A
collection of unidentified blobs was discovered toward the Coma cluster
of galaxies, using the Dragonfly Telephoto Array. One of these puzzling
objects, Dragonfly 44, was studied in detail using the Keck Observatory
and confirmed as an ultra-diffuse galaxy. Even though it is 60,000
light years across, It is so far away that it appears as only a faint
smudge. Credit: P. van Dokkum, R. Abraham, J. Brodie. Hi-res image
Reconstructed
spectrum of light spread out from the ultra-diffuse galaxy,
DragonFly44, as seen by the Keck/LRIS instrument. Dark bands occur where
atoms and molecules absorb the galaxy’s starlight. These bands reveal
the compositions and ages of the stars, and also the distance of the
galaxy. Credit: P. van Dokkum, A. Romanowsky, J. Brodie. Hi-res image
An
ultra-diffuse galaxy, Dragonfly 17, is shown next to other types of
galaxies, to scale. The Andromeda galaxy is a giant spiral like our own
Milky Way, and a dwarf elliptical galaxy, NGC 205, is also shown.
Ultra-diffuse galaxies have the same number of stars as dwarf
ellipticals, but spread out over a much larger region. Credit: B. Schoening, V. Harvey/REU program/NOAO/AURA/NSF, P. van Dokkum/Hubble Space Telescope. Hi-res image
Maunakea, Hawaii – An
international team of researchers led by Pieter van Dokkum at Yale University have
used the W. M. Keck Observatory to confirm the
existence of the most diffuse class of galaxies known in the
universe. These "fluffiest galaxies" are nearly as wide as
our own Milky Way galaxy – about 60,000 light years – yet harbor only one percent
as many stars.
The findings were recently published in the Astrophysical Journal Letters.
“If the Milky Way is a sea of stars, then these
newly discovered galaxies are like wisps of clouds”, said van Dokkum. “We
are beginning to form some ideas about how they were born and it’s remarkable
they have survived at all. They are found in a dense, violent region of
space filled with dark matter and galaxies whizzing around, so we think they
must be cloaked in their own invisible dark matter ‘shields’ that are
protecting them from this intergalactic assault.”
The
team made the latest discovery by combining results from one of the
world's smallest telescopes as well as the largest telescope on Earth.
The Dragonfly Telephoto Array used 14-centimeter state of the art
telephoto lens cameras to produce digital images of the very faint,
diffuse
objects. Keck Observatory’s 10-meter Keck I telescope, with its Low
Resolution
Imaging Spectrograph, then separated the light of one of
the objects into colors that
diagnose its composition and distance.
Finding
the distance was the clinching evidence. The data from Keck Observatory
showed the diffuse "blobs" are very large and very far away, about 300
million light years, rather than small and close by. The blobs can now
safely be called Ultra
Diffuse Galaxies (UDGs).
“If
there are any aliens living on a planet in an ultra-diffuse galaxy, they would
have no band of light across the sky, like our own Milky Way, to tell them they
were living in a galaxy. The night sky would be much emptier of
stars,” said team member Aaron Romanowsky, of San Jose State University.
The
UDGs were found in an area of the sky called the Coma cluster, where thousands
of galaxies have been drawn together in a mutual gravitational dance. “Our
fluffy objects add to the great diversity of galaxies that were previously known,
from giant ellipticals that outshine the Milky Way, to ultra compact dwarfs,” said
University of California, Santa Cruz Professor Jean Brodie.
“The big challenge now is to figure out where these mysterious
objects came from,” said Roberto Abraham, of the University of Toronto.
“Are they ‘failed galaxies’ that started off well and then ran out of
gas? Were they once normal galaxies that got knocked around so much inside
the Coma cluster that they puffed up? Or are they bits of galaxies
that were pulled off and then got lost in space?” The key next
step in understanding UDGs is to to pin down exactly how much dark
matter they have. Making this measurement will be even more challenging
than
the latest work.
The W. M. Keck Observatory operates the
largest, most scientifically productive telescopes on Earth. The two, 10-meter
optical/infrared telescopes near 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 spectrographs and
world-leading laser guide star adaptive optics systems.
The Low Resolution Imaging Spectrometer (LRIS) is a very
versatile visible-wavelength imaging and spectroscopy instrument commissioned
in 1993 and operating at the Cassegrain focus of Keck I. Since it has been
commissioned it has seen two major upgrades to further enhance its
capabilities: addition of a second, blue arm optimized for shorter wavelengths
of light; and the installation of detectors that are much more sensitive at the
longest (red) wavelengths. Each arm is optimized for the wavelengths it covers. This
large range of wavelength coverage, combined with the instrument's high
sensitivity, allows the study of everything from comets (which have interesting
features in the ultraviolet part of the spectrum), to the blue light from star
formation, to the red light of very distant objects. LRIS also records the
spectra of up to 50 objects simultaneously, especially useful for studies of
clusters of galaxies in the most distant reaches, and earliest times, of the
universe.
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 NASA.
Source: W.M. Keck Observatory
