Description: We have constructed a mass-selected sample of M *  〉 10 11 M galaxies at 1 〈  z  〈 3 in the CANDELS UKIDSS UDS and COSMOS fields and have decomposed these systems into their separate bulge and disc components according to their H 160 -band morphologies. By extending this analysis to multiple bands, we have been able to conduct individual bulge and disc component SED fitting which has provided us with stellar-mass and star formation rate estimates for the separate bulge and disc components. Having utilized the new decomposed stellar-mass estimates, we confirm that the bulge components display a stronger size evolution than the discs. The median sizes of the bulge components is 3.09 ± 0.20 times smaller than similarly massive local galaxies over the full 1 〈  z  〈 3 redshift range; for the discs, the corresponding factor is 1.77 ± 0.10. Moreover, by splitting our sample into the passive and star-forming bulge and disc sub-populations and examining their sizes as a fraction of their present-day counter-parts, we find that the star-forming and passive bulges are equally compact, star-forming discs are larger, while the passive discs have intermediate sizes. This trend is not evident when classifying galaxy morphology on the basis of single-Sérsic fits and adopting the overall star formation rates. Finally, by evolving the star formation histories of the passive discs back to the redshifts when the passive discs were last active, we show that the passive and star-forming discs have consistent sizes at the relevant epoch. These trends need to be reproduced by any mechanisms which attempt to explain the morphological evolution of galaxies.

Description: We examine the spheroid growth and star formation quenching experienced by galaxies since z ~ 3 by studying the evolution with redshift of the quiescent and spheroid-dominated fractions of galaxies from the CANDELS (Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey) and GAMA (Galaxy and Mass Assembly) surveys. We compare the observed fractions with predictions from a semi-analytic model which includes prescriptions for bulge growth and AGN feedback due to mergers and disc instabilities. We facilitate direct morphological comparison by converting our model bulge-to-total stellar mass ratios to Sérsic indices. We then subdivide our population into the four quadrants of the specific star formation rate–Sérsic index plane and study the build-up of each of these subpopulations. We find that the fraction of star-forming discs declines steadily, while the fraction of quiescent spheroids builds up over cosmic time. The fractions of star-forming spheroids and quiescent discs are both non-negligible, and stay nearly constant over the period we have studied. Our model is qualitatively successful at reproducing the evolution of the two ‘main’ populations (star-forming discs and quiescent spheroids), and approximately reproduces the relative fractions of all four types, but predicts a stronger decline in star-forming spheroids, and increase in quiescent discs, than is seen in the observations. A model with an additional channel for bulge growth via disc instabilities agrees better overall with the observations than a model in which bulges can grow only through mergers. We also examine the relative importance of these different physical drivers of transformation (major and minor mergers and disc instabilities).

Description: The modern merger hypothesis offers a method of forming a new elliptical galaxy through merging two equal-mass, gas-rich disc galaxies fuelling a nuclear starburst followed by efficient quenching and dynamical stabilization. A key prediction of this scenario is a central concentration of young stars during the brief phase of morphological transformation from highly disturbed remnant to new elliptical galaxy. To test this aspect of the merger hypothesis, we use integral field spectroscopy to track the stellar Balmer absorption and 4000-Å break strength indices as a function of galactic radius for 12 massive ( M *  ≥ 10 10 M ), nearby ( z  ≤ 0.03), visually-selected plausible new ellipticals with blue-cloud optical colours and varying degrees of morphological peculiarities. We find that these index values and their radial dependence correlate with specific morphological features such that the most disturbed galaxies have the smallest 4000-Å break strengths and the largest Balmer absorption values. Overall, two-thirds of our sample are inconsistent with the predictions of the modern merger hypothesis. Of these eight, half exhibit signatures consistent with recent minor merger interactions. The other half have star formation histories similar to local, quiescent early-type galaxies. Of the remaining four galaxies, three have the strong morphological disturbances and star-forming optical colours consistent with being remnants of recent, gas-rich major mergers, but exhibit a weak, central burst consistent with forming ~5 per cent of their stars. The final galaxy possesses spectroscopic signatures of a strong, centrally concentrated starburst and quiescent core optical colours indicative of recent quenching (i.e. a post-starburst signature) as prescribed by the modern merger hypothesis.

Description: Important but rare and subtle processes driving galaxy morphology and star formation may be missed by traditional spiral, elliptical, irregular or Sérsic bulge/disc classifications. To overcome this limitation, we use a principal component analysis (PCA) of non-parametric morphological indicators (concentration, asymmetry, Gini coefficient, M 20 , multimode, intensity and deviation) measured at rest-frame B band (corresponding to HST /WFC3 F 125 W at 1.4 〈 z 〈 2) to trace the natural distribution of massive (〉10 10 M ) galaxy morphologies. PCA quantifies the correlations between these morphological indicators and determines the relative importance of each. The first three principal components (PCs) capture ~75 per cent of the variance inherent to our sample. We interpret the first PC as bulge strength, the second PC as dominated by concentration and the third PC as dominated by asymmetry. Both PC1 and PC2 correlate with the visual appearance of a central bulge and predict galaxy quiescence. PC1 is a better predictor of quenching than stellar mass, as good as other structural indicators (Sérsic-n or compactness). We divide the PCA results into groups using an agglomerative hierarchical clustering method. Unlike Sérsic, this classification scheme separates compact galaxies from larger, smooth protoelliptical systems, and star-forming disc-dominated clumpy galaxies from star-forming bulge-dominated asymmetric galaxies. Distinguishing between these galaxy structural types in a quantitative manner is an important step towards understanding the connections between morphology, galaxy assembly and star formation.

Description: The processes that trigger active galactic nuclei (AGN) remain poorly understood. While lower luminosity AGN may be triggered by minor disturbances to the host galaxy, stronger disturbances are likely required to trigger luminous AGN. Major wet mergers of galaxies are ideal environments for AGN triggering since they provide large gas supplies and galaxy scale torques. There is however little observational evidence for a strong connection between AGN and major mergers. We analyse the morphological properties of AGN host galaxies as a function of AGN and host galaxy luminosity and compare them to a carefully matched sample of control galaxies. AGN are X-ray selected in the redshift range 0.5 〈  z  〈 0.8 and have luminosities 41  log ( L X  [erg s –1 ])  44.5. ‘Fake AGN’ are simulated in the control galaxies by adding point sources with the magnitude of the matched AGN. We find that AGN host and control galaxies have comparable asymmetries, Sérsic indices and ellipticities at rest frame ~950 nm. AGN host galaxies show neither higher average asymmetries nor higher fractions of very disturbed objects. There is no increase in the prevalence of merger signatures with AGN luminosity. At 95 per cent confidence we find that major mergers are responsible for 〈6 per cent of all AGN in our sample as well as 〈40 per cent of the highest luminosity AGN (log ( L X [erg s –1 ]) ~ 43.5). Major mergers therefore either play only a very minor role in the triggering of AGN in the luminosity range studied or time delays are too long for merger features to remain visible.

Description: The rate of major galaxy–galaxy merging is theoretically predicted to steadily increase with redshift during the peak epoch of massive galaxy development (1 ≤ z ≤ 3). We use close-pair statistics to objectively study the incidence of massive galaxies (stellar M 1  〉 2 × 10 10  M ⊙ ) hosting major companions (1 ≤ M 1 / M 2 ≤ 4; i.e. 〈4:1) at six epochs spanning 0 〈  z  〈 3. We select companions from a nearly complete, mass-limited (≥5 × 10 9  M ⊙ ) sample of 23 696 galaxies in the five Cosmic Assembly Near-Infrared Deep Extragalactic Legacy Survey fields and the Sloan Digital Sky Survey. Using 5–50 kpc projected separation and close redshift proximity criteria, we find that the major companion fraction f mc ( z ) based on stellar mass-ratio (MR) selection increases from 6 per cent ( z  ∼ 0) to 16 per cent ( z  ∼ 0.8), then turns over at z  ∼ 1 and decreases to 7 per cent ( z  ∼ 3). Instead, if we use a major F160W flux-ratio (FR) selection, we find that f mc ( z ) increases steadily until z  = 3 owing to increasing contamination from minor (MR 〉 4:1) companions at z  〉 1. We show that these evolutionary trends are statistically robust to changes in companion proximity. We find disagreements between published results are resolved when selection criteria are closely matched. If we compute merger rates using constant fraction-to-rate conversion factors ( C merg,pair  = 0.6 and T obs,pair  = 0.65 Gyr), we find that MR rates disagree with theoretical predictions at z  〉 1.5. Instead, if we use an evolving T obs,pair ( z ) ∝ (1 +  z ) −2 from Snyder et al., our MR-based rates agree with theory at 0 〈  z  〈 3. Our analysis underscores the need for detailed calibration of C merg,pair and T obs,pair as a function of redshift, mass, and companion selection criteria to better constrain the empirical major merger history.

Description: SUMMARY Available apparent polar wander (APW) paths for the 200 Ma configuration of Pangea, just prior to the opening of the Central Atlantic Ocean, differ as much as 10 o in arc length. Here, we add new data from northwest Africa for this time, obtained from the northeast-trending Foum-Zguid and Ighrem dykes ( ca. 200 Ma). These dykes form part of the northern domain of the Central Atlantic Magmatic Province (CAMP), and crosscut the Anti-Atlas Ranges in Morocco, and compositionally correspond to quartz-normative tholeiites intruded in continental lithosphere shortly before the opening of the Central Atlantic Ocean. The Foum-Zguid dyke has been intensively studied, whereas the Ighrem dyke has received less scientific focus. We sampled both dykes for paleomagnetic investigation along 100 km of each dyke (12 sites for Foum-Zguid and 11 for Ighrem, 188 samples included in the final analyses). Rock magnetic experiments indicate a mixture of multidomain and single-domain magnetite and/or low-Ti titanomagnetite particles as the principal remanence carriers. In both dykes, the primary nature of the characteristic remanent magnetization is supported by positive contact tests, related to Fe-metasomatism or baked overprints of the corresponding sedimentary country rocks. The directions of the characteristic magnetization exhibit exclusively normal polarity. Site-mean virtual geomagnetic poles are differently grouped in each dyke, suggesting distinct geomagnetic secular variation records. The Foum-Zguid paleomagnetic pole ( N = 12, PLat = 67.9°N, PLon = 247.9°E, κ= 125, A 95 = 3.9°) plots close to that of Ighrem ( N = 11, PLat = 78.4°N, PLon = 238.2°E, κ= 47, A 95 = 6.7°), confirming those mineralogical and geochemical evidences supporting that they represent dissimilar magmatic stages. Virtual geomagnetic poles dispersion from both dykes ( S = 10.5° 13.0 ° 8.1° ) is in line with those obtained from recent studies of a CAMP-related dyke in Iberia and results from CAMP lavas in the Argana basin. These three new estimates of paleosecular variation at low latitudes around the Triassic–Jurassic boundary are concordant with a recently proposed dispersion curve for the Jurassic but suggest a slightly lower geomagnetic scatter than considered so far. After combining results from both dykes, the resulting paleomagnetic pole ( PLat = 73.0°N, PLon = 244.7°E, N = 23, κ= 55, A 95 = 4.1°) is statistically compared with existing and coeval African paleopoles, and with global synthetic 200 Ma running mean poles in northwest Africa coordinates.

Description: 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.

Description: We present the results of a new and improved study of the morphological and spectral evolution of massive galaxies over the redshift range 1 〈 z 〈 3. Our analysis is based on a bulge–disc decomposition of 396 galaxies with M * 〉 10 11 M uncovered from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) Wide Field Camera 3 (WFC3)/IR imaging within the Cosmological Evolution Survey (COSMOS) and UKIRT Infrared Deep Sky Survey (UKIDSS) UDS survey fields. We find that, by modelling the H 160 image of each galaxy with a combination of a de Vaucouleurs bulge (Sérsic index n = 4) and an exponential disc ( n = 1), we can then lock all derived morphological parameters for the bulge and disc components, and successfully reproduce the shorter-wavelength J 125 , i 814 , v 606 HST images simply by floating the magnitudes of the two components. This then yields sub-divided four-band HST photometry for the bulge and disc components which, with no additional priors, is well described by spectrophotometric models of galaxy evolution. Armed with this information, we are able to properly determine the masses and star formation rates for the bulge and disc components, and find that: (i) from z = 3 to 1 the galaxies move from disc dominated to increasingly bulge dominated, but very few galaxies are pure bulges/ellipticals by z = 1; (ii) while most passive galaxies are bulge dominated, and most star-forming galaxies disc dominated, 18 ± 5 per cent of passive galaxies are disc dominated, and 11 ± 3 per cent of star-forming galaxies are bulge dominated, a result which needs to be explained by any model purporting to connect star formation quenching with morphological transformations; (iii) there exists a small but significant population of pure passive discs, which are generally flatter than their star-forming counterparts (whose axial ratio distribution peaks at b / a ~= 0.7); (iv) flatter/larger discs re-emerge at the highest star formation rates, consistent with recent studies of sub-mm galaxies, and with the concept of a maximum surface density for star formation activity.

Description: The modern merger hypothesis offers a method of forming a new elliptical galaxy through merging two equal-mass, gas-rich disc galaxies fuelling a nuclear starburst followed by efficient quenching and dynamical stabilization. A key prediction of this scenario is a central concentration of young stars during the brief phase of morphological transformation from highly disturbed remnant to new elliptical galaxy. To test this aspect of the merger hypothesis, we use integral field spectroscopy to track the stellar Balmer absorption and 4000-Å break strength indices as a function of galactic radius for 12 massive ( M *  ≥ 10 10 M ), nearby ( z  ≤ 0.03), visually-selected plausible new ellipticals with blue-cloud optical colours and varying degrees of morphological peculiarities. We find that these index values and their radial dependence correlate with specific morphological features such that the most disturbed galaxies have the smallest 4000-Å break strengths and the largest Balmer absorption values. Overall, two-thirds of our sample are inconsistent with the predictions of the modern merger hypothesis. Of these eight, half exhibit signatures consistent with recent minor merger interactions. The other half have star formation histories similar to local, quiescent early-type galaxies. Of the remaining four galaxies, three have the strong morphological disturbances and star-forming optical colours consistent with being remnants of recent, gas-rich major mergers, but exhibit a weak, central burst consistent with forming ~5 per cent of their stars. The final galaxy possesses spectroscopic signatures of a strong, centrally concentrated starburst and quiescent core optical colours indicative of recent quenching (i.e. a post-starburst signature) as prescribed by the modern merger hypothesis.