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 describe the results of an extremely deep, 0.28 deg 2 survey for z ¼ 3:1 Ly emission-line galaxies in the Extended Chandra Deep FieldYSouth. By using a narrowband 5000 8 filter and complementary broadband photometry from the MUSYC survey, we identify a statistically complete sample of 162 galaxies with monochromatic fluxes brighter than 1:5 ; 10 À17 ergs cm À2 s À1 and observer's frame equivalent widths greater than 80 8. We show that the equivalent width distribution of these objects follows an exponential with a rest-frame scale length of w 0 ¼ 76In addition, we show that in the emission line, the luminosity function of Ly galaxies has a faint-end power-law slope of ¼ À1:49 þ0:45 À0:34 , a bright-end cutoff of log L Ã ¼ 42:64 þ0:26 À0:15 , and a space density above our detection thresholds of (1:46 AE 0:12) ; 10 À3 h 3 70 galaxies Mpc À3 . Finally, by comparing the emission-line and continuum properties of the Ly emitters, we show that the star formation rates derived from Ly are $3 times lower than those inferred from the rest-frame UV continuum. We use this offset to deduce the existence of a small amount of internal extinction within the host galaxies. This extinction, coupled with the lack of extremely high equivalent width emitters, argues that these galaxies are not primordial Population III objects, although they are young and relatively chemically unevolved.

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We present first results of a spectroscopic survey targeting K-selected galaxies at using the Gemini z p 2.0-2.7 near-infrared spectrograph (GNIRS). We obtained near-infrared spectra with a wavelength coverage of 1.0-2.5 mm for 26 K-bright galaxies ( ) selected from the Multi-wavelength Survey by Yale-Chile (MUSYC) using K ! 19.7 photometric redshifts. We successfully derived spectroscopic redshifts for all 26 galaxies using rest-frame optical emission lines or the redshifted Balmer/4000 break. Twenty galaxies have spectroscopic redshifts in the range A , for which bright emission lines like Ha and [O iii] fall in atmospheric windows. Surprisingly, we 2.0 ! z ! 2.7 detected no emission lines for nine of these 20 galaxies. The median 2 j upper limit on the rest-frame equivalent width of Ha for these nine galaxies is ∼10 . The stellar continuum emission of these same nine galaxies is best A fitted by evolved stellar population models. The best-fit star formation rate (SFR) is zero for five out of nine galaxies and is consistent with zero within 1 j for the remaining four. Thus, both the Ha measurements and the independent stellar continuum modeling imply that 45% of our K-selected galaxies are not forming stars intensely. This high fraction of galaxies without detected line emission and low SFRs may imply that the suppression of star formation in massive galaxies occurs at higher redshift than is predicted by current cold dark matter (CDM) galaxy formation models. However, obscured star formation may have been missed, and deep mid-infrared imaging is needed to clarify this situation.

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