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 deg² 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.

We use a large sample of galaxies from the Two Micron All Sky Survey (2MASS) and the Sloan Digital Sky Survey (SDSS) to calculate galaxy luminosity and stellar mass functions in the local universe. We estimate corrections for passband shifting and galaxy evolution, as well as present-day stellar mass-to-light (M/L) ratios, by fitting the optical-near-infrared galaxy data with simple models. Accounting for the 8% galaxy overdensity in the SDSS early data release region, the optical and near-infrared luminosity functions we construct for this sample agree with most recent literature optical and near-infrared determinations within the uncertainties. We argue that 2MASS is biased against low surface brightness galaxies and use SDSS plus our knowledge of stellar populations to estimate the true K-band luminosity function. This has a steeper faint end slope and a slightly higher overall luminosity density than the direct estimate. Furthermore, assuming a universally applicable stellar initial mass function (IMF), we find good agreement between the stellar mass function we derive from the 2MASS/SDSS data and that derived by Cole et al. The faint end slope for the stellar mass function is steeper than -1.1, reflecting the low stellar M/L ratios characteristic of low-mass galaxies. We estimate an upper limit to the stellar mass density in the local universe Ω*h = 2.0 ± 0.6 × 10-3 by assuming an IMF as rich in low-mass stars as allowed by observations of galaxy dynamics in the local universe. The stellar mass density may be lower than this value if a different IMF with fewer low-mass stars is assumed. Finally, we examine type-dependence in the optical and near-infrared luminosity functions and the stellar mass function. In agreement with previous work, we find that the characteristic luminosity or mass of early-type galaxies is larger than for later types, and the faint end slope is steeper for later types than for earlier types. Accounting for typing uncertainties, we estimate that at least half, and perhaps as much as 3/4, of the stellar mass in the universe is in early-type galaxies. As an aid to workers in the field, we present in an Appendix the relationship between model stellar M/L ratios and colors in SDSS/2MASS passbands, an updated discussion of near-infrared stellar M/L ratio estimates, and the volume-corrected distribution of g- and K-band stellar M/L ratios as a function of stellar mass.

We have used a suite of simplified spectrophotometric spiral galaxy evolution models to argue that there are substantial variations in stellar mass-to-light ratios (M/Ls) within and among galaxies, amounting to factors of between 3 and 7 in the optical, and factors of 2 in the near-infrared. Under the assumption of a universal spiral galaxy IMF, our models show a strong and robust correlation between stellar M/L and the optical colors of the integrated stellar populations. Using observed maximum disk M/Ls we conclude that a Salpeter Initial Mass Function (IMF) has too many low-mass stars per unit luminosity, but that an IMF similar to the Salpeter IMF at the high-mass end with less low-mass stars (giving stellar M/Ls 30% or more lower than the Salpeter value) is consistent with the maximum disk constraints. We apply the model trends in stellar M/L with color to the Tully-Fisher (TF) relation. We find that the stellar mass TF relation is relatively steep and has modest scatter, and is independent of the passband and color used to derive the stellar masses. The difference in slope between the optical (especially blue) and near-infrared TF relations is due to the combined effects of dust attenuation and stellar M/L variations with galaxy mass. We find that the baryonic TF relation has a slope of 3.5 (with random and systematic errors of +/- 0.2 each). Since we have normalized the stellar M/L to be as high as can possibly be allowed by maximum disk constraints, the slope of the baryonic TF relation will be somewhat shallower than 3.5 if all disks are substantially sub-maximal. [Abridged]Comment: 17 pages; 10 embedded postscript figures; to appear in the Astrophysical Journal on March 20 200