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.

We present the first sample of galaxy clusters selected on the basis of their weak gravitational lensing shear. The shear induced by a cluster is a function of its mass profile and its redshift relative to the background galaxies being sheared; in contrast to more traditional methods of selecting clusters, shear selection does not depend on the cluster's star formation history, baryon content, or dynamical state. Because mass is the property of clusters that provides constraints on cosmological parameters, the dependence on these other parameters could induce potentially important biases in traditionally selected samples. Comparison of a shear-selected sample with optically and X-rayselected samples is therefore of great importance. Here we present the first step toward a new shear-selected sample: the selection of cluster candidates from the first 8.6 deg 2 of the 20 deg 2 Deep Lens Survey (DLS), and tabulation of their basic properties such as redshifts and optical and X-ray counterparts.

The fraction of cluster galaxies that host luminous AGN is an important probe of AGN fueling processes, the cold ISM at the centers of galaxies, and how tightly black holes and galaxies co-evolve. We present a new measurement of the AGN fraction in a sample of 13 clusters of galaxies (M ≥ 10 14 M ) at 1 < z < 1.5 selected from the Spitzer/IRAC Shallow Cluster Survey, as well as the field fraction in the immediate vicinity of these clusters, and combine these data with measurements from the literature to quantify the relative evolution of cluster and field AGN from the present to z ∼ 3. We estimate that the cluster AGN fraction at 1 < z < 1.5 is f A = 3.0 +2.4 −1.4 % for AGN with a rest-frame, hard X-ray luminosity greater than L X,H ≥ 10 44 erg/s. This fraction is measured relative to all cluster galaxies more luminous than M * 3.6 (z) + 1, where M * 3.6 (z) is the absolute magnitude of the break in the galaxy luminosity function at the cluster redshift in the IRAC 3.6µm bandpass. The cluster AGN fraction is 30 times greater than the 3σ upper limit on the value for AGN of similar luminosity at z ∼ 0.25, as well as more than an order of magnitude greater than the AGN fraction at z ∼ 0.75. AGN with L X,H ≥ 10 43 erg/s exhibit similarly pronounced evolution with redshift. In contrast with the local universe, where the luminous AGN fraction is higher in the field than in clusters, the X-ray and MIR-selected AGN fractions in the field and clusters are consistent at 1 < z < 1.5. This is evidence that the cluster AGN population has evolved more rapidly than the field population from z ∼ 1.5 to the present. This environment-dependent AGN evolution mimics the more rapid evolution of star-forming galaxies in clusters relative to the field.

We report on the methodology and first results from the Deep Lens Survey (DLS) transient search. We utilize image subtraction on survey data to yield all sources of optical variability down to 24 th magnitude. Images are analyzed immediately after acquisition, at the telescope and in near-real time, to allow for followup in the case of time-critical events. All classes of transients are posted to the web upon detection. Our observing strategy allows sensitivity to variability over several decades in timescale. The DLS is the first survey to classify and report all types of photometric and astrometric variability detected, including solar system objects, variable stars, supernovae, and short timescale phenomena. Three unusual optical transient events were detected, flaring on thousandsecond timescales. All three events were seen in the B passband, suggesting blue color indices for the phenomena. One event (OT 20020115) is determined to be from a flaring Galactic dwarf star of spectral type dM4. From the remaining two events, we find an overall rate of η = 1.4 events deg −2 day −1 on thousand-second timescales, with a 95% confidence limit of η < 4.3. One of these events (OT 20010326) originated from a compact precursor in the field of galaxy cluster Abell 1836, and its nature is uncertain. For the second (OT 20030305) we find strong evidence for an extended extragalactic host. A dearth of such events in the R passband yields an upper 95% confidence limit on short timescale astronomical variability between 19.5 < M R < 23.4 of η R < 5.2 events deg −2 day −1 . We report also on our ensemble of astrometrically variable objects, as well as an example of photometric variability with an undetected precursor.

Using high-resolution near-infrared spectroscopy with the Keck Telescope, we have detected the radial velocity signatures of the cool secondary components in four optically identified pre-main-sequence, singlelined spectroscopic binaries. All are weak-lined T Tauri stars with well-defined center-of-mass velocities. The mass ratio for one young binary, NTTS 160905À1859, is M 2 /M 1 = 0.18 AE 0.01, the smallest yet measured dynamically for a pre-main-sequence spectroscopic binary. These new results demonstrate the power of infrared spectroscopy for the dynamical identification of cool secondaries. Visible-light spectroscopy, to date, has not revealed any pre-main-sequence secondary stars with masses <0.5 M , while two of the young systems reported here are in that range. We compare our targets with a compilation of the published young, double-lined spectroscopic binaries and discuss our unique contribution to this sample.