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) <

Context. Narrow-band surveys to detect Lyα emitters are powerful tools for identifying high, and very high, redshift galaxies. Although samples are increasing at redshifts z = 3−6, the nature of these galaxies is still poorly known. The number of galaxies detected at redshifts below z ∼ 3 are also small. Aims. We study the properties of z = 2.25 Lyα emitters and compare them with those of z > 3 Lyα emitters.Methods. We present narrow-band imaging made with the MPG/ESO 2.2m telescope and the WFI (Wide Field Imager) detector. Using this data, we have searched for emission-line objects. We find 170 candidate typical Lyα emitters and 17 candidates that we regard as high UV-transmission Lyα emitters. We have derived the magnitudes of these objects in 8 photometric bands from u * to K s , and studied whether they have X-ray and/or radio counterparts. Results. We demonstrate that there has been significant evolution in the properties of Lyα emitters between redshift z ∼ 3 and z = 2.25. The spread in spectral energy distributions (SEDs) at the lower redshift is larger and we detect a significant AGN contribution in the sample. The distribution of the equivalent widths is narrower than at z ∼ 3, with only a few candidates with rest-frame equivalent width above the predicted limit of 240 Å. The star formation rates derived from the Lyα emission compared to those derived from the UV emission are lower by on average a factor of ∼1.8, indicative of a significant absorption by dust. Conclusions. Lyα emitters at redshift z = 2.25 may be more evolved than Lyα emitters at higher redshift. The red SEDs imply more massive, older and/or dustier galaxies at lower redshift than observed at higher redshifts. The decrease in equivalent widths and star formation rates indicate more quiescent galaxies, with in general less star formation than in higher redshift galaxies. At z = 2.25, AGN appear to be more abundant and also to contribute more to the Lyα emitting population.

We analyze the redshift evolution of the mass-metallicity relation in a sample of 110 Damped Lyα absorbers spanning the redshift range z = 0.11 − 5.06 and find that the zero-point of the correlation changes significantly with redshift. The evolution is such that the zero-point is constant at the early phases of galaxy growth (i.e. no evolution) but then features a sharp break at z = 2.6±0.2 with a rapid incline towards lower redshifts such that damped absorbers of identical masses are more metal rich at later times than earlier. The slope of this mass metallicity correlation evolution is 0.35 ± 0.07 dex per unit redshift.We compare this result to similar studies of the redshift evolution of emission selected galaxy samples and find a remarkable agreement with the slope of the evolution of galaxies of stellar mass log(M * /M ⊙ ) ≈ 8.5. This allows us to form an observational tie between damped absorbers and galaxies seen in emission.We use results from simulations to infer the virial mass of the dark matter halo of a typical DLA galaxy and find a ratio (M vir /M * ) ≈ 30.We compare our results to those of several other studies that have reported strong transition-like events at redshifts around z = 2.5 − 2.6 and argue that all those observations can be understood as the consequence of a transition from a situation where galaxies were fed more unprocessed infalling gas than they could easily consume to one where they suddenly become infall starved and turn to mainly processing, or re-processing, of previously acquired gas.

We report on the discovery of a z = 3.16 Lyman-α emitting blob in the Great Observatories Origins Deep Survey (GOODS) South field. The discovery was made with the VLT, through narrow-band imaging. The blob has a total Lyα luminosity of ∼10 43 erg s −1 and a diameter larger than 60 kpc. The available multi-wavelength data in the GOODS field consists of 13 bands from X-rays (Chandra) to infrared (Spitzer). Unlike other known Lyα blobs, this blob shows no obvious continuum counter-parts in any of the broad-bands. In particular, no optical counter-parts are found in deep HST/ACS imaging. For previously published blobs, AGN (Active Galactic Nuclei) or "superwind" models have been found to provide the best match to the data. We here argue that the most probable origin of the extended Lyα emission from this blob is cold accretion onto a dark matter halo.

We study the multi-wavelength properties of a set of 171 Lyα emitting candidates at redshift z = 2.25 found in the COSMOS field, with the aim of understanding the underlying stellar populations in the galaxies. We especially seek to understand what the dust contents, ages and stellar masses of the galaxies are, and how they relate to similar properties of Lyα emitters at other redshifts. The candidates here are shown to have different properties from those of Lyα emitters found at higher redshift, by fitting the spectral energy distributions (SEDs) using a Monte-Carlo Markov-Chain technique and including nebular emission in the spectra. The stellar masses, and possibly the dust contents, are higher, with stellar masses in the range log M * = 8.5−11.0 M and A V = 0.0−2.5 mag. Young population ages are well constrained, but the ages of older populations are typically unconstrained. In 15% of the galaxies only a single, young population of stars is observed. We show that the Lyα fluxes of the best fit galaxies are correlated with their dust properties, with higher dust extinction in Lyα faint galaxies. Testing for whether results derived from a light-weighted stack of objects correlate to those found when fitting individual objects we see that stellar masses are robust to stacking, but ages and especially dust extinctions are derived incorrectly from stacks. We conclude that the stellar properties of Lyα emitters at z = 2.25 are different from those at higher redshift and that they are diverse. Lyα selection appears to be tracing systematically different galaxies at different redshifts.