(Left)
The ultra-diffuse galaxy is rich with globular clusters, which hold the
key to understanding this mysterious object’s origin and mass.
(Right) A closer look at one of the globular clusters within the galaxy,
which are all much brighter than typically seen, with the brightest
emitting almost as much light as the brightest globular cluster within
the Milky Way. The spectrum, obtained by Keck Observatory, shows the
calcium absorption lines used to determine the velocity of this object.
10 clusters were observed, providing the information needed to determine
the mass of the galaxy, revealing its lack of dark matter. Credit: W.M. Keck Observatory/Gemini Observatory/NSF/AURA/J.Miller/J.Pollard
Maunakea, Hawaii – Galaxies and
dark matter go hand in hand; you typically don’t find one without the other. So
when researchers uncovered a galaxy, known as NGC1052-DF2, that is almost
completely devoid of the stuff, they were shocked.
“Finding a galaxy without
dark matter is unexpected because this invisible, mysterious substance is the
most dominant aspect of any galaxy,” said lead author Pieter van Dokkum of Yale
University. “For decades, we thought that galaxies start their lives as blobs
of dark matter. After that everything else happens: gas falls into the dark
matter halos, the gas turns into stars, they slowly build up, then you end up
with galaxies like the Milky Way. NGC1052-DF2 challenges the standard ideas of
how we think galaxies form.”
The research, published in the March
29th issue of the journal Nature, amassed data from Gemini North and W. M. Keck
Observatory, both on Maunakea, Hawaii, the Hubble Space Telescope, and other
telescopes around the world.
Given its large size and
faint appearance, astronomers classify NGC1052-DF2 as an ultra-diffuse galaxy,
a relatively new type of galaxy that was first discovered in 2015. These barely
visible galaxies are surprisingly common. However, no other galaxy of this type
yet-discovered is so lacking in dark matter.
“NGC 1052-DF2 is an oddity,
even among this unusual class of galaxy,” said Shany Danieli, a Yale University
graduate student on the team.
Van Dokkum and his team
first spotted NGC1052-DF2 with the Dragonfly Telephoto Array, a custom-built
telescope in New Mexico that they designed to find these ghostly galaxies.
NGC1052-DF2 stood out in stark contrast when comparisons were made between
images from the Dragonfly Telephoto Array and the Sloan Digital Sky Survey
(SDSS). The Dragonfly images show a faint “blob-like” object, while SDSS data
reveal a collection of relatively bright point-like sources.
To get a closer look at this
inconsistency, the team dissected the light from several of the bright sources
within NGC1052-DF2 using Keck Observatory’s Deep Imaging Multi-Object
Spectrograph (DEIMOS) and Low-Resolution Imaging Spectrometer (LRIS),
identifying 10 globular clusters. These clusters are large compact groups of
stars that orbit the galactic core.
The spectral data obtained
on the Keck telescopes revealed that the globular clusters were moving much
slower than expected. The slower the objects in a system move, the less mass
there is in that system. The team’s calculations show that all of the mass in
the galaxy could be attributed to the mass of the stars, which means there is
almost no dark matter in NGC1052-DF2.
“If there is any dark matter
at all, it’s very little,” van Dokkum explained. “The stars in the galaxy can
account for all of the mass, and there doesn’t seem to be any room for dark
matter.”
“Keck is in a very small
group of telescopes that could even attempt these observations, because you
need a large telescope to measure these accurate velocities,” van Dokkum added.
“Keck also has some of the best spectrographs in the world for measuring the
velocities of faint objects. We had the opportunity to check and make sure we
got the same result within the uncertainties, and that gave us confidence that
we were doing things right.”
To peer even deeper into
this unique galaxy, the team used the Gemini-North Multi Object Spectrograph
(GMOS) to capture detailed images of NGC1052-DF2, assess its structure, and
confirm that the galaxy had no signs of interactions with other galaxies.
“Without the Gemini images
dissecting the galaxy’s morphology we would have lacked context for the rest of
the data,” said Danieli. “Also, Gemini’s confirmation that NGC1052-DF2 is not
currently interacting with another galaxy will help us answer questions about
the conditions surrounding its birth.”
The team’s results demonstrate
that dark matter is separable from galaxies.
“This discovery shows that
dark matter is real – it has its own separate existence apart from other
components of galaxies,” said van Dokkum.
NGC1052-DF2’s globular
clusters and atypical structure has perplexed astronomers aiming to determine
the conditions this galaxy formed under.
“It’s like you take a galaxy
and you only have the stellar halo and globular clusters, and it somehow forgot
to make everything else,” van Dokkum said. “There is no theory that predicted
these types of galaxies. The galaxy is a complete mystery, as everything about
it is strange. How you actually go about forming one of these things is
completely unknown.”
However, researchers do have
some ideas. NGC1052-DF2 resides about 65 million light years away in a
collection of galaxies that is dominated by the giant elliptical galaxy NGC
1052. Galaxy formation is turbulent and violent, and van Dokkum suggests that
the growth of the fledgling massive galaxy billions of years ago perhaps played
a role in NGC1052-DF2’s dark-matter deficiency.
Another idea is that a cataclysmic event within the oddball galaxy, such as the
birth of myriad massive stars, swept out all the gas and dark matter, halting
star formation.
These possibilities are
speculative, however, and don’t explain all of the characteristics of the
observed galaxy, the researchers said.
The team continues the hunt for more dark-matter-deficient galaxies. They are
analyzing Hubble images of 23 other diffuse galaxies. Three of them appear to
share similarities with NGC1052-DF2, which van Dokkum plans to follow up on in
the coming months at Keck Observatory.
“Every galaxy we knew about before has dark matter and they all fall in
familiar categories like spiral or elliptical galaxies,” van Dokkum said. “But
what would you get if there were no dark matter at all? Maybe this is what you
would get.”
Science Contacts
Source: W.M. Keck Observatory
Science Contacts
Astronomy Department
Yale University
Email: pieter.vandokkum@yale.edu
Phone: (203) 432-5048
Shany Danieli
Astronomy Department
Yale University
Phone: (857) 919-3674
Email: shany.danieli@yale.edu
Media Contacts
Mari-Ela Chock
W. M. Keck Observatory
Email: mchock@keck.hawaii.edu
Phone: (808) 554-0567
Jasmin Silva
Gemini Observatory
Email: jsilva@gemini.edu
Desk: (808) 974-2575
Cell: (808) 989-7418
About Deimos
The DEep
Imaging and Multi-Object Spectrograph (DEIMOS) boasts the largest field of view
(16.7arcmin 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. Support for
DEIMOS was generously provided by the National Science Foundation.
About W.M.Keck Observatory
The W. M. Keck Observatory telescopes are
among the most scientifically productive on Earth. The two, 10-meter
optical/infrared telescopes on 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 spectrometers, and
world-leading laser guide star adaptive optics systems.
Some of the data presented herein were
obtained at Keck Observatory, which is a private 501(c) 3 non-profit organization
operated as a scientific partnership among the California Institute of
Technology, the University of California, and the National Aeronautics and
Space Administration.
The Observatory was made possible by the generous
financial support of the W. M. Keck Foundation.
The authors wish to
recognize and acknowledge the very significant cultural role and reverence that
the summit of Maunakea has always had within the indigenous Hawaiian
community. We are most fortunate to have the opportunity to conduct
observations from this mountain.
Article Summary
Astronomers using W. M. Keck
Observatory and Gemini data in Hawai‘i have encountered a galaxy that appears to have
almost no dark matter. Since the Universe is dominated by dark matter, and it
is the foundation upon which galaxies are built, “...this is a game changer,”
according to Principal Investigator Pieter van Dokkum of Yale University.
