1,720,958 research outputs found
Spectroscopic signatures of sub-parsec massive black hole.
No full textThe focus of this thesis is on the electromagnetic signatures associated with massive black hole (MBH) binaries (MBHBs). In current models of structure formation, galaxies assemble in a bottom-up hierarchical fashion. Observations show that MBHs are hosted in the nuclei of almost all nearby massive galaxies. The existence of close MBHBs is therefore expected as the result of galaxy mergers, although clear observational evidences of MBHBs remain elusive. Most of the current sub-pc MBHB candidates have been spectroscopically selected through the Doppler-shift technique. This method consists in the identification of broad emission lines (BELs) shifted with respect to the galaxy reference frame, as traced by the set of narrow lines (NELs). This shift is due to the hypothesized motion of the active MBH orbiting in a close binary. However velocity off-sets between BELs and NELs are not a unique feature of close MBHBs, and cannot be searched for beyond z ∼ 0.8 since most NELs fall out of the wavelength range covered by current optical surveys.
This work presents our contribution to (i) the prediction of new spectroscopic features associated with MBHBs at different evolutionary stages in the aftermath of a gas-rich merger, and to (ii) the search for new secure candidates at different redshifts, ruling out other possible interpretations of their physical nature.
In the first part of this thesis we discuss a MBHB model envisaging a set of narrow lines characterized by the same Doppler-shift displayed by the BELs.
This additional NEL system originates from the lower density gas orbiting in the so called gap region excavated by the gravitational interaction between the sub-pc MBHB and a surrounding thin circum-binary disk. We propose this model to interpret the features observed in the spectrum of the quasar SDSSJ092712.65+294344.0 (SDSSJ0927 hereafter). For the case of this peculiar source we show that the binary hypothesis is favored against the other possible physical scenarios of a recoiling AGN or of a chance superposition of two distinct objects in a galaxy cluster. However further follow up observations are mandatory to monitor on longer timescales (∼ > 10 yr) the periodic drift of the BELs expected over the binary orbital period, and to gather additional information on the quasar host galaxy and its close environment. Our study on SDSSJ0927 contributed to the realization of a systematic search for MBHB candidates at z ∼ < 0.8 through the development of automatic procedures by different groups including ours. The new algorithm implemented by our group will be also adapted to look for new candidates at higher redshifts based on the presence of two displaced peaks in the profile of the broad CIV line interpreted in terms of two active binary MBHs.
In the second part of this work, we devise a new spectroscopic approach to select MBHBs based on peculiarly reduced flux ratios between broad lines of lower and higher ionization potentials. In particular, we show that MBHBs can display anomalously small flux ratios between the MgII and CIV BELs, i.e. FMgII/FCIV ∼ < 0.15, in a limited range of sub-pc separations due to the erosion of the BEL emitting material bound to the secondary active MBH. This erosion is due to the tidal field of the more massive binary companion.
The advantage of our newly proposed technique is twofold: (i) it can select new candidates at higher redshifts, z ∼ 2, where both the considered BELs are visible in the optical spectrum, and (ii) the detection of a reduced FMgII/FCIV would be a strong indication of the presence of a sub-pc MBHB since it is not expected in the other physical scenarios alternative to the binary hypothesis.
We show that it is possible to use our flux ratio criterion to select peculiar AGN spectra at z ∼ 2 from the database of the Sloan Sky Digital Survey.
These peculiar sources can be observed again in the IR band to look for a shift between the broad lines of Hβ, MgII and CIV and the narrow [OIII]λ5007.
We start this new MBHB search with a small sample of 9 targets observed in the NIR-H band with the Very Large Telescope. Two sources are selected as binary candidates because the lower ionization lines of the MgII and of the Hβ have reduced fluxes relative respectively to the higher ionization line of the CIV and of the [OIII], and the three observed BELs show moderate shifts up to 1000 kms−1 relative to the narrow [OIII]. Hence we plan to perform new optical observations of these two sources to confirm and monitor the measured shifts, and to complete the search for MBHBs at z ∼ 2 to check the reliability of our newly proposed spectroscopic approach
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
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Author-wise bibliometric analysis based on entropy.
No full textAuthor-wise bibliometric analysis based on entropy.</p
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