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) <

We measure the sizes of 82 massive ( ) galaxies at utilizing deep HST NICMOS datataken in the GOODS North and South fields. Our sample is almost an order of magnitude larger than previous studies at these redshifts, providing the first statistical study of massive galaxy sizes at , confirming the z 1 2 extreme compactness of these systems. We split our sample into disk-like ( ) and spheroid-like ( ) n ≤ 2 n 1 2 galaxies based on their Sérsic indices, and find that at a given stellar mass disk-like galaxies at are a z ∼ 2.3 factor of 2.6 ‫ע‬ 0.3 smaller than present-day equal-mass systems, and spheroid-like galaxies at the same redshifts are 4.3 ‫ע‬ 0.7 smaller than comparatively massive elliptical galaxies today. At our results are compatible z 1 2 with both a leveling off, or a mild evolution in size. Furthermore, the high density (∼2 # 10 10 M kpc ) of Ϫ3 , massive galaxies at these redshifts, which are similar to present-day globular clusters, possibly makes any further evolution in sizes beyond unlikely. z p 3

We investigate the total major (> 1:4 by stellar mass) and minor (> 1:100 by stellar mass) merger history of a population of 80 massive (M * > 10 11 M ⊙ ) galaxies at high redshifts (z = 1.7 -3). We utilise extremely deep and high resolution HST H-band imaging from the GOODS NICMOS Survey (GNS), which corresponds to rest-frame optical wavelengths at the redshifts probed. We find that massive galaxies at high redshifts are often morphologically disturbed, with a CAS deduced merger fraction f m = 0.23 +/-0.05 at z = 1.7 -3. We find close accord between close pair methods (within 30 kpc apertures) and CAS methods for deducing major merger fractions at all redshifts. We deduce the total (minor + major) merger history of massive galaxies with M * > 10 9 M ⊙ galaxies, and find that this scales roughly linearly with log-stellar-mass and magnitude range. We test our close pair methods by utilizing mock galaxy catalogs from the Millennium Simulation. We compute the total number of mergers to be (4.5 +/-2.9) / < τ m > from z = 3 to the present, to a stellar mass sensitivity threshold of ∼ 1:100 (where τ m is the merger timescale in Gyr which varies as a function of mass). This corresponds to an average mass increase of (3.4 +/-2.2) ×10 11 M ⊙ over the past 11.5 Gyrs due to merging. We show that the size evolution observed for these galaxies may be mostly explained by this merging.

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