ALBERT

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 present a study of galaxies showing mid-infrared variability in data taken in the deepest Spitzer /MIPS 24 μm surveys in the Great Observatory Origins Deep Survey South field. We divide the data set in epochs and subepochs to study the long-term (months–years) and the short-term (days) variability. We use a 2 -statistics method to select active galactic nucleus (AGN) candidates with a probability ≤1 per cent that the observed variability is due to statistical errors alone. We find 39 (1.7 per cent of the parent sample) sources that show long-term variability and 55 (2.2 per cent of the parent sample) showing short-term variability. That is, 0.03 sources arcmin –2 for both, long-term and short-term variable sources. After removing the expected number of false positives inherent to the method, the estimated percentages are 1.0 and 1.4 per cent of the parent sample for the long term and short term, respectively. We compare our candidates with AGN selected in the X-ray and radio bands, and AGN candidates selected by their IR emission. Approximately, 50 per cent of the MIPS (Multiband Imaging Photometer for Spitzer ) 24 μm variable sources would be identified as AGN with these other methods. Therefore, MIPS 24 μm variability is a new method to identify AGN candidates, possibly dust obscured and low-luminosity AGN, that might be missed by other methods. However, the contribution of the MIPS 24 μm variable identified AGN to the general AGN population is small (≤13 per cent) in GOODS-South.

Description: We present a comprehensive study of star-forming (SF) galaxies in the Hubble Space Telescope (HST) Frontier Field recent cluster merger A2744 ( z  = 0.308). Wide-field, ultraviolet–infrared (UV–IR) imaging enables a direct constraint of the total star formation rate (SFR) for 53 cluster galaxies, with SFR UV+IR  = 343 ± 10 M  yr –1 . Within the central 4 arcmin (1.1 Mpc) radius, the integrated SFR is complete, yielding a total SFR UV+IR  = 201 ± 9 M  yr –1 . Focusing on obscured star formation, this core region exhibits a total SFR IR  = 138 ± 8 M  yr –1 , a mass-normalized SFR IR of SFR  = 11.2 ± 0.7 M  yr –1 per 10 14 M and a fraction of IR-detected SF galaxies $f_{\rm SF} = 0.080^{+0.010}_{-0.037}$ . Overall, the cluster population at z  ~ 0.3 exhibits significant intrinsic scatter in IR properties (total SFR IR , T dust distribution) apparently unrelated to the dynamical state: A2744 is noticeably different to the merging Bullet cluster, but similar to several relaxed clusters. However, in A2744 we identify a trail of SF sources including jellyfish galaxies with substantial unobscured SF due to extreme stripping (SFR UV /SFR IR up to 3.3). The orientation of the trail, and of material stripped from constituent galaxies, indicates that the passing shock front of the cluster merger was the trigger. Constraints on star formation from both IR and UV are crucial for understanding galaxy evolution within the densest environments.

Description: We combine multi-wavelength data in the AEGIS-XD and C-COSMOS surveys to measure the typical dark matter halo mass of X-ray selected active galactic nuclei (AGN) [ L X (2–10 keV) 〉 10 42 erg s – 1 ] in comparison with far-infrared selected star-forming galaxies detected in the Herschel/PEP survey (PACS Evolutionary Probe; L IR  〉 10 11 L ) and quiescent systems at z 1. We develop a novel method to measure the clustering of extragalactic populations that uses photometric redshift probability distribution functions in addition to any spectroscopy. This is advantageous in that all sources in the sample are used in the clustering analysis, not just the subset with secure spectroscopy. The method works best for large samples. The loss of accuracy because of the lack of spectroscopy is balanced by increasing the number of sources used to measure the clustering. We find that X-ray AGN, far-infrared selected star-forming galaxies and passive systems in the redshift interval 0.6 〈  z  〈 1.4 are found in haloes of similar mass, log M DMH /(M h –1 ) 13.0. We argue that this is because the galaxies in all three samples (AGN, star-forming, passive) have similar stellar mass distributions, approximated by the J -band luminosity. Therefore, all galaxies that can potentially host X-ray AGN, because they have stellar masses in the appropriate range, live in dark matter haloes of log M DMH /(M h –1 ) 13.0 independent of their star formation rates. This suggests that the stellar mass of X-ray AGN hosts is driving the observed clustering properties of this population. We also speculate that trends between AGN properties (e.g. luminosity, level of obscuration) and large-scale environment may be related to differences in the stellar mass of the host galaxies.

Description: We are undertaking a search for high-redshift low-luminosity Lyman Alpha sources in the SHARDS (Survey for High-z Absorption Red and Dead Sources) survey. Among the pre-selected Lyman Alpha sources two candidates were spotted, located 3.19 arcsec apart, and tentatively at the same redshift. Here, we report on the spectroscopic confirmation with Gran Telescopio Canarias of the Lyman Alpha emission from this pair of galaxies at a confirmed spectroscopic redshifts of z =5.07. Furthermore, one of the sources is interacting/merging with another close companion that looks distorted. Based on the analysis of the spectroscopy and additional photometric data, we infer that most of the stellar mass of these objects was assembled in a burst of star formation 100 Myr ago. A more recent burst (2 Myr old) is necessary to account for the measured Lyman Alpha flux. We claim that these two galaxies are good examples of Lyman Alpha sources undergoing episodic star formation. Besides, these sources very likely constitute a group of interacting Lyman Alpha emitters (LAEs).

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: We present a complete census of all Herschel -detected sources within the six massive lensing clusters of the HST Frontier Fields (HFF). We provide a robust legacy catalogue of 263 sources with Herschel fluxes, primarily based on imaging from the Herschel Lensing Survey and PEP/HerMES Key Programmes. We optimally combine Herschel, Spitzer and WISE infrared (IR) photometry with data from HST , VLA and ground-based observatories, identifying counterparts to gain source redshifts. For each Herschel -detected source we also present magnification factor (μ), intrinsic IR luminosity and characteristic dust temperature, providing a comprehensive view of dust-obscured star formation within the HFF. We demonstrate the utility of our catalogues through an exploratory overview of the magnified population, including more than 20 background sub-LIRGs unreachable by Herschel without the assistance gravitational lensing.