1,720,970 research outputs found
Barred galaxies in cosmological simulations. Tidal perturbations and feedback.
Full text linkBars are truly common objects among disc galaxies. Even though it is now commonly accepted that non-axisymmetric structures could deeply affect the life of isolated disc galaxies, by shaping their stellar and gaseous distribution, some fundamental aspects of their formation and growth are still debated. The following manuscript is divided in two parts, in which I contribute to the study of bar formation and evolution through the use of state-of-the-art hydrodynamical and cosmological simulations. In the first part of my work, I investigate the influence of tidal encounters on either unbarred, or already barred systems. Here, I take advantage of two specific simulations belonging to the Eris suite – ErisBH and Eris2k – which evolve two Milky Way-sized barred galaxies in their cosmological volume. I both analyse the direct output of these simulations, and run a sample of new cosmological zoom-in simulations, by altering the original history of galaxy-satellite interactions in the ErisBH run. In the second part, I focus on the analysis of the effect of the different feedback prescriptions implemented in the ErisBH and Eris2k runs. My conclusions depict a scenario where, once the disc has grown to a mass large enough to sustain global non-axisymmetric modes, bar formation is inevitable and tidal encounters have, a destructive effect on the two-fold overdensity. In addition, the self-gravity of the disc and its interplay with various internal processes, seem to be the main drivers of bar formation and growth. I confirm the importance of accurate feedback prescriptions in cosmological zoom-in simulations on the investigated processes, and propose a possible use of a statistical sample of barred galaxies to quantitatively assess this dependence
The feedback of quasars on their galactic environment
No full textQuasar outflows might either quench (negative) or enhance (positive feedback) star formation in galaxies located in the quasar environment. The possible outcome depend on 4 parameters: The quasar (σ) and satellite (σ*) halo velocity dispersion, their relative distance, d, and satellite disc radius, rd. We find that: (i) small satellites with σ∗<164 σ2/3200km s^−1 have their star formation quenched; (ii) in larger satellites, star formation, and hence UV/far-infrared (FIR) luminosity, is instead boosted by >80 per cent in a burst with typical duration of 5-10 Myr, if the following positive feedback criterion is met: d/rd < 15(Q/η)1/2σ200, where Q ≈ 1 is the satellite disc Toomre parameter; the disruption parameter (see equation 17) must be η > 1 to prevent complete satellite gas removal. We compare our predictions with ALMA data finding that observed satellites of z ≃ 6 QSOs on average form stars at a 3 × higher rate with respect to field galaxies at the same redshift. Further tests of the model are suggested
External versus internal triggers of bar formation in cosmological zoom-in simulations
No full textThe emergence of a large-scale stellar bar is one of the most striking features in disc galaxies. By means of state-of-the-art cosmological zoom-in simulations, we study the formation and evolution of bars in Milky Way-like galaxies in a fully cosmological context, including the physics of gas dissipation, star formation and supernova feedback. Our goal is to characterize the actual trigger of the non-axisymmetric perturbation that leads to the strong bar observable in the simulations at z = 0, discriminating between an internal/secular and an external/tidal origin. To this aim, we run a suite of cosmological zoom-in simulations altering the original history of galaxy-satellite interactions at a time when the main galaxy, though already bar-unstable, does not feature any non-axisymmetric structure yet. We find that the main effect of a late minor merger and of a close fly-by is to delay the time of bar formation and those two dynamical events are not directly responsible for the development of the bar and do not alter significantly its global properties (e.g. its final extension). We conclude that, once the disc has grown to a mass large enough to sustain global non-axisymmetric modes, then bar formation is inevitable
Bar resilience to flybys in a cosmological framework
Full text linkIt has been proposed that close interactions with satellite galaxies can significantly perturb the morphology of the main galaxy. However, the dynamics of an already formed bar following the interaction with the external environment has not been studied in detail in a fully cosmological context. In this work, analysing the cosmological zoom-in simulation Eris2k, we study the effects that a very unequal-mass flyby crossing the stellar disc has on the stability of the pre-existing bar. We characterize the evolution of the bar strength and length showing that the perturbation exerted by the flyby shuffles the orbits of stars for less than one Gyr. After this time, the bar shows a remarkable resilience, reforming with properties comparable to those it had before the interaction. Our work shows that close unequal-mass encounters, the most frequent interactions occurring during the evolution of cosmic structures, have (i) an overall minor impact on the global evolution of the bar in the long term, still (ii) the effect is destructive, and (iii) a very weak interaction is sufficient to dismantle a strong bar leading to its "apparent death". As a consequence, due to the non-negligible duration of the bar-less period, a fraction of observed spiral galaxies classified as non-barred could be prone to bar formation
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
Probing z > 6 massive black holes with gravitational waves
No full textWe investigate the coalescence of massive black hole (MBH≳10^6 M⊙) binaries (MBHBs) at 6 < z < 10 by adopting a suite of cosmological hydrodynamical simulations of galaxy formation, zoomed-in on biased (>3σ) overdense regions (Mh ~ 1012 M⊙ dark matter haloes at z = 6) of the Universe. We first analyse the impact of different resolutions and AGN feedback prescriptions on the merger rate, assuming instantaneous mergers. Then, we compute the halo bias correction factor due to the overdense simulated region. Our simulations predict merger rates that range between 3 and 15 yr−1 at z ~6, depending on the run considered, and after correcting for a bias factor of ~20-30. For our fiducial model, we further consider the effect of delay in the MBHB coalescence due to dynamical friction. We find that 83 per cent of MBHBs will merge within the Hubble time, and 21 per cent within 1 Gyr, namely the age of the Universe at z > 6. We finally compute the expected properties of the gravitational wave (GW) signals and find the fraction of LISA detectable events with high signal-to-noise ratio (SNR > 5) to range between 66 per cent and 69 per cent. However, identifying the electro-magnetic counterpart of these events remains challenging due to the poor LISA sky localization that, for the loudest signals (Mc∼106 M⊙ at z = 6), is around 10 deg^2
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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