“Beyond the Edge of the Milky Way” –Dark Energy Probe Hints at an Enormous Invisible Halo of Dark Matter

Galaxy NGC 474


One early result of the ongoing Dark Energy Survey is the previously untold story revealed by old, giant RR Lyrae pulsating stars, which tell scientists about the region of space beyond the edge of our Milky Way. In this area nearly devoid of stars–the motion of the RR Lyrae stars hints at the presence of an enormous halo of invisible dark matter, which may provide clues to how our galaxy evolved over the last 12 billion years.

One of the Biggest Mysteries in Science

Dark matter, one of the enduring mysteries in science, is “dark” in the sense that it doesn’t emit radiation or hardly interact with anything except via its gravitational attraction. Dark matter may be responsible for some of the observed gravitational waves signals, and seeded supermassive black holes found in the center of the Milky Way and other galaxies, according to a recent study by the Kavli Institute for the Physics and Mathematics of the Universe.

This new discovery was revealed by new The Dark Energy Survey data (DR2) –a massive, public collection of astronomical data and calibrated images from six years of work. This second data release in the Survey’s seven-year history, contains data on nearly 700 million astronomical objects.

“Dark Energy Originates from a Vast Sea of Objects Spread Throughout Cosmic Voids”

Goal –Understanding the Phenomenon of Dark Energy

DR2 is the second release of images and object catalogs from the Dark Energy Survey (DES). It is the culmination of over a half-decade of astronomical data collection and analysis, with the ultimate goal of understanding the accelerating expansion rate of the universe and the phenomenon of dark energy that is thought to be responsible for the expansion. The Dark Energy Survey is a global collaboration that includes Fermi National Accelerator Laboratory (Fermilab), National Center for Supercomputing Applications (NCSA), and NSF’s NOIRLab.

Including a catalog of nearly 700 million astronomical objects, DR2 builds on the 400 million objects cataloged with the survey’s previous data release (DR1), and also improves on it by refining calibration techniques, which, with the deeper combined images from DR2, leads to improved estimates of the amount and distribution of matter in the universe. It is one of the largest astronomical catalogs released to date.

Map of Milky Way’s Dwarf Satellites

In another result, DES scientists used the extensive DR2 galaxy catalog, along with data from the LIGO gravitational wave experiment, to estimate the location of a black hole merger and, independent of other techniques, infer the value of the Hubble constant, a key cosmological parameter. Combining their data with other surveys, DES scientists have also been able to generate a detailed map of the Milky Way’s dwarf satellites, giving researchers insight into how our own galaxy was assembled and how it compares with cosmologists’ predictions.


 Dark Energy Camera (DECam)


The Dark Energy Camera (DECam) is mounted on the Víctor M. Blanco 4-meter Telescope at the Cerro Tololo Inter-American Observatory (CTIO) in north central Chile. Telescope construction started in 1969 with the casting of the primary mirror. The assembly at the Cerro Tololo mountaintop was finished in 1974. Upon completion of construction it was the 3rd largest telescope in the world, behind the 200″ Hale telescope at Palomar Observatory in California and the BTA-6 in southern Russia, and the largest in the Southern Hemisphere (a title that it held for 22 years). Later named in 1995 in honor of Víctor M. Blanco, Puerto Rican astronomer and former director of CTIO. (DOE/LBNL/DECam/R. Hahn/CTIO/NOIRLab/NSF/AURA)

The Dark Energy Phenomenon

In 1998, researchers found that the rate of expansion accelerated with distance, and that the universe was filled with mysterious “dark energy” that has caused the acceleration for 14 billion years—earning them a 2011 Nobel prize. DES was conceived to map hundreds of millions of galaxies and to chart the size of the expanding universe as it accelerates under the influence of dark energy. DES has produced the largest and most accurate dark matter map from galaxy weak lensing to date.

Covering 5000 square degrees of the southern sky, the survey data enable many other investigations in addition to those targeting dark energy, covering a vast range of cosmic distances—from discovering new nearby solar system objects to investigating the nature of the first star-forming galaxies in the early universe.

A Momentous Milestone

“This is a momentous milestone. For six years, the Dark Energy Survey collaboration took pictures of distant celestial objects in the night sky. Now, after carefully checking the quality and calibration of the images captured by the Dark Energy Camera, we are releasing this second batch of data to the public,” said DES Director Rich Kron of Fermilab and the University of Chicago. “We invite professional and amateur scientists alike to dig into what we consider a rich mine of gems waiting to be discovered.”

The primary tool used to collect these images, the Dark Energy Camera (DECam), is mounted on the National Science Foundation-funded Víctor M. Blanco 4-meter Telescope, part of the Cerro Tololo Inter-American Observatory (CTIO) in the Chilean Andes, a Program of NSF’s NOIRLab. Each week from 2013 to 2019, DECam collected thousands of images of the southern sky, unlocking a trove of potential cosmological insights.

Once captured, these images (and the large amount of data surrounding them) were transferred to NCSA for processing via the DES Data Management (DESDM) project. Using the Blue Waters supercomputer at NCSA, the Illinois Campus Cluster, and computational systems at Fermilab, NCSA prepares calibrated data products for research and public consumption.

It took approximately four months to process one year’s worth of data into a searchable, usable catalog. The DES DR2 is hosted at the Community Science and Data Center (CSDC), a Program of NSF’s NOIRLab. CSDC provides software systems, user services, and development initiatives to connect and support the scientific missions of NOIRLab’s telescopes, including the Blanco Telescope at CTIO.

Analyzing Enormous Data Sets

“Because astronomical data sets today are so vast, the cost of handling them is prohibitive for individual researchers or most organizations,” said Robert Nikutta, Project Scientist for Astro Data Lab at CSDC. “CSDC provides open access to big astronomical datasets like DES DR2, and the necessary tools to explore and exploit them—then all it takes is someone from the community with a clever idea to discover new and exciting science.”

The NASA APOD image at the top of the page shows multiple layers of emission that appear strangely complex and unexpected. The cause of the shells is currently unknown, but possibly tidal tails related to debris left over from absorbing numerous small galaxies in the past billion years. Alternatively the shells may be like ripples in a pond, where the ongoing collision with the spiral galaxy just above NGC 474 is causing density waves to ripple through the galactic giant. The featured image dramatically highlights the increasing consensus that at least some elliptical galaxies have formed in the recent past, and that the outer halos of most large galaxies are not really smooth but have complexities induced by frequent interactions with — and accretions of — smaller nearby galaxies. The halo of our own Milky Way Galaxy is one example of such unexpected complexity. CFHT, Coelum, MegaCam, J.-C. Cuillandre (CFHT) & G. A. Anselmi (Coelum)

Maxwell Moe, astrophysicist, NASA Einstein Fellow, University of Arizona, via AAS and AURA



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