1,721,018 research outputs found
A Multi-band view of high-redshift QSOs and future outlook for LSST
No full textActive Galactic Nuclei (AGN) are highly luminous sources (L ∼ 1042 − 1048 erg/s), powered by accretion onto supermassive black holes (SMBHs) with masses ranging from MBH ∼ 106 − 109M⊙, located at the centers of galaxies. The observed tight correlations between SMBH properties and those of their host galaxies has led to the understanding that they co-evolve, influencing one another. Over the past few years, AGN research has increasingly focused on their role in galaxy evolution, although the physical processes underlying this interaction remain still unclear. Growing evidence indicates that AGN-driven outflows can deposit energy and momentum into the interstellar medium of their host galaxies, influencing star formation activity. This suggests these outflows as a potential feedback mechanism in regulating galaxy evolution. This feedback effect is expected to be particularly strong during the Cosmic Noon, at z = 2 − 3, when both AGN accretion and star formation activity are at their peak. As the efficiency in generating powerful outflows appears to scale with AGN luminosity, understanding the coupling between nuclear energy output and the host galaxy is an open issue particularly relevant for the most luminous AGN, known as quasars (hereafter QSOs). Another thriving field of study focuses on understanding the formation of these supermassive black holes. Indeed, due to their extreme luminosities, QSOs can be observed at very early cosmic epochs, reaching back to z = 6 − 7.5, when the Universe was less than 1 Gyr old. These extremely distant QSOs reveal the existence of fully formed SMBHs already in the early Universe, posing a significant challenge to our current models, which struggle to explain how such massive objects could form in such a short span of time. A direct approach to understanding the physical processes within AGN is through their spectral energy distri- bution (SED), which represents their emission across the electromagnetic spectrum. The AGN SED extends from the hard X-ray to the radio bands and is due to the sum of several contributions, arising from distinct regions and from different physical mechanisms. Specifically, gas inflow toward the SMBH via an accretion disk converts gravitational energy into thermal energy through viscous torques, producing the distinctive blue bump in UV and optical emission. This primary radiation is Compton up-scattered to X-ray energies in a region of hot electron gas called the corona. Additionally, a surrounding dusty torus absorbs photons emitted by the corona and the accretion disk, re-emitting them in the near and mid-IR. Dust at lower temperatures (T ≈ 20 − 100 K), located much farther from the nucleus and heated by both hot stars and partially by the AGN itself, generates far-IR emission. Finally, in approximately 10% of AGN, the presence of a relativistic jet accounts for their radio emission. This thesis focuses on high-redshift AGN, specifically on their SED. Initially, we concentrated on the WISSH sample, comprising 85 hyperluminous QSOs at redshifts 2–4. These QSOs, which can drive some of the fastest and most powerful outflows, are ideal targets for exploring the AGN-host galaxy connection. Then, we moved to even higher redshifts, studying the X-ray-to-NIR broadband emission of the HYPERION and E-XQR samples at z = 5.5 − 7.5. These quasars host the most massive SMBHs observed in this redshift range, challenging our current models of SMBH formation. Finally, we address preparatory work for the upcoming Vera Rubin Legacy Survey of Space and Time (LSST), which, with its wide and deep coverage of the southern sky, is expected to discover many new high-z QSOs, increasing the known population by at least an order of magnitude. This thesis is organized as follows: - Chapter 1 provides a brief introduction to AGN, covering their powering mechanism, structure and resulting spectral energy distribution. Sec. 1.5 highlights the particular interest in studying luminous QSOs within the broader AGN population. - Chapter 2 presents the derivation of a X-ray to FIR mean SED for the WISSH sample, which consists of 85 hyperluminous QSOs at Cosmic Noon, and its comparison with other averaged AGN SEDs from the literature. - Chapter 3 is dedicated to investigating the broadband emission of 54 QSOs at the Epoch of Reionization, drawn from the HYPERION and E-XQR-30 samples. 1 - Chapter 4 describes the AGILE pipeline, developed to produce a realistic photometric catalog that closely mirrors the data expected from LSST observations. This chapter particularly focuses on the modeling and assignment of multiwavelength SEDs for AGN in the catalog. - Chapter 5 presents the results of applying machine learning tools on datasets with LSST-like features to efficiently identify luminous, high-redshift QSOs within the vast amount of data that LSST will deliver, and, more generally, to distinguish AGN from inactive galaxies and stars. - Chapter 6 provides a brief summary of the results of the work and outlines future perspectives for continuing the analysis of high-z QSOs
Chasing the observational signatures of seed black holes at z > 7: Candidate statistics
Full text linkSupermassive black holes (SMBHs) of 109–1010 M⊙ were already in place ∼13 Gyr ago, at z > 6. Super-Eddington growth of low-mass BH seeds (∼100 M⊙) or less extreme accretion on to∼105 M⊙ seeds have been recently considered as the main viable routes to these SMBHs. Here, we study the statistics of these SMBH progenitors at z ∼ 6. The growth of low- and high-mass seeds and their host galaxies are consistently followed using the cosmological data constrained model GAMETE/QSOdust, which reproduces the observed properties of high-z quasars, like SDSS J1148+5251. We show that both seed formation channels can be in action over a similar redshift range 15 < z < 18 and are found in dark matter haloes with comparable mass, ∼5 × 107 M⊙. However, as long as the systems evolve in isolation (i.e. no mergers occur), noticeable differences in their properties emerge: At z ≥ 10 galaxies hosting high-mass seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star formation proceeds less efficiently than in low-mass seeds hosts. At z < 10 these differences are progressively erased, as the systems experience minor or major mergers and every trace of the BH origin gets lost
The low-end of the black hole mass function at cosmic dawn
No full textUnderstanding the formation and growth of supermassive black holes (SMBHs) at high redshift represents a major challenge for theoretical models. In this work we investigate the early evolution of the first SMBHs by constraining their distribution in mass and luminosity at z>4. In particular, we focus on the poorly explored low-mass end of the nuclear black hole (BH) distribution down to z≃4, and explore its connection with the nature of the first BH seeds and the processes governing their mass growth. To this aim, we have developed CAT (Cosmic Archaeology Tool), a new semi-analytic model that describes the formation of the first stars and black holes in a self-consistent way and follows the co-evolution of nuclear BHs and their host galaxies for a representative population at z>4. We find that current observational constraints favour models where the growth of BH seeds is Eddington limited and occurs at the Bondi-Hoyle-Lyttleton rate or where super-Eddington accretion occurs via a slim disk during gas rich galaxy mergers. The main difference between these two model variants lies at the low-end of the predicted mass and luminosity functions at 4≤z≤6, where a clear gap appears in the first model, reflecting the stunted growth of light BH seeds formed as remnants of the first stars. Detecting this signature will be extremely challenging even for the future generation of space observatories, such as JWST, Athena and Lynx
Chasing the observational signatures of seed black holes at z > 7: Candidate statistics
Full text linkSupermassive black holes (SMBHs) of 109–1010M were already in place ∼13 Gyr ago, at
z > 6. Super-Eddington growth of low-mass BH seeds (∼100M) or less extreme accretion
on to∼105M seeds have been recently considered as the main viable routes to these SMBHs.
Here, we study the statistics of these SMBH progenitors at z ∼ 6. The growth of low- and
high-mass seeds and their host galaxies are consistently followed using the cosmological data
constrained model GAMETE/QSODUST, which reproduces the observed properties of high-z
quasars, like SDSS J1148+5251.We show that both seed formation channels can be in action
over a similar redshift range 15 < z < 18 and are found in dark matter haloes with comparable
mass, ∼5 × 107M. However, as long as the systems evolve in isolation (i.e. no mergers
occur), noticeable differences in their properties emerge: At z ≥ 10 galaxies hosting high-mass
seeds have smaller stellar mass and metallicity, the BHs accrete gas at higher rates and star
formation proceeds less efficiently than in low-mass seeds hosts. At z < 10 these differences
are progressively erased, as the systems experience minor or major mergers and every trace of
the BH origin gets lost
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 low-end of the black hole mass function at cosmic dawn
Full text linkUnderstanding the formation and growth of supermassive black holes (SMBHs) at high redshift represents a major challenge for theoretical models. In this work, we investigate the early evolution of the first SMBHs by constraining their distribution in mass and luminosity at z > 4. In particular, we focus on the poorly explored low-mass end of the nuclear black hole (BH) distribution down to z ≃ 4, and explore its connection with the nature of the first BH seeds and the processes governing their mass growth. To this aim, we have developed CAT (Cosmic Archaeology Tool), a new semi-analytic model that describes the formation of the first stars and BHs in a self-consistent way and follows the co-evolution of nuclear BHs and their host galaxies for a representative population at z > 4. We find that current observational constraints favour models where the growth of BH seeds is Eddington limited and occurs at the Bondi-Hoyle-Lyttleton rate or where super-Eddington accretion occurs via a slim disc during gas-rich galaxy mergers. The main difference between these two model variants lies at the low end of the predicted mass and luminosity functions at 4 ≤ z ≤ 6, where a clear gap appears in the first model, reflecting the stunted growth of light BH seeds formed as remnants of the first stars. Detecting this signature will be extremely challenging even for the future generation of space observatories, such as JWST, Athena, and Lynx
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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