1,721,602 research outputs found
Assembly of supermassive black hole seeds
No full textWe present a suite of six fully cosmological, three-dimensional simulations of the collapse of an atomic cooling halo in the early Universe. We use the moving-mesh code AREPO with an improved primordial chemistry network to evolve the hydrodynamical and chemical equations. The addition of a strong Lyman-Werner background suppresses molecular hydrogen cooling and permits the gas to evolve nearly isothermally at a temperature of about 8000 K. Strong gravitational torques effectively remove angular momentum and lead to the central collapse of gas, forming a supermassive protostar at the centre of the halo. We model the protostar using two methods: sink particles that grow through mergers with other sink particles, and a stiff equation of state that leads to the formation of an adiabatic core. We impose threshold densities of 10(8), 10(10), and 10(12) cm(-3) for the sink particle formation and the onset of the stiff equation of state to study the late, intermediate, and early stages in the evolution of the protostar, respectively. We follow its growth from masses similar or equal to 10 to similar or equal to 10(5) M-circle dot, with an average accretion rate of (M+) similar or equal to 2 M-circle dot yr(-1) for sink particles, and similar or equal to 0.8-1.4M(circle dot) yr(-1) for the adiabatic cores. At the end of the simulations, the H pi region generated by radiation from the central object has long detached from the protostellar photosphere, but the ionizing radiation remains trapped in the inner host halo, and has thus not yet escaped into the intergalactic medium. Fully coupled, radiation-hydrodynamics simulations hold the key for further progress
Large Dark Matter Content and Steep Metallicity Profile Predicted for Ultradiffuse Galaxies Formed in High-spin Halos
No full textWe study the stellar properties of a sample of simulated ultradiffuse galaxies (UDGs) with stellar mass M ⋆ = 107.5-109 M ⊙, selected from the TNG50 simulation, where UDGs form mainly in high-spin dwarf-mass halos. We divide our sample into star-forming and quenched UDGs, finding good agreement with the stellar assembly history measured in observations. Star-forming UDGs and quenched UDGs with M ⋆ ≥ 108 M ⊙ in our sample are particularly inefficient at forming stars, having 2-10 times less stellar mass than non-UDGs for the same virial mass halo. These results are consistent with recent mass inferences in UDG samples and suggest that the most inefficient UDGs arise from a late assembly of the dark matter mass followed by a stellar growth that is comparatively slower (for star-forming UDGs) or that was interrupted due to environmental removal of the gas (for quenched UDGs). Regardless of efficiency, UDGs are 60% poorer in [Fe/H] than the population of non-UDGs at a fixed stellar mass, with the most extreme objects having metal content consistent with the simulated mass-metallicity relation at z ∼ 2. Quenched UDGs stop their star formation in shorter timescales than non-UDGs of similar mass and are, as a consequence, alpha enhanced with respect to non-UDGs. We identify metallicity profiles in UDGs as a potential avenue to distinguish between different formation paths for these galaxies, where gentle formation as a result of high-spin halos would present well-defined declining metallicity radial profiles while powerful-outflows or tidal stripping formation models would lead to flatter or constant metallicity as a function of radius due to the inherent mixing of stellar orbits
Imprints of temperature fluctuations on the z ∼ 5 Lyman-α forest: a view from radiation-hydrodynamic simulations of reionization
Full text linkReionization leads to large spatial fluctuations in the intergalactic temperature that can persist well after its completion. We study the imprints of such fluctuations on the z similar to 5 Ly alpha forest flux power spectrum using a set of radiation-hydrodynamic simulations that model different reionization scenarios. We find that large-scale coherent temperature fluctuations bring similar to 20-60 per cent extra power at k similar to 0.002 s km(-1), with the largest enhancements in the models where reionization is extended or ends the latest. On smaller scales (k greater than or similar to 0.1 s km(-1)), we find that temperature fluctuations suppress power by less than or similar to 10 per cent. We find that the shape of the power spectrum is mostly sensitive to the reionization mid-point rather than temperature fluctuations from reionization's patchiness. However, for all of our models with reionization mid-points of z <= 8 (z <= 12), the shape differences are <= 20 per cent (<= 40 per cent) because of a surprisingly well-matched cancellation between thermal broadening and pressure smoothing that occurs for realistic thermal histories. We also consider fluctuations in the ultraviolet background, finding their impact on the power spectrum to be much smaller than temperature fluctuations at k >= 0.01 s km(-1). Furthermore, we compare our models to power spectrum measurements, finding that none of our models with reionization mid-points of z < 8 is strongly preferred over another and that all of our models with mid-points of z >= 8 are excluded at 2.5 sigma. Future measurements may be able to distinguish between viable reionization models if they can be performed at lower k or, alternatively, if the error bars on the high-k power can be reduced by a factor of 1.5
Simulating the effect of photoheating feedback during reionization
Full text linkWe present self-consistent radiation hydrodynamic simulations of hydrogen reionization performed with AREPO-RT complemented by a state-of-the-art galaxy formation model. We examine how photoheating feedback, due to reionization, shapes the galaxies properties. Our fiducial model completes reionization by z≈6 and matches observations of the Lyαforest, the cosmic microwave background electron scattering optical depth, the high-redshift ultraviolet(UV) luminosity function, and stellar mass function. Contrary to previous works, photoheating suppresses star formation rates by more than 50 per cent only in haloes less massive than∼10^8.4 M_Sun (∼10^8.8 M_Sun) at z=6 (z=5), suggesting inefficient photoheating feedback from photons within galaxies. The use of a uniform UV background that heats up the gas at z≈10.7 generates an earlier onset of suppression of star formation compared to our fiducial model.This discrepancy can be mitigated by adopting a UV background model with a more realistic reionization history. In the absence of stellar feedback, photoheating alone is only able to quench haloes less massive than ∼ 10^9 M_Sun at z>~5, implying that photoheating feedback is sub-dominant in regulating star formation. In addition, stellar feedback, implemented as a non-local galactic wind scheme in the simulations, weakens the strength of photoheating feedback by reducing the amount of stellar sources. Most importantly, photoheating does not leave observable imprints in the UV luminosity function, stellar mass function, or the cosmic star formation rate density. The feasibility of using these observables to detect imprints of reionization therefore requires further investigation
Origin and evolution of ultra-diffuse galaxies in different environments
Full text linkWe study the formation of ultra-diffuse galaxies (UDGs) using the
cosmological hydrodynamical simulation TNG50 of the Illustris-TNG suite. We
define UDGs as dwarf galaxies in the stellar mass range that are in the most extended tail of
the simulated mass-size relation. This results in a sample of UDGs with
half-mass radii and surface brightness
between and , similar to definitions
of UDGs in observations. The large cosmological volume in TNG50 allows for a
comparison of UDGs properties in different environments, from the field to
galaxy clusters with virial mass . All UDGs in our sample have dwarf-mass haloes () and show the same environmental trends as normal
dwarfs: field UDGs are star-forming and blue while satellite UDGs are typically
quiescent and red. The TNG50 simulation predicts UDGs that populate
preferentially higher spin haloes and more massive haloes at fixed
compared to non-UDG dwarfs. This applies also to most
satellite UDGs, which are actually ``born" UDGs in the field and infall into
groups and clusters without significant changes to their size. We find,
however, a small subset of satellite UDGs () with present-day
stellar size a factor larger than at infall, confirming that tidal
effects, particularly in the lower mass dwarfs, are also a viable formation
mechanism for some of these dwarfs, although subdominant in this simulation.Comment: 17 pages, 14 figures, accepted to 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
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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