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TheUmbrella Galaxytakes its name from a mysterious feature seen on the
left here, that is now found to be debris from a tiny galaxy, only a 50th
its size, shredded apart by gravity. The image is a combination of data
from the 0.5-meter BlackBird Remote Observatory Telescope and Suprime-Cam
on the 8-meter Subaru Telescope.
The inset shows a small cluster of stars embedded in the stream, which
marks the center of the disrupted galaxy. Credit: R. Jay GaBany.
three-dimensional, rotating computer model of the Umbrella galaxy, the
disk of the main galaxy is shown by blue circles. The path of the dwarf
galaxy through space is shown by a green curve. The white dots show
stars that once belonged to the dwarf galaxy but have now been ripped
off by tidal forces into a long stream of stars.Credit: N. Singh/UCSC
MAUNA KEA, HAWAII – Scientists studying a ‘twin’ of
the Milky Way have used the W. M. Keck Observatory and Subaru
Observatory to accurately model how it is swallowing another, smaller
galaxy. Their findings have opened the way to a better understanding of
how structure forms in the universe and are being published in the
Monthly Notices of the Royal Astronomical Society this week.
work, led by Caroline Foster of the Australian Astronomical Observatory,
has used the Umbrella (NGC 4651) galaxy to reveal insights in galactic
The Umbrella lies 62 million light-years away, in the
northern constellation of Coma Berenices. Its faint parasol is composed
of a stellar stream, thought to be the remnants of a smaller galaxy
being pulled apart by the large galaxy’s intense gravitational field.
The Umbrella will eventually absorb this small galaxy completely.
merging of small galaxies into larger ones is common throughout the
universe, but because the shredded galaxies are so faint it has been
hard to extract details in three-dimensions about how such mergers
Using the most powerful optical facilities in the world, the
twin, 10-meter Keck Observatory and the 8-meter Subaru Telescope, near
the summit of Mauna Kea, Foster and her collaborators have determined
enough about the character of the merger to provide a detailed model of
how and when it occurred.
After taking panoramic images of the
Umbrella with Suprime-Cam on Subaru, the scientists used the DEIMOS
instrument, installed on the Keck II telescope, to map out the motions
of the stream and hence determine how the galaxy is being shredded.
stars in the stream are incredibly faint, so it was necessary to use a
proxy technique to measure the speeds of brighter tracer objects moving
along with the stream stars. These bright tracers include globular star
clusters, planetary nebulae (dying stars that glow like neon lights),
and patches of glowing hydrogen gas.
“This is important because
our whole concept about what galaxies are and how they grow has not been
fully verified,” said co-author Aaron Romanowsky, an astronomer at both
San José State University and University of California Observatories.
“We think they are constantly consuming smaller galaxies as part of a
cosmic food chain, all pulled together by a mysterious form of invisible
‘dark matter’. When a galaxy is torn apart, we sometimes get a glimpse
of the hidden vista because the stripping process lights it up. That’s
what occurred here.”
“Through new techniques we have been able to
measure the movements of the stars in the very distant, very faint,
stellar stream in the Umbrella,” Foster said. “This allows us, for the
first time, to reconstruct the history of the system.”
to study streams this far away means that we can reconstruct the
assembly histories of many more galaxies," Romanowsky said. “In turn
that means we can get a handle on how often these ‘minor mergers’ —
thought to be an important way that galaxies grow — actually occur. We
can also map out the orbits of the stellar streams to test the pull of
gravity for exotic effects, much like the Moon going around the Earth
but without having to wait 300 million years for the orbit to complete.”
present work is a follow-up to a 2010 study, led by Dr. David
Martínez-Delgado (University of Heidelberg), which used small robotic
telescopes to image eight isolated spiral galaxies, and found the signs
of mergers — shells, clouds and arcs of tidal debris — in six of them.
W. M. Keck Observatory operates the largest, most scientifically
productive telescopes on Earth. The two, 10-meter optical/infrared
telescopes on the summit of Mauna Kea on the Island of Hawaii feature a
suite of advanced instruments including imagers, multi-object
spectrographs, high-resolution spectrographs, integral-field
spectroscopy and world-leading laser guide star adaptive optics
DEIMOS (the DEep Imaging and Multi-Object Spectrograph)
boasts the largest field of view (16.7 arcmin by 5 arcmin) of any of the
Keck instruments, and the largest number of pixels (64 Mpix). It is
used primarily in its multi-object mode, obtaining simultaneous spectra
of up to 130 galaxies or stars. Astronomers study fields of distant
galaxies with DEIMOS, efficiently probing the most distant corners of
the universe with high sensitivity.
Keck Observatory is a private
501(c)3 non-profit organization and a scientific partnership of the
California Institute of Technology, the University of California and