We present the COSMOS2015 a catalog which contains precise photometric redshifts and stellar masses for more than half a million objects over the 2deg 2 COSMOS field. Including new Y JHK s images from the UltraVISTA-DR2 survey, Y -band from Subaru/Hyper-Suprime-Cam and infrared data from the Spitzer Large Area Survey with the Hyper-Suprime-Cam Spitzer legacy program, this near-infraredselected catalog is highly optimized for the study of galaxy evolution and environments in the early Universe. To maximise catalog completeness for bluer objects and at higher redshifts, objects have been detected on a χ 2 sum of the Y JHK s and z ++ images. The catalog contains ∼ 6 × 10 5 objects in the 1.5 deg 2 UltraVISTA-DR2 region, and ∼ 1.5 × 10 5 objects are detected in the "ultra-deep stripes" (0.62 deg 2 ) at K s ≤ 24.7 (3σ, 3 , AB magnitude). Through a comparison with the zCOSMOSbright spectroscopic redshifts, we measure a photometric redshift precision of σ ∆z/(1+zs) = 0.007 and a catastrophic failure fraction of η = 0.5%. At 3 < z < 6, using the unique database of spectroscopic redshifts in COSMOS, we find σ ∆z/(1+zs) = 0.021 and η = 13.2%. The deepest regions reach a 90% completeness limit of 10 10 M to z = 4. Detailed comparisons of the color distributions, number counts, and clustering show excellent agreement with the literature in the same mass ranges. COSMOS2015 represents a unique, publicly available, valuable resource with which to investigate the evolution of galaxies within their environment back to the earliest stages of the history of the Universe. The COSMOS2015 catalog is distributed via anonymous ftp b and through the usual astronomical archive systems (CDS, ESO, IRSA).

In this paper we describe the first data release of the UltraVISTA near-infrared imaging survey of the COSMOS field. We summarise the key goals and design of the survey and provide a detailed description of our data reduction techniques. We provide stacked, skysubtracted images in Y JHK s and narrow-band filters constructed from data collected during the first year of UltraVISTA observations. Our stacked images reach 5σ AB depths in an aperture of 2 diameter of ∼25 in Y and ∼24 in JHK s bands and all have subarcsecond seeing. To this 5σ limit, our K s catalogue contains 216 268 sources. We carry out a series of quality assessment tests on our images and catalogues, comparing our stacks with existing catalogues. The 1σ astrometric rms in both directions for stars selected with 17.0 < K s (AB) <

The Lyman alpha (Lyα) line of Hydrogen is a prominent feature in the spectra of star-forming galaxies, usually redshifted by a few hundreds of km s −1 compared to the systemic redshift. This large offset hampers follow-up surveys, galaxy pair statistics and correlations with quasar absorption lines when only Lyα is available. We propose diagnostics that can be used to recover the systemic redshift directly from the properties of the Lyα line profile. We use spectroscopic observations of Lyman-Alpha Emitters (LAEs) for which a precise measurement of the systemic redshift is available. Our sample contains 13 sources detected between z ≈ 3 and z ≈ 6 as part of various Multi Unit Spectroscopic Explorer (MUSE) Guaranteed Time Observations (GTO). We also include a compilation of spectroscopic Lyα data from the literature spanning a wide redshift range (z ≈ 0 − 8). First, restricting our analysis to double-peaked Lyα spectra, we find a tight correlation between the velocity offset of the red peak with respect to the systemic redshift, V red peak , and the separation of the peaks. Secondly, we find a correlation between V red peak and the full width at half maximum of the Lyα line. Fitting formulas, to estimate systemic redshifts of galaxies with an accuracy of ≤ 100 km s −1 when only the Lyα emission line is available, are given for the two methods.