Results from the first data release of the Dark Energy Survey include eleven new stellar streams in the Milky Way galaxy.
Free, detailed information on 400 million astronomical objects, anybody? Just visit the website of the Dark Energy Survey (DES) – it’s there for the taking. At a special session of the 231st meeting of the American Astronomical Society in Washington, D.C., scientists presented the first data release (DR1) of the survey, containing observations that were collected between mid-2013 and early 2016. Among the preliminary results: eleven new stellar streams in the Milky Way galaxy and new constraints on cosmological parameters.
The Dark Energy Survey is carried out with the giant Dark Energy Camera (DECam) at the 4-meter Blanco Telescope of the Cerro Tololo Inter-American Observatory (CTIO) in Chile. Built at Fermilab in Chicago, DECam sports 62 sensitive CCDs with a grand total of 570 million pixels. The 4-ton camera has a huge 3-square-degree field of view. The survey’s main goal is to solve the riddle of dark energy – the mystery force behind the accelerating expansion of the universe.
During the first three years of the survey, some 39,000 exposures have been taken at five different wavelength bands, covering a whopping 5,186 square degrees – about one-eighth of the whole sky. According to DES release scientist Matias Carrasco Kind (University of Illinois at Urbana-Champaign), DR1 contains data on 310 million galaxies and 80 million stars brighter than magnitude 22.5. "This is the largest photometric dataset to date," he says.
Thanks to its impressive sensitivity, DES has almost doubled the number of known ultra-faint dwarf galaxies swarming around the Milky Way, from two dozen to more than fifty (some of the new finds still need independent confirmation). At the meeting, Alex Drlica-Wagner (Fermilab) reported a ‘strong correlation’ in sky position with the Large and Small Magellanic Clouds. "30 to 60 percent of the DES satellite galaxies may have a Magellanic origin," he says. "They may have started out as satellites of the two Milky Way companions."
Drlica-Wagner also presented the discovery of eleven new stellar streams – the tidally stretched remains of dwarf galaxies that have been swallowed by the Milky Way galaxy. "We’re carrying out galactic archeology," he says. "This will help in unraveling the evolutionary history of the Milky Way," which has grown over the eons by gobbling up small satellites. The new streams are located between 40,000 and 165,000 light-years away, and were found by selecting old, metal-poor stars on the basis of their colors.
Michael Troxel (Ohio State University) described DES’s preliminary cosmology results. His team's goal is to map the three-dimensional distribution of dark matter by studying the tiny systematic distortions (‘cosmic shear’) in the shapes of distant galaxies due to gravitational lensing.
So far, scientists have analyzed the shapes of 26 million galaxies, observed in DES’s first year of operation, mostly between declinations –40° and –60°. The result is "quite an exciting map," in Troxel’s words. "This constitutes the most significant measurement of cosmic shear in a galaxy survey to date," he says.
Troxel and his colleagues have combined the DES weak lensing results with data from other surveys on so-called baryon acoustic oscillations (BAOs – tell-tale patterns in the spatial distribution of galaxies) and with knowledge of nucleosynthesis (the formation of elements like deuterium) during the big bang. Together, these three parameters yield a value for the Hubble constant (the current expansion rate of the universe) that is completely independent from other measurements.
The new value, 67.2 kilometers per second per megaparsec, is a bit higher than the value derived from measurements of the cosmic background radiation by the European Planck mission, but it’s still substantially lower than what is obtained from observations of Cepheid variable stars and Type Ia supernovae. However, Troxel isn’t too worried about the apparent discrepancy, which, if confirmed, would point to new physics beyond the standard cosmological model. Referring to this canonical model, with dark energy in the form of a cosmological constant (Λ) and with cold dark matter, he says: ‘ΛCDM works.’
One reason for confidence among DES scientists is that other results from their survey – including observations of galaxy clusters – also appear to agree with current cosmological wisdom. After completing the five-year program later this year, the error bars will become even smaller. In the early 2020s, the Large Synoptic Survey Telescope (LSST) will go deeper than DES and cover a much larger part of the sky, while all-sky space missions like Euclid (ESA) and WFIRST (NASA) are expected to provide final answers.