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.

Previously we used the Nearby Supernova Factory sample to show that SNe Ia having locally star-forming environments are dimmer than SNe Ia having locally passive environments. Here we use the Constitution sample together with host galaxy data from GALEX to independently confirm that result. The effect is seen using both the SALT2 and MLCS2k2 lightcurve fitting and standardization methods, with brightness differences of 0.094 ± 0.037 mag for SALT2 and 0.155 ± 0.041 mag for MLCS2k2 with R V = 2.5. When combined with our previous measurement the effect is 0.094 ± 0.025 mag for SALT2. If the ratio of these local SN Ia environments changes with redshift or sample selection, this can lead to a bias in cosmological measurements. We explore this -2issue further, using as an example the direct measurement of H 0 . GALEX observations show that the SNe Ia having standardized absolute magnitudes calibrated via the Cepheid period-luminosity relation using HST originate in predominately star-forming environments, whereas only ∼ 50% of the Hubble-flow comparison sample have locally star-forming environments. As a consequence, the H 0 measurement using SNe Ia is currently overestimated. Correcting for this bias, we find a value of H corr 0 = 70.6 ± 2.6 km s −1 Mpc −1 when using the LMC distance, Milky Way parallaxes and the NGC 4258 megamaser as the Cepheid zeropoint, and 68.8 ± 3.3 km s −1 Mpc −1 when only using NGC 4258. Our correction brings the direct measurement of H 0 within ∼ 1 σ of recent indirect measurements based on the CMB power spectrum.B SF = 0.094 ± 0.031 mag 1 . Since the underlying connection is with star formation rather than the Hα emission itself, we refer to this effect as the star-formation bias, or SF bias for short.R13 connected the SF bias to the host-mass step by noting that few of the Ia in the SNfactory sample occur in low-mass hosts, leading to a shift in mean brightness with host mass that is driven by the changing fraction of star formation. However, this also implies that simply correcting for the host-mass step will not

As part of an ongoing effort to identify, understand and correct for astrophysics biases in the standardization of Type Ia supernovae (SN Ia) for cosmology, we have statistically classified a large sample of nearby SNe Ia into those located in predominantly younger or older environments. This classification is based on the specific star formation rate measured within a projected distance of 1 kpc from each SN location, (LsSFR). This is an important refinement compared to using the local star formation rate directly (Rigault et al. 2013; 2015), as it provides a normalization for relative numbers of available SN progenitors and is more robust against extinction by dust. We find that the SNe Ia in predominantly younger environments are ∆ Y = 0.163 ± 0.029 mag (5.7 σ) fainter than those in predominantly older environments after conventional light-curve standardization. This is the strongest standardized SN Ia brightness systematic connected to host-galaxy environment measured to date. The well-established step in standardized brightnesses between SNe Ia in hosts with lower or higher total stellar masses is smaller, at ∆ M = 0.119 ± 0.032 mag (4.5 σ), for the same set of SNe Ia. When fit simultaneously, the environment age offset remains very significant, with ∆ Y = 0.129 ± 0.032 mag (4.0 σ), while the global stellar mass step is reduced to ∆ M = 0.064 ± 0.029 mag (2.2 σ). Thus, approximately 70% of the variance from the stellar mass step is due to an underlying dependence on environment-based progenitor age. Also, we verify that using the local star formation rate alone is not as powerful as LsSFR at sorting SNe Ia into brighter and fainter subsets. Standardization using only the SNe Ia in younger environments reduces the total dispersion from 0.142 ± 0.008 mag to 0.120 ± 0.010 mag. We show that as environment ages evolve with redshift, a strong bias, especially on measurement of the derivative of the dark energy equation of state, can develop. Fortunately, data to measure and correct for this effect using our local specific star formation rate indicator is likely to be available for many next-generation SN Ia cosmology experiments.