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We present the results of a new, ultra-deep, near-infrared imaging survey executed with the Hawk-I imager at the ESO VLT, of which we make all the data public. This survey, named HUGS (Hawk-I UDS and GOODS Survey), provides deep, high-quality imaging in the K and Y bands over the CANDELS UDS and GOODS-South fields. We describe here the survey strategy, the data reduction process, and the data quality. HUGS delivers the deepest and highest quality K-band images ever collected over areas of cosmological interest, and ideally complements the CANDELS data set in terms of image quality and depth. The seeing is exceptional and homogeneous, confined to the range 0.38"-0.43". In the deepest region of the GOODS-S field, (which includes most of the HUDF) the K-band exposure time exceeds 80 hours of integration, yielding a 1-sigma magnitude limit of ~28.0 mag/sqarcsec. In the UDS field the survey matches the shallower depth of the CANDELS images reaching a 1-sigma limit per sq.arcsec of ~27.3mag in the K band and ~28.3mag in the Y-band, We show that the HUGS observations are well matched to the depth of the CANDELS WFC3/IR data, since the majority of even the faintest galaxies detected in the CANDELS H-band images are also detected in HUGS. We present the K-band galaxy number counts produced by combining the HUGS data from the two fields. We show that the slope of the number counts depends sensitively on the assumed distribution of galaxy sizes, with potential impact on the estimated extra-galactic background light (abridged)., Accepted for publication on Astronomy and Astrophysics

We present the public release of the stellar mass catalogs for the GOODS-S and UDS fields obtained using some of the deepest near-IR images available, achieved as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) project. We combine the effort from ten different teams, who computed the stellar masses using the same photometry and the same redshifts. Each team adopted their preferred fitting code, assumptions, priors, and parameter grid. The combination of results using the same underlying stellar isochrones reduces the systematics associated with the fitting code and other choices. Thanks to the availability of different estimates, we can test the effect of some specific parameters and assumptions on the stellar mass estimate. The choice of the stellar isochrone library turns out to have the largest effect on the galaxy stellar mass estimates, resulting in the largest distributions around the median value (with a semi interquartile range larger than 0.1 dex). On the other hand, for most galaxies, the stellar mass estimates are relatively insensitive to the different parameterizations of the star formation history. The inclusion of nebular emission in the model spectra does not have a significant impact for the majority of galaxies (less than a factor of 2 for ~80% of the sample). Nevertheless, the stellar mass for the subsample of young galaxies (age < 100 Myr), especially in particular redshift ranges (e.g., 2.2 < z < 2.4, 3.2 < z < 3.6, and 5.5 < z < 6.5), can be seriously overestimated (by up to a factor of 10 for < 20 Myr sources) if nebular contribution is ignored., Accepted for publication by ApJ. Replaced to match the final version (two tables and one figure added in the appendix)

The exploration of the relation between galaxy sizes and other physical parameters has provided important clues for understanding galaxy formation. We use the CANDELS Deep+Wide surveys in the GOODS-South, UDS and EGS fields, complemented by data from the HUDF09 program, to address the relation between size and luminosity at z\sim7. We select 153 z-band drop-out galaxies in six different fields characterized by a wide combination of depth and areal coverage, ideally suited to sample without biases the observed size-magnitude plane. Detailed simulations allow us to derive the completeness as a function of size and magnitude and to quantify measurements errors/biases. We find that the half light radius distribution function of z\sim7 galaxies fainter than J=26.6 is peaked at 3) and the Lyman continuum escape fraction of high-z LBGs is relatively low (f_esc, 21 pages, 15 figures, accepted for publication in A&A

We constrain the X-ray emission properties of Lyman break galaxies (LBGs) at z ~ 3-6 using the 2 Ms Chandra Deep Field-North and 1 Ms Chandra Deep Field-South. Large samples of LBGs were discovered using HST as part of the Great Observatories Origins Deep Survey (GOODS). Deep optical and X-ray imaging over the GOODS fields have allowed us to place the most significant constraints on the X-ray properties of LBGs to date. Mean X-ray properties of 449, 1734, 629, and 247 LBGs with z ~ 3, 4, 5, and 6, respectively, were determined using stacking techniques. When stacked, we detect X-ray emission from LBGs at z ~ 3 (7 sigma) and from an optically bright subset (brightest 25%) of LBGs at z ~ 4 (3 sigma); the latter is the highest redshift detection yet for "normal" galaxies in the X-ray band. The effective exposure times for these stacked observations are 0.7 and 0.5 Gs, respectively. The derived average rest-frame 2.0-8.0 keV luminosities are 1.5 and 1.4 x 10^41 erg s^-1, respectively. X-ray emission from these LBGs is likely due to high mass X-ray binaries (HMXBs) and Type II supernovae; the corresponding star formation rates are 85-240 M_sol yr^-1. The X-ray to B-band mean luminosity ratio (Lx/LB) at z~3 is somewhat elevated with respect to that measured for starburst galaxies in the local Universe. When stacking full samples of LBGs at z ~ 4, 5, and 6 we do not obtain significant detections and derive rest-frame 2.0-8.0 keV luminosity upper limits of 0.9, 2.8, and 7.1 x 10^41 erg s^-1, respectively. These upper limits constrain any widespread AGN activity in these objects to be modest at best. Furthermore, we find that ~ 0.5% of our LBGs from z ~ 3-6 are detected individually in the X-ray band. Continued ..., 19 pages, 5 figures. AJ, in press