1,721,095 research outputs found
The Asiago-ESO/RASS QSO Survey. I. The Catalog and the Local QSO Luminosity Function
Full text linkThis paper presents the first results of a survey for bright quasars (V30°. The photometric database is derived from the Guide Star and USNO catalogs. Quasars are identified on the basis of their X-ray emission measured in the ROSAT All-Sky Survey. The surface density of quasars brighter than 15.5 mag turns out to be (10+/-2)×10-3 deg-2, about 3 times higher than that estimated by the PG survey. The quasar optical luminosity function (LF) at 0.04<z<=0.3 is computed and shown to be consistent with a luminosity-dependent luminosity evolution of the type derived by La Franca & Cristiani in the range 0.3<z<=2.2. The predictions of semianalytical models of hierarchical structure formation agree remarkably well with the present observations
The rise of the CIV mass density at z < 2.5
Full text linkThe cosmic evolution of the metal content of the intergalactic medium puts stringent constraints on the properties of galactic outflows and on the nature of UV background. In this paper, we present a new measure of the redshift evolution of the mass density of C IV, Omega(C) (IV), in the interval 1.5 less than or similar to z less than or similar to 4 based on a sample of more than 1500 C IV lines with column densities 10(12) less than or similar to N(C IV) less than or similar to 10(15) cm(-2). This sample more than doubles the absorption redshift path covered in the range z < 2.5 by previous samples. The result shows a significant increase of Omega(C IV) towards the lower redshifts at variance with the previously pictured constant behaviour
Tracing the gas at redshift 1.7-3.5 with the Lyα forest: the FLO approach
Full text linkWe present FLO (From Lines to Over-densities), a new technique to reconstruct the hydrogen density field for the Lya forest lines observed in high resolution QSO spectra. The method is based on the hypothesis that the Lya lines arise in the low to intermediate density intergalactic gas and that the Jeans length is the typical size of the Lya absorbers. The reliability of FLO is tested against mock spectra obtained from cosmological simulations. The recovering algorithm gives satisfactory results in the range from the mean density to over-densities of ~30 and reproduces correctly the correlation function of the density field and the 1D power spectrum on scales between ~20 and 60 comoving Mpc. A sample of Lya forests from 22 high resolution QSO spectra is analysed, covering the redshift range 1.7<z<3.5. For each line of sight, we fit Voigt profiles to the lines of the Lya forest, providing the largest, homogeneous sample of fitted Lya lines ever studied. The line number density evolution with redshift follows a power-law relation: dn/dz=(166 +/- 4) [(1+z)/3.5]^{(2.8 +/- 0.2)} (1 sigma errors). The two-point correlation function of lines shows a signal up to separations of ~2 comoving Mpc; weak lines (log N(HI)<13.8) also show a significant clustering but on smaller scales (r<1.5 comoving Mpc). We estimate with FLO the hydrogen density field toward the 22 observed lines of sight. The redshift distribution of the average densities computed for each QSO is consistent with the cosmic mean hydrogen density in the analysed redshift range. The two-point correlation function and the 1D power spectrum of the delta field are estimated. The correlation function shows clustering signal up to ~4 comoving Mpc
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
The QUBRICS Survey
No full textFinding the brightest QSOs at high-z is important both for constraining cosmic evolution and fundamental physics. However, in the Southern Hemisphere, the number of bright QSOs is still relatively scarce. The advent of recent databases, including SkyMapper, Gaia, DESI, offers a golden opportunity to fill in this gap. The QUBRICS survey (QUasars as BRIght beacons for Cosmology in the Southern hemisphere) has been active since 2019, exploiting these databases through machine learning techniques (e.g. CCA, PRF, XGB) and so far over 400 new, bright (i 2.5) QSOs have been spectroscopically confirmed. In this poster we highlight some of the multiple scientific applications enabled by such a dataset like (i) improving the estimate of the luminosity function; (ii) analyzing absorption features along the line of sight and (iii) improving the performance of the selection algorithms. In the future, QSOs confirmed by this survey will be the targets of subsequent studies using higher resolution spectrographs like MIKE, ESPRESSO, WINERED and will be prime targets for foreground galaxy redshift surveys with LLAMAS
The probabilistic random forest applied to the selection of quasar candidates in the QUBRICS survey
Full text linkThe number of known, bright (i 2.5) QSOs in the Southern hemisphere is considerably lower than the corresponding number in the Northern hemisphere due to the lack of multiwavelength surveys at δ 2.5 QSOs. The performances of the PRF, currently comparable to those of the CCA, are expected to improve as the number of high-z QSOs available for the training sample grows: results are however already promising, despite this being one of the first applications of this method to an astrophysical context
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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