1,720,989 research outputs found
Spectral ageing in the lobes of cluster-centre FR II radio galaxies
Get PDFRecent investigations have shown that many parameters and assumptions made in the application of spectral-ageing models to FR II radio galaxies (e.g. injection index, uniform magnetic field, non-negligible cross-lobe age variations) may not be as reliable as previously thought. In this paper we use new Very Large Array observations, which allow spectral curvature at GHz frequencies to be determined in much greater detail than has previously been possible, to investigate two cluster-centre radio galaxies, 3C 438 and 3C 28. We find that for both sources the injection index is much steeper than the values traditionally assumed, consistent with our previous findings. We suggest that the Tribble model of spectral ageing provides the most convincing description when both goodness-of-fit and physically plausibility are considered, but show that even with greatly improved coverage at GHz frequencies, a disparity exists in cluster-centre FR IIs when spectral ages are compared to those determined from a dynamical viewpoint. We find for 3C 438 that although the observations indicate the lobes are expanding, its energetics suggest that the radiating particles and magnetic field at equipartition cannot provide the necessary pressure to support the lobes, similar to other cluster-centre source such as Cygnus A. We confirm that small-scale, cross-lobe age variations are likely to be common in FR II sources and should be properly accounted for when undertaking spectral-ageing studies. Contrary to the assumption of some previous studies, we also show that 3C 28 is an FR II (rather than FR I) source, and suggest that it is most likely a relic system with the central engine being turned off between 6 and 9 Myr ago
Centaurus A: constraints on the nature of the giant lobe filaments from XMM-Newton observations
Get PDFWe report on deep XMM-Newton observations of the vertex filament in the southern giant lobe of the Fanaroff–Riley class I radio galaxy Centaurus A. We find no X-ray excess from the filament region and place a 3? upper limit on the 1 keV flux density of the filament of 9.6 nJy. This directly constrains the electron density and magnetic field strength in the filament. For the first time in an individual filament, we show that so long as the particle index ?2, the excess in synchrotron emissivity cannot be produced purely by excess electrons: the filament magnetic field strength must be higher than in the giant lobes as a whole, and close to or above the equipartition value for the filament. The filaments are not significantly overpressured with respect to the surrounding lobe with a pressure provided by relativistic electrons
Galaxy cluster X-ray luminosity scaling relations from a representative local sample (REXCESS)
Get PDFWe examine the X-ray luminosity scaling relations of 31 nearby galaxy clusters from the Representative XMM-Newton Cluster Structure Survey (REXCESS). The objects are selected only in X-ray luminosity, optimally sampling the cluster luminosity function. Temperatures range from 2 to 9 keV, and there is no bias toward any particular morphological type. To reduce measurement scatter we extract pertinent values in an aperture corresponding to R500, estimated using the tight correlation between (the product of gas mass and temperature) and total mass. The data exhibit power law relations between bolometric X-ray luminosity and temperature, and total mass, all with slopes that are significantly steeper than self-similar expectations. We examine the possible causes for the steepening, finding that structural variations have little effect and that the primary driver appears to be a systematic variation of the gas content with mass. Scatter about the relations is dominated in all cases by the presence of cool cores. The natural logarithmic scatter about the raw X-ray luminosity-temperature relation is about 70 per cent, and about the X-ray luminosity- relation it is 40 per?cent. Systems with more morphological substructure show similar scatter about scaling relations than clusters with less substructure, due to the preponderance of cool core systems in the regular cluster subsample. Cool core and morphologically disturbed systems occupy distinct regions in the residual space with respect to the best fitting mean relation, the former lying systematically at the high luminosity side, the latter lying systematically at the low luminosity side. Simple exclusion of the central regions serves to reduce the scatter about the scaling relations by more than a factor of two. The scatter reduces by a similar amount with the use of the central gas density as a third parameter. Using as a total mass proxy, we derive a Malmquist bias-corrected local luminosity-mass relation and compare with other recent determinations. Our results indicate that luminosity can be a reliable mass proxy with controllable scatter, which has important implications for upcoming all-sky cluster surveys, such as those to be undertaken with Planck and eROSITA, and ultimately for the use of the cluster population for cosmological purposes.<br/
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe 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
Get PDF“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
Get PDFWe 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 epoch and environmental dependence of radio-loud active galaxy feedback
No full textThis thesis contains the first systematic X-ray investigation of the relationships between the properties of different types of radio-loud AGN and their large-scale environments, using samples at two distinct redshifts to isolate the effects of evolution. I used X-ray observations of the galaxy clusters hosting the radio galaxies to characterise the properties of the environments and compared them with the low-frequency radio properties of the AGN. I found a strong relationship between radio luminosity and ICM richness for lowexcitation radio galaxies (LERGs) but not for high-excitation radio galaxies (HERGs), and tentative evidence of evolution of the environment for HERGs, but not for LERGs.I then used the X-ray emission from the lobes of the FRII galaxies in the samples to calculate their internal pressures, finding them to be at slightly higher pressures than predicted by equipartition and near pressure balance with their external environment at the lobe mid-points. I made the first estimates of lower limits for lobe-tip Mach numbers for a large sample of lobes; suggesting that about half the lobe tips were driving strong shocks.Finally I verified that the correlation between LERG radio luminosity and cluster X-ray luminosity is not caused by a common correlation with black hole mass, and also found no evidence that HERG properties are affected by ICM richness, adding evidence for theories of local fuelling for HERGs. I found evidence that HERGs should have a strong relationship between jet power and radio luminosity, but that LERG jet power must be subject to factors additional to black hole mass. I also found evidence that the same mass black hole is associated with lower radio luminosities in FRI morphology galaxies than in FRIIs, perhaps as a result of the higher proton content of FRI jets leading to lower radiative efficiency
The universal galaxy cluster pressure profile from a representative sample of nearby systems (REXCESS) and the Y_SZ - M_500 relation
No full textWe investigate the regularity of cluster pressure profiles with REXCESS, a representative sample of 33 local (z < 0.2) clusters drawn from the REFLEX catalogue and observed with XMM-Newton. The sample spans a mass range of 1014 < M500 < 1015 , where M500 is the mass corresponding to a density contrast of 500. We derive an average profile from observations scaled by mass and redshift according to the standard self-similar model, and find that the dispersion about the mean is remarkably low, at less than 30 per cent beyond 0.2 R500, but increases towards the center. Deviations about the mean are related to both the mass and the thermo-dynamical state of the cluster. Morphologically disturbed systems have systematically shallower profiles while cooling core systems are more concentrated. The scaled profiles exhibit a residual mass dependence with a slope of ~0.12, consistent with that expected from the empirically-derived slope of the M500 – YX relation; however, the departure from standard scaling decreases with radius and is consistent with zero at R500. The scatter in the core and departure from self-similar mass scaling is smaller compared to that of the entropy profiles, showing that the pressure is the quantity least affected by dynamical history and non-gravitational physics. Comparison with scaled data from several state of the art numerical simulations shows good agreement outside the core. Combining the observational data in the radial range [0.03–1] R500 with simulation data in the radial range [1–4] R500, we derive a robust measure of the universal pressure profile, that, in an analytical form, defines the physical pressure profile of clusters as a function of mass and redshift up to the cluster “boundary”. Using this profile and direct spherical integration of the observed pressure profiles, we estimate the integrated Compton parameter Y and investigate its scaling with M500 and LX, the soft band X-ray luminosity. We consider both the spherically integrated quantity, Ysph(R), proportional to the gas thermal energy, and the cylindrically integrated quantity, Ycyl(R)=YSZ DA2, which is directly related to the Sunyaev-Zel'dovich (SZ) effect signal. From the low scatter of the observed Ysph(R500) – YX relation we show that variations in pressure profile shape do not introduce extra scatter into the Ysph(R500) – M500 relation as compared to that from the YX – M500 relation. The Ysph(R500) – M500 and Ysph(R500) – LX relations derived from the data are in excellent agreement with those expected from the universal profile. This profile is used to derive the expected YSZ – M500 and YSZ – LX relations for any aperture
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe 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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