DESI Imaging Legacy Surveys Publish Seventh Data Release

Aaron Meisner, Arjun Dey & Stephanie Juneau (NOAO), on behalf of the DESI imaging Legacy Surveys team

The DESI imaging Legacy Surveys team announces the publication of its seventh data release (DR7), featuring source catalogs from images covering more than a quarter of the sky. The images were obtained using the Dark Energy Camera (DECam) on the 4-m Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile. This new DECam Legacy Survey (DECaLS) DR7 data set will help enable target selection for the upcoming Dark Energy Spectroscopic Instrument (DESI), while also providing a much deeper multi-purpose successor to the Sloan Digital Sky Survey imaging catalog over a similar footprint.

DECaLS DR7 consists primarily of DECam optical measurements in the g, r and z passbands. Through an innovative “forced photometry” approach, the DECaLS source catalog includes, for each source, custom mid-infrared fluxes measured from the all-sky survey conducted by the WISE satellite. The DECaLS source positions are tied to the high-fidelity astrometry provided by the Gaia spacecraft.

An image of the interacting galaxy pair UGC 6073 (also known as Arp 198) from the Legacy Surveys’ Data Release 7 Gallery. The system lies at a distance of 125 Mpc from the Milky Way in the constellation of Leo. The white horizontal scale bar represents one arcminute.

DECaLS DR7 is the result of major observational and computational efforts by the Legacy Surveys team. Beginning in 2014, more than 40 observers from 17 institutions collected roughly 120 nights of DECaLS data. In total, DECaLS DR7 processed over 40,000 DECam exposures, corresponding to approximately 70 trillion pixels of input. The entire DESI imaging data set now includes 1.2 billion unique sources, from which DESI will select approximately 30 million to study in further detail. The data were processed at the National Optical Astronomy Observatory in Tucson, Arizona and the National Energy Research Scientific Computing Center in Berkeley, California.

The Legacy Surveys team is committed to open data practices, and plans its next data release (DR8) for January 2019. DR8 will for the first time jointly process DECam data together with more northerly images from the Kitt Peak-based Mayall z-band Legacy Survey (MzLS) and Beijing-Arizona Sky Survey (BASS), filling out the entire DESI spectroscopic footprint. DESI’s footprint is currently covered by the combination of Legacy Surveys DR6 and DR7. An overview of the full DESI imaging effort is provided in Dey et al. (2018).

Potential DR7 users are encouraged to explore the data set via the Legacy Survey sky viewer, which allows for seamless interactive zooming and panning throughout the entire data set (see image below). The DECaLS DR7 gallery highlights objects drawn from Halton Arp’s Atlas of Peculiar Galaxies (see image above).

Click the image to play an animation that illustrates the online sky viewer tool as it zooms in to a cluster of galaxies, and then back out to a wider field of view. You can also view the visualization in HD.

The DR7 data products are available through [1] the Legacy Survey website; [2] the NOAO Science Archive; and [3] the NOAO Data Lab. The NOAO Science Archive provides access to both the DR7 raw and calibrated images. The NOAO Data Lab provides tools to access databases containing the catalogs. The Data Lab tools enable complex user queries and analyses of the data using a Jupyter Notebook server, a Simple Image Access (SIA) service and a TAP handle (which allows, for example, users to connect to the databases via tools such as TOPCAT). Example Jupyter Notebooks are also provided. Data Lab also provides opportunities for combined analyses using other datasets (such as Dark Energy Survey (DES) DR1, the NOAO Source Catalog (NSC), and Gaia DR2). Precomputed tables that crossmatch Legacy Surveys to other catalogs are provided by Data Lab for these purposes. User feedback can be communicated via the Data Lab Helpdesk or email (datalab@noao.edu). A comprehensive list of Legacy Surveys acknowledgments is available from Data Lab.