Overview of the DESI Legacy Imaging Surveys (1804.08657v2)

Abstract: The DESI Legacy Imaging Surveys are a combination of three public projects (the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall z-band Legacy Survey) that will jointly image approximately 14,000 deg^2 of the extragalactic sky visible from the northern hemisphere in three optical bands (g, r, and z) using telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory. The combined survey footprint is split into two contiguous areas by the Galactic plane. The optical imaging is conducted using a unique strategy of dynamically adjusting the exposure times and pointing selection during observing that results in a survey of nearly uniform depth. In addition to calibrated images, the project is delivering a catalog, constructed by using a probabilistic inference-based approach to estimate source shapes and brightnesses. The catalog includes photometry from the grz optical bands and from four mid-infrared bands (at 3.4, 4.6, 12 and 22 micorons) observed by the Wide-field Infrared Survey Explorer (WISE) satellite during its full operational lifetime. The project plans two public data releases each year. All the software used to generate the catalogs is also released with the data. This paper provides an overview of the Legacy Surveys project.

• The paper details a coordinated survey covering 14,000 deg² with dynamic exposure adjustments and strategic targeting to ensure uniform imaging depth.
• It employs advanced multi-band imaging in the g, r, and z bands, integrated with WISE mid-infrared data, to create a comprehensive source catalog.
• The study demonstrates robust photometric and astrometric calibration using Pan-STARRS1 and Gaia DR1, enhancing DESI spectroscopic targeting for cosmological research.

An Overview of the DESI Legacy Imaging Surveys

The DESI Legacy Imaging Surveys represent a concerted effort to map the extragalactic sky across approximately 14,000 square degrees visible from the northern hemisphere. This initiative is a collaboration of three public projects: the Dark Energy Camera Legacy Survey, the Beijing-Arizona Sky Survey, and the Mayall $z$-band Legacy Survey. The surveys utilize telescopes at the Kitt Peak National Observatory and the Cerro Tololo Inter-American Observatory to cover the sky in the $g$, $r$, and $z$ optical bands. A distinctive aspect of these surveys is their strategy of dynamically adjusting exposure times and selecting observational points, thereby maintaining a survey of nearly uniform depth.

Key Elements of the Survey

1. Survey Design and Coverage:
   • The combined survey divides its coverage into two large areas separated by the Galactic plane. Survey regions have been carefully selected to avoid areas of high stellar density and high Galactic extinction. The optical survey design balances the need for coverage with an efficient observational strategy aimed at achieving an expansive footprint.
2. Data Acquisition & Depth:
   • Each survey employs a specific telescope fitted with an advanced camera. For example, DECaLS utilizes the Dark Energy Camera on the Blanco telescope to cover 9,000 deg$^2$, BASS uses the 90Prime Camera on the Bok 2.3-m telescope, and MzLS employs the Mosaic-3 camera on the Mayall 4-m telescope. The surveys deliver 5$\sigma$ detection depths in the optical bands of roughly $g=24.0$, $r=23.4$, and $z=22.5$ AB magnitudes.
3. Integration with Mid-Infrared Data:
   • Complementary mid-infrared data are provided by the WISE satellite, adding four bands: 3.4, 4.6, 12, and 22$\mu$m. The surveys use a probabilistic inference-based approach to compile a catalog of source shapes and brightnesses that incorporate WISE data, enhancing the scientific potential of the project by extending the spectral coverage.
4. Photometric and Astrometric Calibration:
   • Photometric calibration is achieved by reference to the Pan-STARRS1 survey, adjusted to align with Gaia DR1 astrometry, ensuring accurate and precise positional data that exceeds an RMS uncertainty of 95 mas.
5. Data Processing and Analysis:
   • The imaging data undergo processing using the NOAO Community Pipelines, and catalogs are generated via The Tractor software, a sophisticated modeling tool that handles multi-band direct image data in a coherent and consistent approach. The catalogs are employed in the public data releases, which occur twice annually.

Implications and Future Directions

The DESI Legacy Imaging Surveys aim to provide critical data for the Dark Energy Spectroscopic Instrument (DESI) Survey by supplying precise targeting information. This data is vital for comprehensive cosmological analyses, including understanding the spatial distribution of galaxies, quasars, and the intergalactic medium. The survey's precision and scale will enhance the measurement of cosmic structures, enabling precise constraints on cosmological parameters such as the equation of state of dark energy.

The survey also offers immense value beyond DESI, enriching multiple areas of astrophysics by enabling studies of large-scale structure, the evolution of galaxies, and providing a detailed map of the cosmic web. The results can refine models of galaxy formation and evolution through synergy with existing and future spectroscopic data.

Given the project's significant contribution toward understanding dark energy and structures in the universe, future developments could involve even deeper and broader surveys, potentially including additional bands or next-generation telescopes, to further refine cosmological models and investigate the underlying physics of dark energy with unprecedented accuracy.