ALBERT

Description: We use cosmological simulations to study a characteristic evolution pattern of high-redshift galaxies. Early, stream-fed, highly perturbed, gas-rich discs undergo phases of dissipative contraction into compact, star-forming systems (‘blue’ nuggets) at z  ~ 4–2. The peak of gas compaction marks the onset of central gas depletion and inside-out quenching into compact ellipticals (red nuggets) by z  ~ 2. These are sometimes surrounded by gas rings or grow extended dry stellar envelopes. The compaction occurs at a roughly constant specific star formation rate (SFR), and the quenching occurs at a constant stellar surface density within the inner kpc ( 1 ). Massive galaxies quench earlier, faster, and at a higher 1 than lower mass galaxies, which compactify and attempt to quench more than once. This evolution pattern is consistent with the way galaxies populate the SFR-size–mass space, and with gradients and scatter across the main sequence. The compaction is triggered by an intense inflow episode, involving (mostly minor) mergers, counter-rotating streams or recycled gas, and is commonly associated with violent disc instability. The contraction is dissipative, with the inflow rate 〉SFR, and the maximum 1 anticorrelated with the initial spin parameter. The central quenching is triggered by the high SFR and stellar/supernova feedback (maybe also active galactic nucleus feedback) due to the high central gas density, while the central inflow weakens as the disc vanishes. Suppression of fresh gas supply by a hot halo allows the long-term maintenance of quenching once above a threshold halo mass, inducing the quenching downsizing.

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: We present new measurements of the evolution of the X-ray luminosity functions (XLFs) of unabsorbed and absorbed active galactic nuclei (AGNs) out to z  ~ 5. We construct samples containing 2957 sources detected at hard (2–7 keV) X-ray energies and 4351 sources detected at soft (0.5–2 keV) energies from a compilation of Chandra surveys supplemented by wide-area surveys from ASCA and ROSAT . We consider the hard and soft X-ray samples separately and find that the XLF based on either (initially neglecting absorption effects) is best described by a new flexible model parametrization where the break luminosity, normalization, and faint-end slope all evolve with redshift. We then incorporate absorption effects, separately modelling the evolution of the XLFs of unabsorbed (20 〈 log N H  〈 22) and absorbed (22 〈 log N H  〈 24) AGNs, seeking a model that can reconcile both the hard- and soft-band samples. We find that the absorbed AGN XLF has a lower break luminosity, a higher normalization, and a steeper faint-end slope than the unabsorbed AGN XLF out to z  ~ 2. Hence, absorbed AGNs dominate at low luminosities, with the absorbed fraction falling rapidly as luminosity increases. Both XLFs undergo strong luminosity evolution which shifts the transition in the absorbed fraction to higher luminosities at higher redshifts. The evolution in the shape of the total XLF is primarily driven by the changing mix of unabsorbed and absorbed populations.

Description: We present star formation histories (SFHs) for a sample of 104 massive (stellar mass M  〉 10 10 M ) quiescent galaxies (MQGs) at z = 1.0–1.5 from the analysis of spectrophotometric data from the Survey for High- z Absorption Red and Dead Sources (SHARDS) and HST /WFC3 G102 and G141 surveys of the GOODS-North field, jointly with broad-band observations from ultraviolet (UV) to far-infrared (far-IR). The sample is constructed on the basis of rest-frame UVJ colours and specific star formation rates (sSFRs = SFR/Mass). The spectral energy distributions (SEDs) of each galaxy are compared to models assuming a delayed exponentially declining SFH. A Monte Carlo algorithm characterizes the degeneracies, which we are able to break taking advantage of the SHARDS data resolution, by measuring indices such as MgUV and D4000. The population of MQGs shows a duality in their properties. The sample is dominated (85 per cent) by galaxies with young mass-weighted ages, $\overline{t_{\rm M}}$  〈 2 Gyr, short star formation time-scales, 〈〉 ~ 60–200 Myr, and masses log( M /M ) ~ 10.5. There is an older population (15 per cent) with $\overline{t_{\rm M}}$ = 2–4 Gyr, longer star formation time-scales, 〈〉 ~ 400 Myr, and larger masses, log( M /M ) ~ 10.7. The SFHs of our MQGs are consistent with the slope and the location of the main sequence of star-forming galaxies at z  〉 1.0, when our galaxies were 0.5–1.0 Gyr old. According to these SFHs, all the MQGs experienced a luminous infrared galaxy phase that lasts for ~500 Myr, and half of them an ultraluminous infrared galaxy phase for ~100 Myr. We find that the MQG population is almost assembled at z  ~ 1, and continues evolving passively with few additions to the population.

Description: Chandra data in the COSMOS, AEGIS-XD and 4 Ms Chandra Deep Field South are combined with multiwavelength photometry available in those fields to determine the rest-frame U  –  V versus V  –  J colours of X-ray AGN hosts in the redshift intervals 0.1 〈  z  〈 0.6 (mean $\overline{z}=0.40$ ) and 0.6 〈  z  〈 1.2 (mean $\overline{z}=0.85$ ). This combination of colours provides an effective and least model-dependent means of separating quiescent from star-forming, including dust reddened, galaxies. Morphological information emphasizes differences between AGN populations split by their U  –  V versus V  –  J colours. AGN in quiescent galaxies consist almost exclusively of bulges, while star-forming hosts are equally split between early- and late-type hosts. The position of AGN hosts on the U  –  V versus V  –  J diagram is then used to set limits on the accretion density of the Universe associated with evolved and star-forming systems independent of dust induced biases. It is found that most of the black hole growth at z 0.40 and 0.85 is associated with star-forming hosts. Nevertheless, a non-negligible fraction of the X-ray luminosity density, about 15–20 per cent, at both $\overline{z}=0.40$ and 0.85, is taking place in galaxies in the quiescent region of the U  –  V versus V  –  J diagram. For the low-redshift sub-sample, 0.1 〈  z  〈 0.6, we also find tentative evidence, significant at the 2 level, that AGN split by their U  –  V and V  –  J colours have different Eddington ratio distributions. AGN in blue star-forming hosts dominate at relatively high Eddington ratios. In contrast, AGN in red quiescent hosts become increasingly important as a fraction of the total population towards low Eddington ratios. At higher redshift, z  〉 0.6, such differences are significant at the 2 level only for sources with Eddington ratios 10 – 3 . These findings are consistent with scenarios in which diverse accretion modes are responsible for the build-up of supermassive black holes at the centres of galaxies. We compare these results with the predictions of the galform semi-analytic model for the cosmological evolution of AGN and galaxies. This model postulates two black hole fuelling modes, the first is linked to star formation events and the second takes place in passive galaxies. galform predicts that a substantial fraction of the black hole growth at z  〈 1 is associated with quiescent galaxies, in apparent conflict with the observations. Relaxing the strong assumption of the model that passive AGN hosts have zero star formation rate could bring those predictions in better agreement with the data.

Description: We probe the merging channel of massive galaxies over the z  = 0.3–1.3 redshift window by studying close pairs in a sample of 238 galaxies with stellar mass 10 11  M , from the SHARDS (Survey for High-z Absorption Red and Dead Sources) survey. SHARDS provides medium-band photometry equivalent to low-resolution optical spectra ( R  ~ 50), allowing us to obtain extremely accurate photometric redshifts (median | z |/(1 +  z ) ~ 0.55 per cent) and to improve the constraints on the age distribution of the stellar populations. Our data set is volume limited, probing merger progenitors with mass ratios 1:100 (μ M sat / M cen  = 0.01) out to z  = 1.3. A strong correlation is found between the age difference of host and companion galaxy and stellar mass ratio, from negligible age differences in major mergers to age differences ~4 Gyr for 1:100 minor mergers. However, this correlation is simply a reflection of the mass–age trend in the general population. The dominant contributor to the growth of massive galaxies corresponds to mass ratios μ 0.3, followed by a decrease in the fractional mass growth rate linearly proportional to log μ, at least down to μ ~ 0.01, suggesting a decreasing role of mergers involving low-mass companions, especially if dynamical friction time-scales are taken into account. A simple model results in an upper limit for the average mass growth rate of massive galaxies of ( M / M )/ t  ~ 0.08 ± 0.02 Gyr –1 , over the z 1 range, with an ~70 per cent fractional contribution from (major) mergers with μ 0.3. The majority of the stellar mass contributed by mergers does not introduce significantly younger populations, in agreement with the small radial age gradients observed in present-day early-type galaxies.

Description: We investigate the relationship between active galactic nucleus (AGN) activity and host galaxy properties using a sample of massive galaxies at z ~ 2 in the Chandra Deep Field- South (CDFS). A sample of 268 galaxies with M *  〉 10 10.5 M at 1.4 〈  z  〈 3 are selected from Hubble Space Telescope wide field camera 3 (WFC3) H -band observations in CDFS taken as part of the cosmic assembly near-infrared deep extragalactic legacy survey (CANDELS) survey. We find that a large fraction (22.0 ± 2.5 per cent) are detected in the 4 Ms Chandra /Advanced CCD Image Spectrometer observations in the field, implying a high AGN content in these massive galaxies. To investigate further the relationship between these AGN and their hosts, we create four subsamples, based on their star formation rates (star-forming versus quiescent) and galaxy size (compact versus extended), following Barro et al. and perform X-ray spectral fitting. We find a clear effect whereby the AGN in compact galaxies – be they star forming or quiescent – show significantly higher luminosities and levels of obscuration than the AGN in extended galaxies. These results provide clear evidence for two modes of black hole growth in massive galaxies at high redshift. The dominant growth mode is a luminous, obscured phase which occurs overwhelmingly in compact galaxies while another lower luminosity, unobscured phase is predominantly seen in extended galaxies. Both modes could produce AGN feedback, with violent transformative feedback in the former and a gentler ‘maintenance mode’ produced by the latter.

Description: We analyse the stellar populations in the host galaxies of 53 X-ray selected optically dull active galactic nuclei (AGN) at 0.34 〈 z 〈 1.07 with ultradeep ( m AB = 26.5, 3) optical medium-band ( R ~ 50) photometry from the Survey for High-z Absorption Red and Dead Sources (SHARDS). The spectral resolution of SHARDS allows us to consistently measure the strength of the 4000 Å break, D n (4000), a reliable age indicator for stellar populations. We confirm that most X-ray selected moderate-luminosity AGN ( L X 〈 10 44  erg s –1 ) are hosted by massive galaxies (typically M * 〉10 10.5 M ) and that the observed fraction of galaxies hosting an AGN increases with the stellar mass. A careful selection of random control samples of inactive galaxies allows us to remove the stellar mass and redshift dependences of the AGN fraction to explore trends with several stellar age indicators. We find no significant differences in the distribution of the rest-frame U – V colour for AGN hosts and inactive galaxies, in agreement with previous results. However, we find significantly shallower 4000 Å breaks in AGN hosts, indicative of younger stellar populations. With the help of a model-independent determination of the extinction, we obtain extinction-corrected U – V colours and light-weighted average stellar ages. We find that AGN hosts have younger stellar populations and higher extinction compared to inactive galaxies with the same stellar mass and at the same redshift. We find a highly significant excess of AGN hosts with D n (4000) ~ 1.4 and light-weighted average stellar ages of 300–500 Myr, as well as a deficit of AGN in intrinsic red galaxies. We interpret failure in recognizing these trends in previous studies as a consequence of the balancing effect in observed colours of the age–extinction degeneracy.

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: 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.