The DESI Legacy Imaging Surveys (http://legacysurvey.org/) 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 ≈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 μm) observed by the Wide-field Infrared Survey Explorer 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 H I mass function (HiMF) for galaxies in the local universe is constructed from the results of the Arecibo H I Strip Survey, a blind extragalactic survey in the 21cm line. The survey consists of two strips covering in total ∼ 65 square degrees of sky, with a depth of cz = 7400 km s −1 and was optimized to detect column densities of neutral gas N HI > 10 18 cm −2 (5σ). The survey yielded 66 significant extragalactic signals of which approximately 50% are cataloged galaxies. No free floating H I clouds without stars are found. VLA follow-up observations of all signals have been used to obtain better measurements of the positions and fluxes and allow an alternate determination of the achieved survey sensitivity. The resulting HiMF has a shallow faint end slope (α ≈ 1.2), and is consistent with earlier estimates computed for the population of optically selected gas rich galaxies. This implies that there is not a large population of gas rich low luminosity or low surface brightness galaxies that has gone unnoticed by optical surveys. The influence of large scale structure on the determination of the HiMF from the Arecibo H I Strip Survey is tested by numerical experiments and was not found to affect the resulting HiMF significantly. The cosmological mass density of H I at the present time determined from the survey, Ω HI (z = 0) = (2.0 ± 0.5) × 10 −4 h −1 , is in good agreement with earlier estimates. We determine lower limits to the average 1 current address: Qualcomm Inc., 6455 Lusk Blvd., San Diego, CA 92121 -2column densities N HI of the galaxies detected in the survey and find that none of the galaxies have N HI < 10 19.7 cm −2 , although there are no observational selection criteria against finding lower density systems. Eight percent of the signals detected in the original survey originated in groups of galaxies, whose signals chanced to coincide in frequency.

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We present a luminosity function for low surface brightness (LSB) galaxies identified in the APM survey of Impey et al. (1996). These galaxies have central surface brightnesses (µ(0)) in B in the range 22.0 ≤ mu(0) ≤ 25.0. Using standard maximum-likelihood estimators, we determine that the best-fit Schechter function parameters for this luminosity function (LF) are α = −1.42, M * = −18.34, and φ * = 0.0036, assuming H 0 = 100 h 100 km s −1 Mpc −1 . We compare the luminosity and number densities derived from this luminosity function to those obtained from other recent field galaxy studies and find that surveys which do not take account of the observation selection bias imposed by surface brightness are missing a substantial fraction of the galaxies in the local universe. Under our most conservative estimates, our derivation of the LF for LSB galaxies suggests that the CfA redshift survey has missed at least one third of the local galaxy population. This overlooked fraction is not enough by itself to explain the large number of faint blue galaxies observed at moderate redshift under no-evolution models, but it does help close the gap between local and moderate-redshift galaxy counts.