Dark Energy Survey

Survey

DES imaged 5,000 square degrees of the southern sky in a footprint that overlaps with the South Pole Telescope and Stripe 82 (in large part avoiding the Milky Way). The survey took 758 observing nights spread over six annual sessions between August and February to complete, covering the survey footprint ten times in five photometric bands (g, r, i, z, and Y). The survey reached a depth of 24th magnitude in the i band over the entire survey area. Longer exposure times and faster observing cadence were made in five smaller patches totaling 30 square degrees to search for supernovae.

First light was achieved on 12 September 2012; after a verification and testing period, scientific survey observations started in August 2013. The last observing session was completed on 9 January 2019.

Other surveys using DECam

After completion of the Dark Energy Survey, the Dark Energy Camera was used for other sky surveys:

• Dark Energy Camera Legacy Survey (DECaLS) covers the sky below 32°Declination, not including the Milky Way. This survey covers over 9000 square degrees.
• The DESI Legacy Imaging Surveys (Legacy Surveys), as of data release 10, includes DECaLS, BASS and MzLS. It also incorporating additional DECam data, which means that it covers almost the entire extragalactic southern sky, including parts of the Magellanic Clouds. The purpose of the Legacy Surveys is to find targets for the Dark Energy Spectroscopic Instrument.
• Dark Energy Camera Plane Survey (DECaPS), covers the Milky Way in the southern sky.

Observing

Each year from August through February, observers will stay in dormitories on the mountain. During a weeklong period of work, observers sleep during the day and use the telescope and camera at night. There will be some DES members working at the telescope console to monitor operations while others are monitoring camera operations and data process.

For the wide-area footprint observations, DES takes roughly two minutes for each new image: The exposures are typically 90 seconds long, with another 30 seconds for readout of the camera data and slewing to point the telescope at its next target. Despite the restrictions on each exposure, the team also need to consider different sky conditions for the observations, such as moonlight and cloud cover.

In order to get better images, DES team use a computer algorithm called the "Observing Tactician" (ObsTac) to help with sequencing observations. It optimizes among different factors, such as the date and time, weather conditions, and the position of the moon. ObsTac automatically points the telescope in the best direction, and selects the exposure, using the best light filter. It also decides whether to take a wide-area or time-domain survey image, depending on whether or not the exposure will also be used for supernova searches.

Results

Cosmology

Dark Energy Group published several papers presenting their results for cosmology. Most of these cosmology results coming from its first-year data and the third-year data. Their results for cosmology were concluded with a Multi-Probe Methodology, which mainly combine the data from Galaxy-Galaxy Lensing, different shape of weak lensing, cosmic shear, galaxy clustering and photometric data set.

For the first-year data collected by DES, Dark Energy Survey Group showed the Cosmological Constraints results from Galaxy Clustering and Weak Lensing results and cosmic shear measurement. With Galaxy Clustering and Weak Lensing results, ${\displaystyle S_8={\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}={0.773}_{-0.020}^{+0.026}}$ and ${\displaystyle {\mathrm{\Omega }}_m={0.267}_{-0.017}^{+0.030}}$ for ΛCDM, ${\displaystyle S_8={0.782}_{-0.024}^{+0.036}}$, ${\displaystyle {\mathrm{\Omega }}_m={0.284}_{-0.030}^{+0.033}}$ and ${\displaystyle \omega =-{0.82}_{-0.20}^{+0.21}}$ at 68% confidence limits for ωCMD. Combine the most significant measurements of cosmic shear in a galaxy survey, Dark Energy Survey Group showed that ${\displaystyle {\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}={0.782}_{-0.027}^{+0.027}}$ at 68% confidence limits and ${\displaystyle {\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}={0.777}_{-0.038}^{+0.036}}$ for ΛCDM with ${\displaystyle \omega =-{0.95}_{-0.36}^{+0.33}}$. Other cosmological analyses from first year data showed a derivation and validation of redshift distribution estimates and their uncertainties for the galaxies used as weak lensing sources. The DES team also published a paper summarize all the Photometric Data Set for Cosmology for their first-year data.

For the third-year data collected by DES, they updated the Cosmological Constraints to ${\displaystyle {\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}={0.759}_{-0.025}^{+0.023}}$ for the ΛCDM model with the new cosmic shear measurements. From third-year data of Galaxy Clustering and Weak Lensing results, DES updated the Cosmological Constraints to ${\displaystyle S_8={\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}={0.776}_{-0.017}^{+0.017}}$ and ${\displaystyle {\mathrm{\Omega }}_m={0.339}_{-0.031}^{+0.032}}$ in ΛCDM at 68% confidence limits, ${\displaystyle S_8={\sigma }_8({\mathrm{\Omega }}_m/0.3{)}^{0.5}={0.775}_{-0.024}^{+0.026}}$, ${\displaystyle {\mathrm{\Omega }}_m={0.352}_{-0.041}^{+0.035}}$ and ${\displaystyle \omega =-{0.98}_{-0.20}^{+0.32}}$ in ωCDM at 68% confidence limits. Similarly, the DES team published their third-year observations for photometric data set for cosmology comprising nearly 5000 deg2 of grizY imaging in the south Galactic cap, including nearly 390 million objects, with depth reaching S/N ~ 10 for extended objects up to ${\displaystyle i_{AB}}$ ~ 23.0, and top-of-the-atmosphere photometric uniformity < 3mmag.

Weak lensing

Weak lensing was measured statistically by measuring the shear-shear correlation function, a two-point function, or its Fourier Transform, the shear power spectrum. In April 2015, the Dark Energy Survey released mass maps using cosmic shear measurements of about 2 million galaxies from the science verification data between August 2012 and February 2013. In 2021 weak lensing was used to map the dark matter in a region of the southern hemisphere sky, in 2022 together with galaxy clustering data to give new cosmological constrains. and in 2023 with data from the Planck telescope and South Pole telescope to give once new improved constraints.

Another big part of weak lensing result is to calibrate the redshift of the source galaxies. In December 2020 and June 2021, DES team published two papers showing their results about using weak lensing to calibrate the redshift of the source galaxies in order to mapping the matter density field with gravitational lensing.

Gravitational waves

After LIGO detected the first gravitational wave signal from GW170817, DES made follow-up observations of GW170817 using DECam. With DECam independent discovery of the optical source, DES team establish its association with GW170817 by showing that none of the 1500 other sources found within the event localization region could plausibly be associated with the event. DES team monitored the source for over two weeks and provide the light curve data as a machine-readable file. From the observation data set, DES concluded that the optical counterpart they have identified near NGC 4993 is associated with GW170817. This discovery ushers in the era of multi-messenger astronomy with gravitational waves and demonstrates the power of DECam to identify the optical counterparts of gravitational-wave sources.

Dwarf galaxies

In March 2015, two teams released their discoveries of several new potential dwarf galaxy candidates found in Year 1 DES data. In August 2015, the Dark Energy Survey team announced the discovery of eight additional candidates in Year 2 DES data. Later on, Dark Energy Survey team found more dwarf galaxies. With more Dwarf Galaxy results, the team was able to take a deep look about more properties of the detected Dwarf Galaxy such as the chemical abundance, the structure of stellar population, and Stellar Kinematics and Metallicities. In Feb 2019, the team also discovered a sixth star cluster in the Fornax Dwarf Spheroidal Galaxy and a tidally Disrupted Ultra-Faint Dwarf Galaxy.

Baryon acoustic oscillations

The signature of baryon acoustic oscillations (BAO) can be observed in the distribution of tracers of the matter density field and used to measure the expansion history of the Universe. BAO can also be measured using purely photometric data, though at less significance. DES team observation samples consists of 7 million galaxies distributed over a footprint of 4100 deg² with 0.6 < z_photo < 1.1 and a typical redshift uncertainty of 0.03(1+z). From their statistics, they combine the likelihoods derived from angular correlations and spherical harmonics to constrain the ratio of comoving angular diameter distance ${\displaystyle D_m(Z_{\text{eff}}=0.835)/r_d=18.92\pm 0.51}$ at the effective redshift of our sample to the sound horizon scale at the drag epoch.

Type Ia supernova observations

In May 2019, Dark Energy Survey team published their first cosmology results using Type Ia supernovae. The supernova data was from DES-SN3YR. The Dark Energy Survey team found Ωm = 0.331 ± 0.038 with a flat ΛCDM model and Ωm = 0.321 ± 0.018, w = −0.978 ± 0.059 with a flat wCDM model. Analyzing the same data from DES-SN3YR, they also found a new current Hubble constant, ${\displaystyle H_0=67.1\pm 1.3\mathrm{k}\mathrm{m}{\mathrm{s}}^{-1}\mathrm{M}\mathrm{p}{\mathrm{c}}^{-1}}$. This result has an excellent agreement with the Hubble constant measurement from Planck Satellite Collaboration in 2018. In June 2019, there a follow-up paper was published by DES team discussing the systematic uncertainties, and validation of using the supernovae to measure the cosmology results mentioned before. The team also published their photometric pipeline and light curve data in another paper published in the same month.

Minor planets

Several minor planets were discovered by DeCam in the course of The Dark Energy Survey, including high-inclination trans-Neptunian objects (TNOs).

The MPC has assigned the IAU code W84 for DeCam's observations of small Solar System bodies. As of October 2019, the MPC inconsistently credits the discovery of nine numbered minor planets, all of them trans-Neptunian objects, to either "DeCam" or "Dark Energy Survey". The list does not contain any unnumbered minor planets potentially discovered by DeCam, as discovery credits are only given upon a body's numbering, which in turn depends on a sufficiently secure orbit determination.