Research Byte

Published in the RSAA Lunations
Vol1 Issue12 1–31 January 2021

After almost two decades since it was first envisaged, and seven years of collecting data, both in Chile and Australia, the Dark Energy Survey (DES) has made public its second data release consisting of images covering 5000 sq. degrees in five broadband filters.

DES started collecting data in 2012 using DECam, a wide-field imager covering almost 4 sq. degrees on the 4-metre Victor M. Blanco Telescope at Cerro Tololo in northern Chile. At the same time, targets observed with DECam were followed spectroscopically with the AAT through a program called OzDES. Both programs ran concurrently and ended in January 2019, although the AAT is still following some DES targets. 

The second data release includes reduced images and co-add source catalogues from the six years of full science operations (2013-2018). There are almost 700 million objects down to 24th magnitude in the catalogues. The median image quality is 0.9”, the photometric accuracy is 10 mmag, and the internal astrometric precision is 27 mas.

The main aim of DES is to constrain the properties of dark energy by measuring the expansion history of Universe and the growth of structure using four astronomical probes: weak gravitational lensing; galaxy clusters; large scale structure; and type Ia supernovae. Results from the first year of data were published in 2019, and results from the first three years of data will be published this year. Excitingly, there is tension with the results from other probes, indicating either that the concordance model (a flat universe in which there is ordinary matter, dark matter and dark energy) is wrong (or incomplete), or we have not fully accounted for sources of systematic uncertainty in the analysis. 

The analysis of the full survey is currently underway and includes a sample of 2,000 distant type Ia supernova, the largest sample of its kind. RSAA PhD scholars Georgie Taylor Patrick Armstrong are working with these data as part of their theses.

The data from DES are used for other science goals. A good example is the DES reverberation mapping program, which has so far recovered time lags between the accretion disk and the broad line region in AGN for around a hundred AGN, increasing by factors of several the number of AGN for which such measurements are available. RSAA PhD scholar Umang Malik is leading this work.    

DES was only possible through the efforts of hundreds of scientists in dozens of countries over two decades. A photo showing the people involved in DES is shown below. They blazed the trail for future surveys that combine wide field imaging with wide field, multi-object spectroscopy.

Chris Lidman

^{Picture: The group photo at the 2015 DES collaboration at the University of Michigan}