From 2013 to 2019, the Dark Energy Survey (DES) used DECam on the 4-meter Blanco Telescope at Cerro Tololo Inter-American Observatory in Chile to measure the shapes of more than 150 million galaxies across about 5,000 square degrees of sky, roughly one-eighth of the full sky. These observations provided key constraints on the distribution of mass and the properties of dark energy, and highlighted mild inconsistencies between measurements in the nearby universe from galaxy surveys and early-universe constraints from the cosmic microwave background (CMB).
A new UChicago-led effort, described in a series of papers in The Open Journal of Astrophysics, roughly doubles the number of galaxies with measured shapes by analyzing DECam observations taken outside the DES footprint. The project, called the Dark Energy Camera All Data Everywhere (DECADE) cosmic shear project, repurposes archival images that were originally collected for a wide range of science goals and not specifically for weak lensing, creating an independent galaxy shape catalog over thousands of square degrees.
Gravitational lensing, in which mass along the line of sight deflects light, is a key tool for mapping matter in the universe, including dark matter. In the weak lensing regime, the shapes of individual background galaxies are only slightly distorted, so the signal emerges statistically from very large samples. Chihway Chang, associate professor of Astronomy and Astrophysics at UChicago and lead of the DECADE weak lensing project, noted that these measurements track how matter clumps over cosmic time and across different regions of the sky.
"Weak lensing measurements are best at probing the 'clumpiness' of matter," said Dhayaa Anbajagane, a PhD student in Astronomy and Astrophysics who is lead analyst and first author on the DECADE papers. "Quantifying this clumpiness sheds light on the origin and evolution of structures like galaxies and galaxy clusters. This is loosely akin to measuring the distribution of people (the matter) living across a region and using that to understand features such as the landscape's topography or the location or age of urban areas (factors that influence the origin and evolution of structures)."
For the DECADE project, the team measured shapes for more than 100 million galaxies and estimated their distances using redshift, which indicates how much each galaxy's light has shifted toward longer wavelengths as the universe expands. The redshift measurements give recession speeds and allow the researchers to infer distances from Earth, which, combined with the shape information, enables a three-dimensional view of how matter is distributed.
Using these shapes and distances, the team fit the LCDM cosmological model, which includes dark energy, dark matter, ordinary matter, neutrinos, and radiation, to test whether the observed growth of structure matches theoretical expectations. Chang noted that this model has undergone many tests over the past decade and that the new DECADE data add another independent check on its validity.
The DECADE analysis finds that the growth of structure inferred from weak lensing is consistent with predictions of the LCDM model and with previous lensing studies. "In addition, when comparing our constraints with those derived and extrapolated from the early universe's CMB, we also agree well," said Chang. "This last point has been a source of debate over the past five or so years, and with our new results, we can say that we do not see tension between weak lensing and CMB."
By combining DECADE lensing measurements with those from DES, the collaboration assembled a galaxy lensing data set that uses about 270 million galaxies over roughly 13,000 square degrees of sky, close to one-third of the full sky. "We are also able to combine the DECADE lensing measurements with those of DES, resulting in a galaxy lensing analysis that uses the largest number of galaxies (270 million) covering the widest patch of sky (13,000 square degrees) to date," said Anbajagane. "Given this large amount of data, we can make particularly conservative choices in our analysis - such as only making or using the measurements we trust most, rather than all useful or possible measurements - and still make a measurement with enough precision to meaningfully inform our comparisons with the CMB."
The DECADE project provides an independent test of the consistency between CMB and weak lensing measurements on a sky region that does not overlap DES, using data that were not originally optimized for cosmology. Alex Drlica-Wagner, a Fermilab scientist and UChicago associate professor who led the DECADE observing campaign, said that it was not clear at the outset that the DECADE dataset would be suitable for cosmological analysis but that the results demonstrate robust performance. "It was not clear that the DECADE dataset would be of sufficient quality to perform a cosmological analysis, but we have shown that it can indeed produce robust results," he added.
"One unique result from this work has to do with choices we make on image quality," said Anbajagane. Conventional weak lensing-focused surveys typically collect tens of thousands of dedicated images over many years and discard a significant fraction that do not meet strict quality thresholds. "The DECADE project is unique as it repurposes archival data - images originally taken by the astronomy community for a wide variety of science goals, from studying dwarf galaxies to stars to distant galaxy clusters - and uses significantly more permissive criteria for image quality. Our work shows robust lensing analyses can be done even if we do not have lensing-dedicated imaging campaigns," he said.
This strategy could influence how astronomers design and use future weak lensing analyses, including those from the Vera C. Rubin Legacy Survey of Space and Time (Rubin LSST). Surveys may be able to include a larger fraction of their images than originally planned, which would improve the precision of lensing-based cosmological measurements. The DECADE team's ability to rely on archival DECam images depended on careful visual inspection of the data, a task led by PhD student Chin Yi Tan, to ensure that the images met the standards needed for reliable cosmic shear measurements.
The completed DECADE plus DES catalog, now available to the community, spans roughly one-third of the sky and contains 270 million galaxies, and scientists are already using it for additional studies. Researchers have applied the imaging to investigations of dwarf galaxies and to mapping mass on large scales, while the UChicago team and collaborators at the Kavli Institute for Cosmological Physics are developing further analysis methods that will use the same data set to address other questions in cosmology.
Scientists from the University of Chicago, Fermilab, the National Center for Supercomputing Applications at the University of Illinois Urbana-Champaign, Argonne National Laboratory, the University of Wisconsin-Madison, and institutions worldwide contributed to the DECADE analysis. Chang noted that having many of the project's components located close together on campus strengthened collaboration and accelerated progress, and that the interactions among the different groups led to outcomes that went beyond the project's initial goals.
Research Report:The DECADE cosmic shear project I: A new weak lensing shape catalog of 107 million galaxies
Related Links
University of Chicago
Stellar Chemistry, The Universe And All Within It
| Subscribe Free To Our Daily Newsletters |
| Subscribe Free To Our Daily Newsletters |
