1,721,019 research outputs found
Scrutinising evidence for the triggering of Active Galactic Nuclei in the outskirts of massive galaxy clusters at z≈1
No full textEnvironmental effects are believed to play an important yet poorly understood role in triggering accretion events onto the supermassive black holes (SMBHs) of galaxies (Active Galactic Nuclei; AGN). Massive clusters, which represent the densest structures in the Universe, provide an excellent laboratory to isolate environmental effects and study their impact on black hole growth. In this work, we critically review observational evidence for the preferential activation of SMBHs in the outskirts of galaxy clusters. We develop a semi-empirical model under the assumption that the incidence of AGN in galaxies is independent of environment. We demonstrate that the model is broadly consistent with recent observations on the AGN halo occupation at z=0.2, although it may overpredict satellite AGN in massive halos at that low redshift. We then use this model to interpret the projected radial distribution of X-ray sources around high redshift (z≈1) massive (>5×1014M⊙) clusters, which show excess counts outside their virial radius. Such an excess naturally arises in our model as a result of sample variance. Up to 20% of the simulated projected radial distributions show excess counts similar to the observations, which are however, because of background/foreground AGN and hence, not physically associated with the cluster. Our analysis emphasises the importance of projection effects and shows that current observations of z≈1 clusters remain inconclusive on the activation of SMBHs during infall
Cosmic evolution of the incidence of Active Galactic Nuclei in massive clusters: simulations versus observations
Full text linkThis paper explores the role of small-scale environment (<1 Mpc) in modulating accretion events on to supermassive black holes by studying the incidence of active galactic nuclei (AGNs) in massive clusters of galaxies. A flexible, data-driven semi-empirical model is developed based on a minimal set of parameters and under the zero-order assumption that the incidence of AGNs in galaxies is independent of environment. This is used to predict how the fraction of X-ray selected AGN among galaxies in massive dark matter haloes (≳3×1014M⊙) evolves with redshift and reveal tensions with observations. At high redshift, z ∼ 1.2, the model underpredicts AGN fractions, particularly at high X-ray luminosities, LX(2−10keV)≳1044ergs−1. At low redshift, z ∼ 0.2, the model estimates fractions of moderate luminosity AGN (LX(2−10keV)≳1043ergs−1) that are a factor of 2–3 higher than the observations. These findings reject the zero-order assumption on which the semi-empirical model hinges and point to a strong and redshift-dependent influence of the small-scale environment on the growth of black holes. Cluster of galaxies appear to promote AGN activity relative to the model expectation at z ∼ 1.2 and suppress it close to the present day. These trends could be explained by the increasing gas content of galaxies towards higher redshift combined with an efficient triggering of AGNs at earlier times in galaxies that fall on to clusters
Multi-Wavelength Surveys of Extreme Infrared Populations
Full text linkThis Thesis presents a study of galaxies and quasars from the viewpoint of their
optical, infrared and X-ray properties by combining optical data from Gemini and
WIYN with near to far-IR and optical data from the SWIRE survey and X-ray
data from Chandra. This work represents the largest existing optical spectroscopic
survey in ELAIS-N1 with ∽300 reliable spectroscopic redshifts and the largest Xray
survey, in the same field, which has extended the previous X-ray coverage in
ELAIS-N1 by a factor of 12 and has detected more than 600 X-ray sources. Optical
spectroscopy is used both to calibrate photometric redshift techniques and distinguish
between star forming galaxies and quasars. The merged X-ray, optical and
infrared catalogue is used to determine spectral energy distributions and correctly
identify and characterize AGN, star forming galaxies and the link between black
hole growth and star formation in the host galaxy
Semi-empirical models of X-ray AGN in galaxy clusters
No full textThe realisation that all galaxies host a supermassive black hole (SMBH) at their centres had a profound impact on our current understanding of galaxy formation and evolution. These SMBH grow through the accretion of surrounding material that makes them shine as active galactic nuclei (AGN). Different mechanisms have been proposed to ignite such accretion events, however, which of them is the dominant is still debated. In particular, the environment is expected to play a crucial role as it does in galaxy evolution, e.g., galaxy properties depend on which structures of the cosmic web they inhabit. In this thesis, we use a semi-empirical (SEM) data-driven approach to study the effects of the environment on AGN triggering. The advantage of this approach is the use of state-of-the-art observations to construct realistic mock catalogues instead of using physically motivated recipes that need to be calibrated. Hence, SEM is well-suited for hypothesis testing with a reduced parameter space compared to other modelling strategies. The explicit assumption of our model is that SMBH growth is independent of the halo mass. We compare model predictions against the latest observations on the incidence of AGN in massive galaxy clusters at different cosmic times. We find that AGN activity at low redshift is suppressed in clusters with respect to low-dense regions, however, at high redshift this trend is reversed with AGN being promoted in high-density regions. Enhancement of AGN activity likely happens during infall, before galaxies lose their cold gas. We test claims of preferential activation of AGN in the outskirts of massive galaxy clusters at redshift z ∼ 1. We find that the observed projected overdensities of AGN in the outskirts of high redshift clusters could be explained by cosmic variance, but we cannot reject a physical interpretation. Processes such as ram pressure or increased interaction rates due to lower velocity dispersion could promote AGN activity in the outskirts. Future work includes comparing our results with physically motivated models (e.g., hydrodynamical simulations) and analysing AGN activity in a new cluster sample spanning 0.2 < z < 1 to refine our understanding of environmental impacts on AGN activity
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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