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.

The influence of a galaxy’s environment on its evolution has been studied and compared extensively in the literature, although differing techniques are often used to define environment. Most methods fall into two broad groups: those that use nearest neighbours to probe the underlying density field and those that use fixed apertures. The differences between the two inhibit a clean comparison between analyses and leave open the possibility that, even with the same data, different properties are actually being measured. In this work, we apply 20 published environment definitions to a common mock galaxy catalogue constrained to look like the local Universe. We find that nearest‐neighbour‐based measures best probe the internal densities of high‐mass haloes, while at low masses the interhalo separation dominates and acts to smooth out local density variations. The resulting correlation also shows that nearest‐neighbour galaxy environment is largely independent of dark matter halo mass. Conversely, aperture‐based methods that probe superhalo scales accurately identify high‐density regions corresponding to high‐mass haloes. Both methods show how galaxies in dense environments tend to be redder, with the exception of the largest apertures, but these are the strongest at recovering the background dark matter environment. We also warn against using photometric redshifts to define environment in all but the densest regions. When considering environment, there are two regimes: the ‘local environment’ internal to a halo best measured with nearest neighbour and ‘large‐scale environment’ external to a halo best measured with apertures. This leads to the conclusion that there is no universal environment measure and the most suitable method depends on the scale being probed.

No abstract

We use a sample of 8298 galaxies observed as part of the Hubble Space Telescope (HST) H160‐band GOODS NICMOS Survey (GNS) to construct the galaxy stellar mass function both as a function of redshift and as stellar mass up to z= 3.5. Our mass functions are constructed within the redshift range z= 1–3.5 and consist of galaxies with stellar masses of M*= 1012 M⊙ down to nearly dwarf galaxy masses of M*= 108.5 M⊙ in the lowest redshift bin. We discover that a significant fraction of all massive M* > 1011 M⊙ galaxies are in place up to the highest redshifts we probe, with a decreasing fraction of lower mass galaxies present at all redshifts. This is an example of ‘galaxy mass downsizing’, and is the result of massive galaxies forming before lower mass ones, and not just simply ending their star formation earlier as in traditional downsizing scenarios, whose effect is seen at z < 1.5. By fitting Schechter functions to our mass functions we find that the faint‐end slope ranges from α=−1.36 to −1.73, which is significantly steeper than what is found in previous investigations of the mass function at high redshift. We demonstrate that this steeper mass function better matches the stellar mass added due to star formation, thereby alleviating some of the mismatch between these two measures of the evolution of galaxy mass. We furthermore examine the stellar mass function divided into blue/red systems, as well as for star‐forming and non‐star‐forming galaxies. We find a similar mass downsizing present for both blue/red and star‐forming/non‐star forming galaxies, and further find that red galaxies dominate at the high‐mass end of the mass function, but that the low‐mass galaxies are mostly all blue, and therefore blue galaxies are creating the steep mass functions observed at z > 2. We furthermore show that, although there is a downsizing such that high‐mass galaxies are nearer their z= 0 values at high redshift, this turns over at masses M*∼ 1010 M⊙, such that the lowest mass galaxies are more common than galaxies at slight higher masses, creating a ‘dip’ in the observed galaxy mass function. We argue that the galaxy assembly process may be driven by different mechanisms at low and high masses, and that the efficiency of the galaxy formation process is lowest at masses M*∼ 1010 M⊙ at 1 < z < 3. Finally, we calculate the integrated stellar mass density for the total, blue and red populations. We find the integrated stellar mass density of the total and blue galaxy population is consistent with being constant over z= 1–2, while the red population shows an increase in integrated stellar mass density over the same redshift range.