ALBERT

Description: Starting from the results regarding a nonvacuum technique to fabricate CIGS thin films for solar cells by means of single-step electrodeposition, we focus on the methodological problems of modeling at cell structure and photovoltaic module levels. As a matter of fact, electrodeposition is known as a practical alternative to costly vacuum-based technologies for semiconductor processing in the photovoltaic device sector, but it can lead to quite different structural and electrical properties. For this reason, a greater effort is required to ensure that the perspectives of the electrical engineer and the material scientist are given an opportunity for a closer comparison and a common language. Derived parameters from ongoing experiments have been used for simulation with the different approaches, in order to develop a set of tools which can be used to put together modeling both at single cell structure and complete module levels.

Description: Biomass is one of the most promising renewable energy sources. Abundantly, the potential as an alternative source to meet the world energy demand has been widely acknowledged. Gasification is one of the most efficient processes concerning thermochemical conversion, having as objective the production of a gas with useful energy power, known as producer gas. In order to optimize thermochemical processes such as the combustion of gases and subsequent gas mixture, computer modeling is becoming an important tool. Aiming to improve the performance of a combustion chamber, previously coupled to a downdraft gasifier, a thermofluidynamic model was elaborated and validated, using the concepts of computational fluid dynamics (CFD). It was reported that temperature, pressure, and velocity distributions of the computational model showed good consistency with experimental data, which allows using this model to predict the performance of this type of combustion chambers.

Description: We have investigated the gas content of a sample of several hundred AGN host galaxies at z  〈 1 and compared it with a sample of inactive galaxies, matched in bins of stellar mass and redshift. Gas masses have been inferred from the dust masses, obtained by stacked Herschel far-IR and sub-mm data in the GOODS and COSMOS fields, under reasonable assumptions and metallicity scaling relations for the dust-to-gas ratio. We find that AGNs are on average hosted in galaxies much more gas rich than inactive galaxies. In the vast majority of stellar mass bins, the average gas content of AGN hosts is higher than that in inactive galaxies. The difference is up to a factor of 10 higher in low-stellar-mass galaxies, with a significance of 6.5. In almost half of the AGN sample, the gas content is three times higher than that in the control sample of inactive galaxies. Our result strongly suggests that the probability of having an AGN activated is simply driven by the amount of gas in the host galaxy; this can be explained in simple terms of statistical probability of having a gas cloud falling into the gravitational potential of the black hole. The increased probability of an AGN being hosted by a star-forming galaxy, identified by previous works, may be a consequence of the relationship between gas content and AGN activity, found in this paper, combined with the Schmidt–Kennicutt law for star formation.

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 present X-shooter at Very Large Telescope observations of a sample of 10 luminous, X-ray obscured quasi-stellar objects (QSOs) at z  ~ 1.5 from the XMM -COSMOS survey, expected to be caught in the transitioning phase from starburst to active galactic nucleus (AGN)-dominated systems. The main selection criterion is X-ray detection at bright fluxes ( L X   10 44  erg s –1 ) coupled to red optical-to-near-infrared-to-mid-infrared colours. Thanks to its large wavelength coverage, X-shooter allowed us to determine accurate redshifts from the presence of multiple emission lines for five out of six targets for which we had only a photometric redshift estimate, with an 80 per cent success rate, significantly larger than what is observed in similar programs of spectroscopic follow-up of red QSOs. We report the detection of broad and shifted components in the [O iii ] 5007, 4959 complexes for six out of eight sources with these lines observable in regions free from strong atmospheric absorptions. The full width at half-maximum (FWHM) associated with the broad components are in the range FWHM ~ 900–1600 km s –1 , larger than the average value observed in Sloan Digital Sky Survey type 2 AGN samples at similar observed [O iii ] luminosity, but comparable to those observed for QSO/ultraluminous infrared galaxies systems for which the presence of kpc scale outflows has been revealed through integral field unit spectroscopy. Although the total outflow energetics (inferred under reasonable assumptions) may be consistent with winds accelerated by stellar processes, we favour an AGN origin for the outflows given the high outflow velocities observed ( v  〉 1000 km s –1 ) and the presence of strong winds also in objects undetected in the far-infrared.

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: We present the most complete study to date of the X-ray emission from star formation in high-redshift (median z  = 0.7; z  〈 1.5), IR-luminous ( L IR  = 10 10 –10 13  L ) galaxies detected by Herschel 's PACS and SPIRE instruments. For our purpose, we take advantage of the deepest X-ray data to date, the Chandra Deep Fields (North and South). Sources which host AGN are removed from our analysis by means of multiple AGN indicators. We find an AGN fraction of 18 ± 2 per cent amongst our sample and note that AGN entirely dominate at values of log [ L X / L IR ] 〉 –3 in both hard and soft X-ray bands. From the sources which are star formation dominated, only a small fraction are individually X-ray detected and for the bulk of the sample we calculate average X-ray luminosities through stacking. We find an average soft X-ray to infrared ratio of log 〈 L SX / L IR 〉 = –4.3 and an average hard X-ray to infrared ratio of log 〈 L HX / L IR 〉 = –3.8. We report that the X-ray/IR correlation is approximately linear through the entire range of L IR and z probed and, although broadly consistent with the local ( z  〈 0.1) one, it does display some discrepancies. We suggest that these discrepancies are unlikely to be physical, i.e. due to an intrinsic change in the X-ray properties of star-forming galaxies with cosmic time, as there is no significant evidence for evolution of the L X / L IR ratio with redshift. Instead, they are possibly due to selection effects and remaining AGN contamination. We also examine whether dust obscuration in the galaxy plays a role in attenuating X-rays from star formation, by investigating changes in the L X / L IR ratio as a function of the average dust temperature. We conclude that X-rays do not suffer any measurable attenuation in the host galaxy.

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 examine the star formation in the outer halo of NGC 1275, the central galaxy in the Perseus cluster (Abell 426), using far-ultraviolet and optical images obtained with the Hubble Space Telescope . We have identified a population of very young, compact star clusters with typical ages of a few Myr. The star clusters are organized on multiple kiloparsec scales. Many of these star clusters are associated with ‘streaks’ of young stars, the combination of which has a cometary appearance. We perform photometry on the star clusters and diffuse stellar streaks, and fit their spectral energy distributions to obtain ages and masses. These young stellar populations appear to be normal in terms of their masses, luminosities and cluster formation efficiency; 〈10 per cent of the young stellar mass is located in star clusters. Our data suggest star formation is associated with the evolution of some of the giant gas filaments in NGC 1275 that become gravitationally unstable on reaching and possibly stalling in the outer galaxy. The stellar streaks then could represent stars moving on ballistic orbits in the potential well of the galaxy cluster. We propose a model where star-forming filaments, switched on ~50 Myr ago and are currently feeding the growth of the NGC 1275 stellar halo at a rate of –2 to 3 M  yr –1 . This type of process may also build stellar haloes and form isolated star clusters in the outskirts of youthful galaxies.