Gemini Images Galaxy That Is 99.99 Percent Dark Matter

The dark galaxy Dragonfly 44. The image on the left is a wide view of the galaxy taken with the Gemini North telescope using the Gemini Multi-Object Spectrograph (GMOS). The close-up on the right is from the same very deep image, revealing the large, elongated galaxy, and halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy. Dragonfly 44 is very faint for its mass, and consists almost entirely of Dark Matter. Credit: Pieter van Dokkum, Roberto Abraham, Gemini, Sloan Digital Sky Survey. PNG image

MAUNAKEA, Hawaii — Using the world's most powerful telescopes, an international team of astronomers has discovered a massive galaxy that consists almost entirely of Dark Matter. Using the W. M. Keck Observatory and the Gemini North telescope – both on Maunakea, Hawaii – the team found a galaxy whose mass is almost entirely Dark Matter. The findings are being published in The Astrophysical Journal Letters today.

Even though it is relatively nearby, the galaxy, named Dragonfly 44, had been missed by astronomers for decades because it is very dim. It was discovered just last year when the Dragonfly Telephoto Array observed a region of the sky in the constellation Coma. Upon further scrutiny, the team realized the galaxy had to have more than meets the eye: it has so few stars that it quickly would be ripped apart unless something was holding it together.

To determine the amount of Dark Matter in Dragonfly 44, astronomers used the DEIMOS instrument installed on Keck II to measure the velocities of stars for 33.5 hours over a period of six nights so they could determine the galaxy’s mass. The team then used the Gemini Multi-Object Spectrograph (GMOS) on the 8-meter Gemini North telescope on Maunakea in Hawaii to reveal a halo of spherical clusters of stars around the galaxy’s core, similar to the halo that surrounds our Milky Way Galaxy.

“Motions of the stars tell you how much matter there is, van Dokkum said. “They don’t care what form the matter is, they just tell you that it’s there. In the Dragonfly galaxy stars move very fast. So there was a huge discrepancy: using Keck Observatory, we found many times more mass indicated by the motions of the stars, then there is mass in the stars themselves.”

The mass of the galaxy is estimated to be a trillion times the mass of the Sun – very similar to the mass of our own Milky Way galaxy. However, only one hundredth of one percent of that is in the form of stars and "normal" matter; the other 99.99 percent is in the form of dark matter. The Milky Way has more than a hundred times more stars than Dragonfly 44.

Finding a galaxy with the mass of the Milky Way that is almost entirely dark was unexpected. "We have no idea how galaxies like Dragonfly 44 could have formed,” Roberto Abraham, a co-author of the study, said. "The Gemini data show that a relatively large fraction of the stars is in the form of very compact clusters, and that is probably an important clue. But at the moment we're just guessing."

“This has big implications for the study of Dark Matter,” van Dokkum said. “It helps to have objects that are almost entirely made of Dark Matter so we don’t get confused by stars and all the other things that galaxies have. The only such galaxies we had to study before were tiny. This finding opens up a whole new class of massive objects that we can study.

“Ultimately what we really want to learn is what Dark Matter is,” van Dokkum said. “The race is on to find massive dark galaxies that are even closer to us than Dragonfly 44, so we can look for feeble signals that may reveal a Dark Matter particle.”

Additional co-authors are Shany Danieli, Allison Merritt, and Lamiya Mowla of Yale, Jean Brodie of the University of California Observatories, Charlie Conroy of Harvard, Aaron Romanowsky of San Jose State University, and Jielai Zhang of the University of Toronto.

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 Maunakea 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.

DEIMOS (DEep Imaging Multi-Object Spetrograph) boasts the largest field of view (16.7 arcmin by 5 arcmin) of any of the Keck Observatory 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 NASA.

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Science Contact:

Pieter van Dokkum
Yale University
New Haven, Connecticut, USA
Tel: +1-203-432-3000
E-mail: pieter.vandokkum@yale.edu 

Media Contact:

Steve Jefferson
W. M. Keck Observatory
(808) 881-3827
sjefferson@keck.hawaii.edu

Source:   W.M. Keck Observatory

Scientists at Keck Discover the Fluffiest Galaxies

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