1,720,958 research outputs found
The impact of primordial black holes on the high redshift Universe
Full text linkRecent measurements of the cosmic X-ray and radio backgrounds (CXB/CRB, respectively), obtained with Chandra and ARCADE2, report signals in excess of those expected from known sources. Similarly, measurements of the near infrared background (NIRB) angular power spectrum on angular scales θ ≥ 1 arcmin exceeds by roughly two order of magnitudes predictions from known galaxy populations. The nature of the sources producing the CXB/CRB excesses, and the NIRB fluctuations remains unknown, suggesting the presence of a yet undiscovered population of emitters. Interestingly, the NIRB has been found to cross-correlate with the soft-X-ray background (SXB), possibly justified by X-ray emission from elusive, high redshift accreting black holes (BHs). In this context, the most recent James Webb Space Telescope (JWST) observations have revealed the presence of z ∼ 6-11 active galactic nuclei (AGN) powered by accreting MBHs (M_{BH} ∼ 10^{6-8} M_{sun}). The existence of these sources, along with supermassive black holes (SMBHs, M_{BH} ∼ 10^{8-10} M_{sun}) powering z ∼ 6-7.5 quasars, poses a puzzle for current theoretical models of BH formation and evolution. It is indeed still unclear both the nature of the seeds from which these SMBHs are formed and their ability to grow fast enough to assemble an SMBH in less than 1 Gyr (the age of the Universe at z ∼ 6). These results can be interpreted as requiring either massive (M_{BH} ∼ 10^{4-6} M_{sun}) seeds and/or less extreme BHs experiencing bursts of super-Eddington accretion. In this Thesis, we tackle the aforementioned puzzles by ascribing their solution to a population of accreting primordial black holes (PBHs). PBHs are black holes that are expected to have formed during the radiation dominated era from the collapse of overdense regions, and have been considered as potential dark matter (DM) candidates. In particular, the questions we would like to answer are:Can PBHs be the sources of the observed backgrounds excess?Can PBHs lead to the formation of SMBHs seeds?This thesis describes the PBH theoretical model we developed and presents the analysis we carried out to answer the aforementioned two open questions
Primordial Black Holes as Near-Infrared Background sources
Full text linkThe near infrared background (NIRB) is the collective light from unresolved
sources observed in the band 1-10 m. The measured NIRB angular power
spectrum on angular scales arcmin exceeds by roughly two
order of magnitudes predictions from known galaxy populations. The nature of
the sources producing these fluctuations is still unknown. Here we test
primordial black holes (PBHs) as sources of the NIRB excess. Considering PBHs
as a cold dark matter (DM) component, we model the emission of gas accreting
onto PBHs in a cosmological framework. We account for both accretion in the
intergalactic medium (IGM) and in DM haloes. We self consistently derive the
IGM temperature evolution, considering ionization and heating due to X-ray
emission from PBHs. Besides CDM, we consider a model that accounts for
the modification of the linear matter power spectrum due to the presence of
PBHs; we also explore two PBH mass distributions, i.e. a -function and
a lognormal distribution. For each model, we compute the mean intensity and the
angular power spectrum of the NIRB produced by PBHs with mass
1-. In the limiting case in which the entirety of DM
is made of PBHs, the PBH emission contributes <1 per cent to the observed
NIRB fluctuations. This value decreases to <0.1 per cent if current
constraints on the abundance of PBHs are taken into account. We conclude that
PBHs are ruled out as substantial contributors to the NIRB
Blue monsters : why are JWST super-early, massive galaxies so blue?
Full text linkThe recent James Webb Space Telescope tentative discovery of a population of super-early (redshift z > 10), relatively massive (stellar mass M* = 108-109M⊙) and evolved (metallicity Z ≈ 0.1 Z⊙) galaxies, which nevertheless show blue (β ≃ -2.6) spectra, and very small dust attenuation (AV ≲ 0.02), challenges our interpretation of these systems. To solve the puzzle, we propose two solutions in which dust is either (a) ejected by radiation pressure, or (b) segregated with respect to UV-emitting regions. We clarify the conditions for which the two scenarios apply, and show that they can be discriminated by ALMA observations, such as the recent non-detection of the 88μm dust continuum in GHZ2 (z ≃ 12) favouring dust ejection
Cosmic radiation backgrounds from primordial black holes
Full text linkRecent measurements of the cosmic X-ray and radio backgrounds (CXB/CRB,
respectively) obtained with Chandra and ARCADE2 report signals in excess of
those expected from known sources, suggesting the presence of a yet
undiscovered population of emitters. We investigate the hypothesis that such
excesses are due to primordial black holes (PBHs) which may constitute a
substantial fraction of dark matter (DM). We present a novel semi-analytical
model which predicts X-ray and radio emission due to gas accretion onto PBHs,
assuming that they are distributed both inside DM halos and in the
intergalactic medium (IGM). Our model includes a self-consistent treatment of
heating/ionization feedback on the surrounding environment. We find that (i)
the emission from PBHs accreting in the IGM is subdominant at all times (); (ii) most of the CXB/CRB emission
comes from PBHs in DM mini-halos () at early epochs
(). While a small fraction () of DM in the form
of PBHs can account for the total observed CXB excess, the CRB one cannot be
explained by PBHs. Our results set the strongest existing constraint on in the mass range . Finally, we comment on the implications of our results on the global
21cm signal.Comment: 13 pages, 10 figures, 2 tables. Accepted for publication in MNRA
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
Interpreting ALMA non-detections of JWST super-early galaxies
Full text linkRecent attempts to detect [O iii] 88μm emission from super-early (z > 10) galaxy candidates observed by JWST have been unsuccessful. Non-detections can be either due to wrong photometric redshifts or to the faintness of the line in such early systems. By using zoom-in simulations, we show that if redshifts of these galaxies are confirmed, they are faint and mostly fall below the local metal-poor [OIII]−SFR relation as a result of their low ionization parameter, Uion ≲ 10−3. Such low Uion values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample (log[L[OIII]/L⊙]=8.4, Uion ≈ 0.1) could be detected by ALMA in only 2.8 h.Recent attempts to detect [O iii] 88, μm emission from super-early (z > 10) galaxy candidates observed by JWST have been unsuccessful. Non-detections can be either due to wrong photometric redshifts or to the faintness of the line in such early systems. By using zoom-in simulations, we show that if redshifts of these galaxies are confirmed, they are faint and mostly fall below the local metal-poor [O, small III}]-SFR relation as a result of their low ionization parameter, Uion 10-3. Such low Uion values are found in galaxies that are in an early assembly stage, and whose stars are still embedded in high-density natal clouds. However, the most luminous galaxy in our sample (log [L[O, small III]L⊙] = 8.4, Uion ≈ 0.1) could be detected by ALMA in only 2.8 h
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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