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.

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We have designed the Extremely Luminous Quasar Survey (ELQS) to provide a highly complete census of unobscured UV-bright quasars during the cosmic noon, z = 2.8 − 5.0. Here we report the discovery of 70 new quasars in the ELQS South Galactic Cap (ELQS-S) quasar sample, doubling the number of known extremely luminous quasars in 4, 237.3 deg 2 of the SDSS footprint. These observations conclude the ELQS and we present the properties of the full ELQS quasar catalog, containing 407 quasars over 11, 838.5 deg 2 . Our novel ELQS quasar selection strategy resulted in unprecedented completeness at the bright end and allowed us to discover 109 new quasars in total. This marks an increase of ∼ 36% (109/298) to the known population at these redshifts and magnitudes, while we further are able to retain a selection efficiency of ∼ 80%. On the basis of 166 quasars from the full ELQS quasar catalog, who adhere to the uniform criteria of the 2MASS point source catalog, we measure the bright-end quasar luminosity function (QLF) and extend it one magnitude brighter than previous studies. Assuming a single power law with exponential density evolution for the functional form of the QLF, we retrieve the best fit parameters from a maximum likelihood analysis. We find a steep bright-end slope of β ≈ −4.1 and we can constrain the bright-end slope to β ≤ −3.4 with 99% confidence. The density is well modeled by the exponential redshift evolution, resulting in a moderate decrease with redshift (γ ≈ −0.4).

We present the North Galactic Cap sample of the Extremely Luminous Quasar Survey (ELQS-N), which targets quasars with M 1450 < −27 at 2.8 ≤ z < 5 in an area of ∼ 7600 deg 2 of the Sloan Digital Sky Survey (SDSS) footprint with 90°< RA < 270°. Based on a near-infrared/infrared JKW2 color cut, the ELQS selection efficiently uses random forest methods to classify quasars and to estimate photometric redshifts; this scheme overcomes some of the difficulties of pure optical quasar selection at z ≈ 3. As a result, we retain a completeness of > 70% over z ∼ 3.0 − 5.0 at m i 17.5, limited toward fainter magnitudes by the depth of the Two Micron All Sky Survey (2MASS). The presented quasar catalog consists of a total of 270 objects, of which 39 are newly identified in this work with spectroscopy obtained at the Vatican Advanced Technology Telescope and the MMT 6.5 m telescope. In addition to the high completeness, which allowed us to discover new quasars in the already well-surveyed SDSS North Galactic Cap, the efficiency of our selection is relatively high at ∼ 79%. Using 120 objects of this quasar sample we are able to extend the previously measured optical quasar luminosity function (QLF) by one magnitude toward the bright end at 2.8 ≤ z ≤ 4.5. A first analysis of the QLF suggests a relatively steep bright-end slope of β ≈ −4 for this sample. This result contrasts with previous results in the same redshift range, which find a much flatter slope around β ∼ −2.5, but agrees with recent measurements of the bright-end slope at lower and higher redshifts. Our results constrain the bright-end slope at z = 2.8 − 4.5 to β < −2.94 with a 99% confidence.

The dominant baryonic component of galaxy clusters is hot gas whose distribution is commonly probed through X-ray emission arising from thermal bremsstrahlung. The density profile thus obtained has been traditionally modeled with a β-profile, a simple function with only three parameters. However, this model is known to be insufficient for characterizing the range of cluster gas distributions, and attempts to rectify this shortcoming typically introduce additional parameters to increase the fitting flexibility. We use cosmological and physical considerations to obtain a family of profiles for the gas with fewer parameters than the β-model but which better accounts for observed gas profiles over wide radial intervals. a prescription for a new family of profiles that satisfies a set of simple physical assumptions and that has sufficient fitting flexibility to improve upon -but as many or fewer parameters as -the standard β-model. BACKGROUND Gas Density ProfilesGalaxy cluster gas density profiles have been traditionally described by the three-parameter β-model,