1,721,447 research outputs found
Revisiting the Oldest Stars as Cosmological Probes: New Constraints on the Hubble Constant
Full text linkDespite the tremendous advance of observational cosmology, the value of the Hubble constant (H (0)) is still controversial (the so-called "Hubble tension") because of the inconsistency between local/late-time measurements and those derived from the cosmic microwave background. As the age of the Universe is very sensitive to H (0), we explored whether the present-day oldest stars could place independent constraints on the Hubble constant. To this purpose, we selected from the literature the oldest objects (globular clusters, stars, white dwarfs, and ultrafaint and dwarf spheroidal galaxies) with accurate age estimates. Adopting a conservative prior on their formation redshifts (11 & LE; z (f) & LE; 30) and assuming & omega;( M ) = 0.3 & PLUSMN; 0.02, we developed a method based on Bayesian statistics to estimate the Hubble constant. We selected the oldest objects (>13.3 Gyr) and estimated H (0) both for each of them individually and for the average ages of homogeneous subsamples. Statistical and systematic uncertainties were properly taken into account. The constraints based on individual ages indicate that H (0) < 70.6 km s(-1) Mpc(-1) when selecting the most accurate estimates. If the ages are averaged and analyzed independently for each subsample, the most stringent constraints imply H (0) < 73.0 km s(-1) Mpc(-1) with a probability of 90.3% and errors around 2.5 km s(-1) Mpc(-1). We also constructed an "accuracy matrix" to assess how the constraints on H (0) become more stringent with further improvements in the accuracy of stellar ages and & omega;( M ). The results show the great potential of the oldest stars as independent and competitive cosmological probes not limited to just the Hubble constant
An analytical late–Universe approach to the weaving of modern cosmology
Full text linkCombining cosmological probes has consolidated the standard cosmological model with per cent precision, but some tensions have recently emerged when certain parameters are estimated from the local or primordial Universe. The origin of this behaviour is still under debate; however, it is crucial to study as many probes as possible to cross-check the results with independent methods and provide additional pieces of information to the cosmological puzzle. In this work, by combining several late-Universe probes (0 < z < 10), namely, Type Ia supernovae, baryon acoustic oscillations, cosmic chronometers, and gamma-ray bursts, we aim to derive cosmological constraints independently of local or early-Universe anchors. To test the standard cosmological model and its various extensions, considering an evolving dark energy equation of state and the curvature as a free parameter, we analyse each probe individually and all their possible permutations. Assuming a flat Lambda cold dark matter (Lambda CDM) model, the full combination of probes provides H-0=67.2(-3.2)(+3.4) km s(-1) Mpc(-1) and Omega(m) = 0.325 +/- 0.015 [68 per cent confidence level (C.L.)]. Considering a flat wCDM model, we measure omega(0)=-0.91(-0.08)(+0.07) (68 per cent C.L.), while by relaxing the flatness assumption (Lambda CDM model, 95 per cent C.L.) we obtain Omega(k )= 0.125(-0.165)(+0.167). Finally, we analytically characterize the degeneracy directions and the relative orientation of the probes' contours. By calculating the figure-of-merit, we quantify the synergies among independent methods, estimate the constraining power of each probe, and identify which provides the best contribution to the inference process. Pending the new cosmological surveys, this study confirms the exigency for new emerging probes in the landscape of modern cosmology
An unexpected case of bifid esophagus
No full textWe propose a case of a 84 years old male patient who
presented to our department of Otorhinolaryngology for
worsening dysphagia since 3 month, that was annoy-
ing but not conditioning alimentation. Anamnesis did
not suggest any risk factor or co-morbidity that could
explain the disturb. In particular, no history of surgi-
cal interventions to gastrointestinal tract, no neurologi-
cal or geriatric disorders, no history of cancer
Massive and old quiescent galaxies at high redshift
Full text linkAims: Questions of how massive quiescent galaxies rapidly assembled and how abundant they are at high redshift are increasingly important in the study of galaxy formation. Looking at these systems can shed light on the processes of galaxy mass assembly and quenching of the star formation at early epochs. In order to address these questions, we aim to identify and characterize massive quiescent galaxies from z ̃ 2.5 out to the highest redshifts at which these systems can be found. The final purpose is to compare the results with the predictions of state-of-the-art semi-analytical models of galaxy formation and evolution.
Methods: We defined observer-frame color-color diagrams to optimally select quiescent galaxies at z > 2.5 and applied them to the COSMOS2015 catalog. We refined the spectral energy distribution (SED) fitting analysis for the selected candidates to confirm their quiescent nature, then derived their number density, mass density, and stellar mass functions. Finally, we compared the results with previous observations and some current semi-analytic models.
Results: We selected candidates for quiescent galaxies in the redshift range 2.5 ≲ z ≲ 4.5 from the COSMOS2015 catalog by means of two color-color diagrams. The additional SED fitting analysis allowed us to select 128 galaxies, consistent with being massive (log(M*/M☉)≥10.6), old (ages ≳0.5 Gyr), and quiescent (log(sSFR [yr-1]) ≤ -10.5) objects at high redshift (2.5 < z < 4.5). Their number and mass densities are in fair agreement with previous observations and, if confirmed, show a discrepancy with current semi-analytical models of galaxy formation and evolution, that underpredict the number of massive quiescent systems up to a factor of ̃12 at 2.5 ≤ z < 3.0 and ̃10 at z ̃ 4.0. The evolution of the stellar mass functions (SMFs) of these systems is similar to previous estimates and indicates a disagreement with models, particularly with regard to the shape of the SMF.
Conclusions: The present results add further evidence to the possibility that massive and quiescent galaxies can exist out to at least z ̃ 4. If future spectroscopic observations carried out with, for example, the James Webb Space Telecope (JWST), confirm the substantial presence of such a population, further work on modeling the stellar mass assembly, as well as supermassive black hole accretion and feedback processes at early cosmic epochs, is needed to understand how these systems formed, evolved, and quenched their star formatio
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
Galaxies in the act of quenching star formation
Full text linkDetecting galaxies when their star formation is being quenched is crucial to understand the mechanisms driving their evolution. We identify for the first time a sample of quenching galaxies selected just after the interruption of their star formation by exploiting the [O III] λ5007/H α ratio and searching for galaxies with undetected [O III]. Using a sample of ̃174000 star-forming galaxies extracted from the SDSS-DR8 at 0.04 ≤ z< 0.21, weidentify the ̃300 quenching galaxy best candidates with low [O III]/H α, out of ̃26000 galaxies without [O III] emission. They have masses between 10^{9.7} and 10^{10.8} M_{⊙}, consistently with the corresponding growth of the quiescent population at these redshifts. Their main properties (i.e. star-formation rate, colours, and metallicities) are comparable to those of the star-forming population, coherently with the hypothesis of recent quenching, but preferably reside in higher-density environments. Most candidates have morphologies similar to star-forming galaxies, suggesting that no morphological transformation has occurred yet. From a survival analysis we find a low fraction of candidates ( ̃0.58 per cent of the star-forming population), leading to a short quenching time-scale of tQ ̃ 50 Myr and an e-folding time for the quenching history of τQ ̃ 90 Myr, and their upper limits of tQ < 0.76 Gyr and τQ <1.5 Gyr, assuming as quenching galaxies 50 per cent of objects without [O III] ( ̃ 7.5 per cent). Ourresults are compatible with a 'rapid' quenching scenario of satellites galaxies due to the final phase of strangulation or ram-pressure stripping. This approach represents a robust alternative to methods used so far to select quenched galaxies (e.g. colours, specific star-formation rate, or post-starburst spectra)
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