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We investigate the relationship between environment and galaxy evolution in the redshift range $0.5

We present a large-scale galaxy structure Cl J021734-0513 at z ∼ 0.65 discovered in the UKIDSS UDS field, made of ∼ 20 galaxy groups and clusters, spreading over 10 Mpc. We report on a VLT/VIMOS spectroscopic follow-up program that, combined with past spectroscopy, allowed us to confirm four galaxy clusters (M200 ∼ 1014 M⊙) and a dozen associated groups and star-forming galaxy overdensities. Two additional filamentary structures at z ∼ 0.62 and z ∼ 0.69 and foreground and background clusters at 0.6 < z < 0.7 were also confirmed along the line-of-sight. The structure subcomponents are at different formation stages. The clusters have a core dominated by passive galaxies and an established red sequence. The remaining structures are a mix of star-forming galaxy overdensities and forming groups. The presence of quiescent galaxies in the core of the latter shows that ‘preprocessing’ has already happened before the groups fall into their more massive neighbours. Our spectroscopy allows us to derive spectral index measurements e.g. emission/absorption line equivalent widths, strength of the 4000Å break, valuable to investigate the star formation history of structure members. Based on these line measurements, we select a population of ‘poststarburst’ galaxies. These galaxies are preferentially found within the virial radius of clusters, supporting a scenario in which their recent quenching could be prompted by gas stripping by the dense intracluster medium. We derive stellar age estimates using MCMC-based spectral fitting for quiescent galaxies and find a correlation between ages and colours/stellar masses which favours a top-down formation scenario of the red sequence. A catalogue of ∼ 650 redshifts in UDS will be released alongside the paper.

We present a comparison of the observed evolving galaxy stellar mass functions with the predictions of eight semi-analytic models and one halo occupation distribution model. While most models are able to fit the data at low redshift, some of them struggle to simultaneously fit observations at high redshift. We separate the galaxies into ‘passive’ and ‘star-forming’ classes and find that several of the models produce too many low-mass star-forming galaxies at high redshift compared to observations, in some cases by nearly a factor of 10 in the redshift range 2.5 < z < 3.0. We also find important differences in the implied mass of the dark matter haloes the galaxies inhabit, by comparing with halo masses inferred from observations. Galaxies at high redshift in the models are in lower mass haloes than suggested by observations, and the star formation efficiency in low-mass haloes is higher than observed. We conclude that many of the models require a physical prescription that acts to dissociate the growth of low-mass galaxies from the growth of their dark matter haloes at high redshift.

We examine the fraction of massive (M* > 1010 M) compact star-forming galaxies (cSFGs) that host an active galactic nucleus (AGN) at z ~ 2. These cSFGs are likely the direct progenitors of the compact quiescent galaxies observed at this epoch, which are the first population of passive galaxies to appear in large numbers in the early Universe. We identify cSFGs that host an AGN using a combination of Hubble WFC3 imaging and Chandra X-ray observations in four fields: the Chandra Deep Fields, the Extended Groth Strip, and the UKIDSS Ultra Deep Survey field. We find that - 39.2+ % 3.6 3.9 (65/166) of cSFGs at 1.4 < z < 3.0 host an X-ray detected AGN. This fraction is 3.2 times higher than the incidence of AGN in extended star-forming galaxies with similar masses at these redshifts. This difference is significant at the 6.2s level. Our results are consistent with models in which cSFGs are formed through a dissipative contraction that triggers a compact starburst and concurrent growth of the central black hole. We also discuss our findings in the context of cosmological galaxy evolution simulations that require feedback energy to rapidly quench cSFGs. We show that the AGN fraction peaks precisely where energy injection is needed to reproduce the decline in the number density of cSFGs with redshift. Our results suggest that the first abundant population of massive quenched galaxies emerged directly following a phase of elevated supermassive black hole growth and further hints at a possible connection between AGN and the rapid quenching of star formation in these galaxies.

We present a new method to classify the broad-band optical–near-infrared spectral energy distributions (SEDs) of galaxies using three shape parameters (super-colours) based on a principal component analysis of model SEDs. As well as providing a compact representation of the wide variety of SED shapes, the method allows for easy visualization of information loss and biases caused by the incomplete sampling of the rest-frame SED as a function of redshift. We apply the method to galaxies in the United Kingdom Infrared Telescope Infrared Deep Sky Survey Ultra Deep Survey with 0.9 < z < 1.2, and confirm our classifications by stacking rest-frame optical spectra for a fraction of objects in each class. As well as cleanly separating a tight red sequence from star-forming galaxies, three unusual populations are identifiable by their unique colours: very dusty star-forming galaxies with high metallicity and old mean stellar age; post-starburst galaxies which have formed _10 per cent of their mass in a recent unsustained starburst event; and metal-poor quiescent dwarf galaxies.We find that quiescent galaxies account for 45 per cent of galaxies with logM∗/M_ > 11, declining steadily to 13 per cent at logM∗/M_ = 10. The properties and mass function of the poststarburst galaxies are consistent with a scenario in which gas-rich mergers contribute to the growth of the low- and intermediate-mass range of the red sequence.

We investigate the relationship between the quenching of star formation and the structural transformation of massive galaxies, using a large sample of photometrically-selected poststarburst galaxies in the UKIDSS UDS field. We find that post-starburst galaxies at highredshift (z > 1) show high Sérsic indices, significantly higher than those of active star-forming galaxies, but with a distribution that is indistinguishable from the old quiescent population. We conclude that the morphological transformation occurs before (or during) the quenching of star formation. Recently quenched galaxies are also the most compact; we find evidence that massive post-starburst galaxies (M_ > 1010:5 M_) at high redshift (z > 1) are on average smaller than comparable quiescent galaxies at the same epoch. Our findings are consistent with a scenario in which massive passive galaxies are formed from three distinct phases: (1) gas-rich dissipative collapse to very high densities, forming the proto-spheroid; (2) rapid quenching of star formation, to create the “red nugget” with post-starburst features; (3) a gradual growth in size as the population ages, perhaps as a result of minor mergers.

We investigate the prevalence of AGN in the high-redshift protocluster Cl 0218.3–0510 at z = 1.62. Using imaging from the Chandra X-ray Telescope, we find a large overdensity of AGN in the protocluster; a factor of 23±9 times the field density of AGN. Only half of this AGN overdensity is due to the overdensity of massive galaxies in the protocluster (a factor of 11±2), as we find that 17+6−5% of massive galaxies (M∗ > 1010M⊙) in the protocluster host an X-ray luminous AGN, compared to 8±1% in the field. This corresponds to an enhancement of AGN activity in massive protocluster galaxies by a factor of 2.1±0.7 at 1.6s significance. We also find that the AGN overdensity is centrally concentrated, located within 3 arcmin and most pronounced within 1 arcmin of the centre of the protocluster. Our results confirm that there is a reversal in the local anti-correlation between galaxy density and AGN activity, so there is an enhancement of AGN in high-redshift protoclusters. We compare the properties of AGN in the protocluster to the field and find no significant differences in the distributions of their stellar mass, X-ray luminosity, or hardness ratio. We therefore suggest that triggering mechanisms are similar in both environments, and that the mechanisms simply occur more frequently in denser environments.

We present quantified visual morphologies of approximately 48 000 galaxies observed in three Hubble Space Telescope legacy fields by the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) and classified by participants in the Galaxy Zoo project. 90 per cent of galaxies have z ≤ 3 and are observed in rest-frame optical wavelengths by CANDELS. Each galaxy received an average of 40 independent classifications, which we combine into detailed morphological information on galaxy features such as clumpiness, bar instabilities, spiral structure, and merger and tidal signatures. We apply a consensus-based classifier weighting method that preserves classifier independence while effectively down-weighting significantly outlying classifications. After analysing the effect of varying image depth on reported classifications, we also provide depth-corrected classifications which both preserve the information in the deepest observations and also enable the use of classifications at comparable depths across the full survey. Comparing the Galaxy Zoo classifications to previous classifications of the same galaxies shows very good agreement; for some applications, the high number of independent classifications provided by Galaxy Zoo provides an advantage in selecting galaxies with a particular morphological profile, while in others the combination of Galaxy Zoo with other classifications is a more promising approach than using any one method alone. We combine the Galaxy Zoo classifications of ‘smooth’ galaxies with parametric morphologies to select a sample of featureless discs at 1 ≤ z ≤ 3, which may represent a dynamically warmer progenitor population to the settled disc galaxies seen at later epochs.

Due to significant galaxy contamination and impurity in stellar mass selected samples (up to 95 per cent from z = 0–3), we examine the star formation history, quenching time-scales, and structural evolution of galaxies using a constant number density selection with data from the United Kingdom Infra-Red Deep Sky Survey Ultra-Deep Survey field. Using this methodology, we investigate the evolution of galaxies at a variety of number densities from z= 0–3. We find that samples chosen at number densities ranging from 3 × 10−4 to 10−5 galaxies Mpc−3 (corresponding to z ∼ 0.5 stellar masses of M∗ = 1010.95−11.6 M0) have a star-forming blue fraction of ∼50 per cent at z ∼ 2.5, which evolves to a nearly 100 per cent quenched red and dead population by z ∼ 1. We also see evidence for number density downsizing, such that the galaxies selected at the lowest densities (highest masses) become a homogeneous red population before those at higher number densities. Examining the evolution of the colours for these systems furthermore shows that the formation redshift of galaxies selected at these number densities is zform > 3. The structural evolution through size and S´ersic index fits reveal that while there remains evolution in terms of galaxies becoming larger and more concentrated in stellar mass at lower redshifts, the magnitude of the change is significantly smaller than for a mass-selected sample. We also find that changes in size and structure continues at z < 1, and is coupled strongly to passivity evolution.We conclude that galaxy structure is driving the quenching of galaxies, such that galaxies become concentrated before they become passive.

We present a new method for tracing the evolution of brightest cluster galaxies (BCGs) from z ∼ 2 to z ∼ 0. We conclude on the basis of semi-analytical models that the best method to select BCG progenitors at z ∼ 2 is a hybrid environmental density and stellar mass ranking approach. Ultimately, we are able to retrieve 45 per cent of BCG progenitors. We apply this method on the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey, Ultra Deep Survey data to construct a progenitor sample at high redshift. We furthermore populate the comparisons in local Universe by using Sloan Digital Sky Survey data with statistically likely contamination to ensure a fair comparison between high and low redshifts. Using these samples we demonstrate that the BCG sizes have grown by a factor of ∼3.2 since z ∼ 2, and BCG progenitors are mainly late-type galaxies, exhibiting less concentrated profiles than their early type local counterparts. We find that BCG progenitors have more disturbed morphologies. In contrast, local BCGs have much smoother profiles. Moreover, we find that the stellar masses of BCGs have grown by a factor of ∼2.5 since z ∼ 2, and the star formation rate of BCG progenitors has a median value of 13.5 Mʘ yr‾¹, much higher than their quiescent local descendants. We demonstrate that over z = 1–2 star formation and merging contribute equally to BCG mass growth. However, merging plays a dominant role in BCG assembly at z ≲ 1. We also find that BCG progenitors at high z are not significantly different from other galaxies of similar mass at the same epoch. This suggests that the processes which differentiate BCGs from normal massive elliptical galaxies must occur at z ≲ 2.

We use a large sample of ∼350,000 galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major merging histories of massive galaxies (>1010 M⊙) at 0.0051010 M⊙ and at a constant number density of n>10−4 Mpc−3, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger timescale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at z<3.5, accruing an additional 1-4 ×1010 M⊙ in the process. Major merger accretion rate densities of ∼2×10−4 M⊙ yr−1 Mpc−3 are found for number density selected samples, indicating that direct progenitors of local massive (>1011M⊙) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. (2014) semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find major mergers become a comparable source of stellar mass growth compared to star-formation at z<1, but is 10-100 times smaller than the SFR density at higher redshifts.

We use a large sample of ∼350,000 galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major merging histories of massive galaxies (>1010 M⊙) at 0.0051010 M⊙ and at a constant number density of n>10−4 Mpc−3, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger timescale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at z<3.5, accruing an additional 1-4 ×1010 M⊙ in the process. Major merger accretion rate densities of ∼2×10−4 M⊙ yr−1 Mpc−3 are found for number density selected samples, indicating that direct progenitors of local massive (>1011M⊙) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. (2014) semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find major mergers become a comparable source of stellar mass growth compared to star-formation at z<1, but is 10-100 times smaller than the SFR density at higher redshifts.

We use a large sample of ∼350,000 galaxies constructed by combining the UKIDSS UDS, VIDEO/CFHT-LS, UltraVISTA/COSMOS and GAMA survey regions to probe the major merging histories of massive galaxies (>1010 M⊙) at 0.0051010 M⊙ and at a constant number density of n>10−4 Mpc−3, and find rates a factor of 2-3 smaller than previous works, although this depends strongly on the assumed merger timescale and likelihood of a close-pair merging. Galaxies undergo approximately 0.5 major mergers at z<3.5, accruing an additional 1-4 ×1010 M⊙ in the process. Major merger accretion rate densities of ∼2×10−4 M⊙ yr−1 Mpc−3 are found for number density selected samples, indicating that direct progenitors of local massive (>1011M⊙) galaxies have experienced a steady supply of stellar mass via major mergers throughout their evolution. While pair fractions are found to agree with those predicted by the Henriques et al. (2014) semi-analytic model, the Illustris hydrodynamical simulation fails to quantitatively reproduce derived merger rates. Furthermore, we find major mergers become a comparable source of stellar mass growth compared to star-formation at z<1, but is 10-100 times smaller than the SFR density at higher redshifts.

We present a study on the stellar mass growth of the progenitors of local massive galaxies with a variety of number density selections with n≤1×10−4 Mpc−3 (corresponding to M*=1011.24 M⊙ at z=0.3) in the redshift range 0.3

The formation of bars in disc galaxies is a tracer of the dynamical maturity of the population. Previous studies have found that the incidence of bars in discs decreases from the local Universe to z ∼ 1, and by z>1 simulations predict that bar features in dynamically mature discs should be extremely rare. Here, we report the discovery of strong barred structures in massive disc galaxies at z ∼ 1.5 in deep rest-frame optical images from the Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey. From within a sample of 876 disc galaxies identified by visual classification in Galaxy Zoo, we identify 123 barred galaxies. Selecting a subsample within the same region of the evolving galaxy luminosity function (brighter than L*), we find that the bar fraction across the redshift range 0.5 ≤ z ≤ 2 ( $f_{{\rm bar}} = 10.7^{+6.3}_{-3.5}$ per cent after correcting for incompleteness) does not significantly evolve. We discuss the implications of this discovery in the context of existing simulations and our current understanding of the way disc galaxies have evolved over the last 11 billion years