1,720,994 research outputs found
L'EVOLUZIONE DEGLI OGGETTI COMPATTI E DELLE LORO GALASSIE OSPITI NEL TEMPO COSMICO
No full textGravitational wave detections allow us to construct a demography of compact objects. These are exotic systems such as stellar-mass black holes and neutron stars. The LIGO-Virgo-KAGRA collaboration has gathered about 90 event candidates with high probability of astrophysical origin, all of them possibly associated with binary compact object mergers. In this Thesis, I explore the evolution of binary compact objects across cosmic time. This topic has become increasingly important in recent years, as the next generation of ground-based detectors is expected to detect binary black hole mergers up to z ∼ 100.
In this work, I evaluate the astrophysical rates and the properties of the host galaxies of binary black holes (BBHs), black hole-neutron star binaries (BHNSs) and binary neutron stars (BNSs). Determining their merger rate density evolution is challenging because of various sources of uncertainty. To address this issue, I have developed a code called cosmoRate, which combines catalogues of merging compact objects with an observation-based estimate of the metallicity-specific star formation rate density. This code is highly adaptable and allows for the exploration of different regions of the parameter space that significantly impact the merger rate density. For example, I took into account different formation channels of binary compact objects, such as dynamics or isolated binary evolution. I have found that the merger rate density of dynamically formed BBHs in young star clusters is higher than that of isolated BBHs. This is not the case for BNSs formed in young star clusters, which are less likely to merge because dynamics inhibits their formation.
Focusing on the isolated formation scenario, I examined the major sources of uncertainty that affect the merger rate density. For example, the BNS merger rate ranges from ∼ 10^3 to ∼ 20 Gpc^−3 yr^−1 at redshift z ∼ 0, when the common envelope efficiency parameter is varied from α = 7 to 0.5 in our population synthesis code MOBSE. In contrast, the local merger rates of BBHs and BHNSs only change by a factor of ∼ 2 − 3. The main source of uncertainty for the BBH merger rate density is the uncertainty in stellar metallicity evolution, which leads to a variation of (at least) one order of magnitude.
The host galaxies of gravitational-wave sources contain valuable insights about the formation and evolution of compact object mergers. To study this topic, I developed a code called galaxyRate, which uses observational scaling relations to estimate the astrophysical rates and host galaxy properties. I obtained the properties of the formation galaxy population from the galaxy stellar mass function, the star- forming main sequence of galaxies, and either the mass metallicity relation or the fundamental metallicity relation. I found that the evolution of the BBH merger rate density is significantly impacted by the choice of both the galaxy main sequence and the metallicity evolution. For example, the BBH merger rate density increases more rapidly when using the mass metallicity relation. I also found that, in general, BBHs tend to form in low-mass, metal-poor galaxies and merge in high-mass, metal-rich galaxies.
The next-generation ground-based detectors will explore gravitational-wave sources at high redshift. In preparation for this, I evaluated the merger rate density and evolving mass spectrum of BBHs formed from the first generation of stars (Population III stars). I thoroughly examined a variety of initial conditions, including variations in the orbital properties and initial mass function of Pop. III binary systems, as well as four models of the formation history of Pop. III stars. My analysis revealed that the median merger rate density is 4 Gpc^−3 yr^−1 at z = 8−10, and 0.1 Gpc^−3 yr^−1 at z = 0, with an uncertainty of about four orders of magnitude.Gravitational wave detections allow us to construct a demography of compact objects. These are exotic systems such as stellar-mass black holes and neutron stars. The LIGO-Virgo-KAGRA collaboration has gathered about 90 event candidates with high probability of astrophysical origin, all of them possibly associated with binary compact object mergers. In this Thesis, I explore the evolution of binary compact objects across cosmic time. This topic has become increasingly important in recent years, as the next generation of ground-based detectors is expected to detect binary black hole mergers up to z ∼ 100.
In this work, I evaluate the astrophysical rates and the properties of the host galaxies of binary black holes (BBHs), black hole-neutron star binaries (BHNSs) and binary neutron stars (BNSs). Determining their merger rate density evolution is challenging because of various sources of uncertainty. To address this issue, I have developed a code called cosmoRate, which combines catalogues of merging compact objects with an observation-based estimate of the metallicity-specific star formation rate density. This code is highly adaptable and allows for the exploration of different regions of the parameter space that significantly impact the merger rate density. For example, I took into account different formation channels of binary compact objects, such as dynamics or isolated binary evolution. I have found that the merger rate density of dynamically formed BBHs in young star clusters is higher than that of isolated BBHs. This is not the case for BNSs formed in young star clusters, which are less likely to merge because dynamics inhibits their formation.
Focusing on the isolated formation scenario, I examined the major sources of uncertainty that affect the merger rate density. For example, the BNS merger rate ranges from ∼ 10^3 to ∼ 20 Gpc^−3 yr^−1 at redshift z ∼ 0, when the common envelope efficiency parameter is varied from α = 7 to 0.5 in our population synthesis code MOBSE. In contrast, the local merger rates of BBHs and BHNSs only change by a factor of ∼ 2 − 3. The main source of uncertainty for the BBH merger rate density is the uncertainty in stellar metallicity evolution, which leads to a variation of (at least) one order of magnitude.
The host galaxies of gravitational-wave sources contain valuable insights about the formation and evolution of compact object mergers. To study this topic, I developed a code called galaxyRate, which uses observational scaling relations to estimate the astrophysical rates and host galaxy properties. I obtained the properties of the formation galaxy population from the galaxy stellar mass function, the star- forming main sequence of galaxies, and either the mass metallicity relation or the fundamental metallicity relation. I found that the evolution of the BBH merger rate density is significantly impacted by the choice of both the galaxy main sequence and the metallicity evolution. For example, the BBH merger rate density increases more rapidly when using the mass metallicity relation. I also found that, in general, BBHs tend to form in low-mass, metal-poor galaxies and merge in high-mass, metal-rich galaxies.
The next-generation ground-based detectors will explore gravitational-wave sources at high redshift. In preparation for this, I evaluated the merger rate density and evolving mass spectrum of BBHs formed from the first generation of stars (Population III stars). I thoroughly examined a variety of initial conditions, including variations in the orbital properties and initial mass function of Pop. III binary systems, as well as four models of the formation history of Pop. III stars. My analysis revealed that the median merger rate density is 4 Gpc^−3 yr^−1 at z = 8−10, and 0.1 Gpc^−3 yr^−1 at z = 0, with an uncertainty of about four orders of magnitude
Modelling the host galaxies of binary compact object mergers with observational scaling relations
Full text linkThe merger rate density evolution of binary compact objects and the
properties of their host galaxies carry crucial information to understand the
sources of gravitational waves. Here, we present galaxyRate, a new code that
estimates the merger rate density of binary compact objects and the properties
of their host galaxies, based on observational scaling relations. We generate
our synthetic galaxies according to the galaxy stellar mass function. We
estimate the metallicity according to both the mass-metallicity relation (MZR)
and the fundamental metallicity relation (FMR). Also, we take into account
galaxy-galaxy mergers and the evolution of the galaxy properties from the
formation to the merger of the binary compact object. We find that the merger
rate density changes dramatically depending on the choice of the star-forming
galaxy main sequence, especially in the case of binary black holes (BBHs) and
black hole neutron star systems (BHNSs). The slope of the merger rate density
of BBHs and BHNSs is steeper if we assume the MZR with respect to the FMR,
because the latter predicts a shallower decrease of metallicity with redshift.
In contrast, binary neutron stars (BNSs) are only mildly affected by both the
galaxy main sequence and metallicity relation. Overall, BBHs and BHNSs tend to
form in low-mass metal-poor galaxies and merge in high-mass metal-rich
galaxies, while BNSs form and merge in massive galaxies. We predict that
passive galaxies host at least ~5-10%, ~15-25%, and ~15-35% of all BNS, BHNS
and BBH mergers in the local Universe.Comment: 21 pages, 22 figures (including appendices), 3 tables, published 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
New insights on binary black hole formation channels after GWTC-2: young star clusters versus isolated binaries
No full textVariations 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
- …
