As a consequence of spending
much of its life in a dwarf galaxy Kim 2 could have largely escaped the
destructive influence of tidal forces, thus helping it to survive until
the present epoch.
There are many Milky Way globular clusters formerly and currently
associated with satellite dwarf galaxies. It is possible that a
significant fraction of the ancient satellite dwarf galaxies were
completely disrupted by the tidal field of the Milky Way while the high
density of the globular clusters allowed them to survive in our galaxy’s
halo. Indeed, Kim 2 is found close to the vast polar structure of
Milky Way satellite galaxies, a disc-like region surrounding the Milky
Way where satellite galaxies and young halo clusters preferentially
congregate. A similar distribution of satellite galaxies is also found
in the neighbouring Andromeda Galaxy
A large fraction of the Milky Way’s halo is thought to be populated with
optically elusive satellite galaxies and star clusters. New discoveries
of satellite galaxies and globular clusters will therefore provide
valuable information about the formation and the structure of the Milky
Way. Previous surveys like the Sloan Digital Sky Survey have contributed
to many new discoveries in the northern sky. However, most of the
southern sky still remains unexplored to date. The detection of Kim 2
suggests that there are a substantial number of interesting astronomical
objects waiting to be discovered in the southern hemisphere and the
Stromlo Milky Way Satellite Survey team plans to continue searching for
them.
The team's paper, accepted for publication in the Astrophysical Journal, is available as a preprint at http://arxiv.org/abs/1502.03952.
* The Stromlo Milky Way Satellite Survey is led by Australian National
University’s Associate Professor Helmut Jerjen. The research team
includes Dongwon Kim, Antonino Milone, Dougal Mackey, and Gary Da Costa
(all from the Australian National University). See project website at: http://www.mso.anu.edu.au/~jerjen/SMS_Survey.html
-
Peter Michaud
Public Information and Outreach
Gemini Observatory, Hilo, HI
Email: pmichaud@gemini.edu
Cell: (808) 936-6643
- Dongwon Kim
Australia National University
Email: dongwon.kim@anu.edu.au
Office: +61 2 6125 8022
- Helmut Jerjen
Australia National University
Email: helmut.jerjen@anu.edu.au
Office: +61 2 6125 8038
Source: Gemini Observatory
