1,721,096 research outputs found
Constraining the UV emissivity of AGN throughout cosmic time via X-ray surveys
Full text linkThe cosmological process of hydrogen (H i) reionization in the intergalactic medium is thought to be driven by UV photons emitted by star-forming galaxies and ionizing active galactic nuclei (AGN). The contribution of quasars (QSOs) to H i reionization at z > 4 has been traditionally believed to be quite modest. However, this view has been recently challenged by new estimates of a higher faint-end UV luminosity function (LF). To set firmer constraints on the emissivity of AGN at z < 6, we here make use of complete X-ray-selected samples including deep Chandra and new Cosmic Evolution Survey data, capable to efficiently measure the 1 Ryd comoving AGN emissivity up to z ∼ 5–6 and down to 5 mag fainter than probed by current optical surveys, without any luminosity extrapolation. We find good agreement between the logNH ≲ 21-22 cm−2 X-ray LF and the optically selected QSO LF at all redshifts for M1450 ≤ −23. The full range of the logNH ≲ 21-22 cm−2 LF (M1450 ≤ −17) was then used to quantify the contribution of AGN to the critical value of photon budget needed to keep the Universe ionized. We find that the contribution of ionizing AGN at z = 6 is as small as 1–7 per cent, and very unlikely to be greater than 30 per cent, thus excluding an AGN-dominated reionization scenario
No significant evolution of relations between black hole mass and galaxy total stellar mass up to z ∼ 2.5
No full textWe investigate the cosmic evolution of the ratio between black hole (BH) mass (M BH) and host galaxy total stellar mass (M stellar) out to z ~ 2.5 for a sample of 100 X-ray-selected moderate-luminosity, broad-line active galactic nuclei (AGNs) in the Chandra-COSMOS Legacy Survey. By taking advantage of the deep multiwavelength photometry and spectroscopy in the COSMOS field, we measure in a uniform way the galaxy total stellar mass using an spectral energy distribution decomposition technique and the BH mass based on broad emission line measurements and single-epoch virial estimates. Our sample of AGN host galaxies has total stellar masses of 1010−12 M ⊙, and BH masses of 107.0–9.5 M ⊙. Combining our sample with the relatively bright AGN samples from the literature, we find no significant evolution of the M BH–M stellar relation with the BH-to-host total stellar mass ratio of M BH/M stellar ~ 0.3% at all redshifts probed. We conclude that the average BH-to-host stellar mass ratio appears to be consistent with the local value within the uncertainties, suggesting a lack of evolution of the M BH–M stellar relation up to z ~ 2.5
Hidden Active Galactic Nuclei in Early-type Galaxies
Full text linkWe present a stacking analysis of the complete sample of early-Type galaxies (ETGs) in the Chandra COSMOS (C-COSMOS) survey, to explore the nature of the X-ray luminosity in the redshift and stellar luminosity ranges and . Using established scaling relations, we subtract the contribution of X-ray binary populations to estimate the combined emission of hot ISM and active galactic nuclei (AGNs). To discriminate between the relative importance of these two components, we (1) compare our results with the relation observed in the local universe for hot gaseous halos emission in ETGs, and (2) evaluate the spectral signature of each stacked bin. We find two regimes where the non-stellar X-ray emission is hard, consistent with AGN emission. First, there is evidence of hard, absorbed X-ray emission in stacked bins including relatively high z (∼1.2) ETGs with average high X-ray luminosity (). These luminosities are consistent with the presence of highly absorbed "hidden" AGNs in these ETGs, which are not visible in their optical-IR spectra and spectral energy distributions. Second, confirming the early indication from our C-COSMOS study of X-ray detected ETGs, we find significantly enhanced X-ray luminosity in lower stellar mass ETGs (), relative to the local relation. The stacked spectra of these ETGs also suggest X-ray emission harder than expected from gaseous hot halos. This emission is consistent with inefficient accretion onto . © 2016. The American Astronomical Society. All rights reserved
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
High Resolution Deep X-ray Surveys: A Unique Probe of the High-Z Universe
No full textUnderstanding the origin and growth of the supermassive black holes SMBHs that lie at the centers of most, if not all, galaxies at all redshifts is crucial for obtaining the full picture of both BH and galaxy evolution. Enormous progress has been made in the understanding of the galaxy population in the Universes first few Gyrs. On the contrary, there is still the puzzle that giant SMBHs already exist at z=6-7, implying massive seed black holes or highly super-Eddington accretion at the earliest times. This is because the search for z6 Active Galactic Nuclei i.e., accreting SMBHs is still limited to the optically selected sources from the SDSS or CFHQS surveys, which probe the unobscured accretion. Instead, X-ray surveys can probe the obscured high-z, low mass population, which represent the bulk of rapid early SMBH growth, and are therefore essential for obtain an unbiased sample of the accreting SMBHs. I will present the most recent results on the high redshift Universe as we know it from the current Chandra X-ray surveys and discuss what we can expect from planned and proposed future X-ray observatories. High spatial resolution is crucial to locate faint X-ray sources and identify their counterpart in future deep optical images. Increasing the surveys sensitivity to the limits allowed by the X-ray Surveyor, we will be able to collect sizable samples of z6 low luminosity and low mass AGN to study their early growth. Comparison with galaxy formation and evolution at similar redshift will provide the means to understand the SMBH-galaxy relation at the earliest epochs.Session 1 - Compact Objects : Neutron Stars and the Birth and Evolution of Black Holes
Chair: Chryssa Kouvelioto
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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